# Shop built sparkplugs for model engines



## Brian Rupnow (Jan 26, 2022)

I am in the process of making a small sparkplug for a model internal combustion engine. It seems that Corian is the best material for the insulator, but I'm having a hard time finding Corian locally. Does anyone know of a different material than Corian that will work suitably? I need 1/4" diameter material, but if I can find it in scrap rectangular pieces I can turn it in the lathe. I can't find a source on the internet that sells Corian in 1/4" round sections or I would buy some on-line. any help would be appreciated.---Brian


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## werowance (Jan 26, 2022)

some have used teflon instead.  would that be an option?

Making mini sparkplugs. | Home Model Engine Machinist Forum


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## werowance (Jan 26, 2022)

and Ebay has several listings for Machinable Ceramic.  a bit pricey for the rod version though.  sheet versions not as bad


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## jkimberln (Jan 26, 2022)

Corian is just a 70-80% mixture of Alumina and acrylic.  For us, that is very fine alumina grinding compound and liquid acrylic (methyl methacrylate).  In principle you could just mold your own insulators (and put in the center electrode).  Liquid acrylic is available at most art supply stores.  The alumina you might already have in the shop, at least, I do.  The problem I see with this is making the mold and getting the mix to release from it once it gets hard.  I don't know if you could machine it in a lathe since it is brittle, but others do it so I guess you could also.  If you had the mix, you could pack a brass tube with the stuff, let it get semi hard, then just shove it out to make rods for machining later.  Quite possibly, you could substitute any epoxy for the acrylic.  JB Weld?


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## olympic (Jan 26, 2022)

I have some Corian sticks here, more than I need.

If you're interested, PM me with your address and colour choice, and I will send some along.

I' m in Ontario, so mailing is easy.


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## Brian Rupnow (Jan 26, 2022)

Okay, thanks for the offers. I just found a local cabinet making place about three miles from me. They gave me enough Corian to build about ten thousand sparkplugs.


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## Jasonb (Jan 26, 2022)

You won't find it in rounds as it is only made in sheets. Often sold in sticks for the pen turning hobby or offcuts from a company that fabricate worktops and countertops.


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## Madsciguy (Jan 26, 2022)

Brian, I have some 1/2" thick Macor you can have. If it's suitable, just send me your address.


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## Brian Rupnow (Jan 26, 2022)

Thanks, but I'm good now. That piece I scored earlier today will last me till I die.


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## Harglo (Jan 27, 2022)

Brian Rupnow said:


> I am in the process of making a small sparkplug for a model internal combustion engine. It seems that Corian is the best material for the insulator, but I'm having a hard time finding Corian locally. Does anyone know of a different material than Corian that will work suitably? I need 1/4" diameter material, but if I can find it in scrap rectangular pieces I can turn it in the lathe. I can't find a source on the internet that sells Corian in 1/4" round sections or I would buy some on-line. any help would be appreciated.---Brian


Brain
Try a counter top shop for some corian. Teflon in a small OD an hole in the middle might compress an spread out some. Ive used some corian for these little plugs an have some crack donna no why unless the heat. There is machinable ceramic not sure if you have to harden it for plug use.
Harvey


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## DKGrimm (Jan 27, 2022)

Brian,
Corian is nice to work with and makes beautiful spark plugs.  I found one limitation, though, through experience.  I made some long-reach 10mm spark plugs for my Red Wing to reach through the casting well into the combustion chamber (see photo).  I had a lot of carbon buildup on the plug in that engine in the past, so I made them "high heat," with a long, tapered insulator.  (Sharp electrode:  another story -- trying to make it run with a low-energy magneto).  It did run, I'm happy to say, but I smelled burning plastic with the first puffs of exhaust, and by the end of 20 seconds, it quit.  The tip insulator was burned completely away.

I have had much better luck with Corian in the combustion chamber with an ignitor for the same engine.  This was in the very same combustion chamber, but the exposed portion was much smaller and close to metal for heat sinking.  That ignitor has run for the full duration of at least 25 shows with no degradation whatsoever.

Regards,

Don


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## roncohudd (Jan 27, 2022)

Brian Rupnow said:


> I am in the process of making a small sparkplug for a model internal combustion engine. It seems that Corian is the best material for the insulator, but I'm having a hard time finding Corian locally. Does anyone know of a different material than Corian that will work suitably? I need 1/4" diameter material, but if I can find it in scrap rectangular pieces I can turn it in the lathe. I can't find a source on the internet that sells Corian in 1/4" round sections or I would buy some on-line. any help would be appreciated.---Brian


Maybe delrin.


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## Brian Rupnow (Jan 27, 2022)

Okay---we're on our way to a sparkplug. Here you can see the detail drawing of part#1. You can also see that it took me two attempts to actually make the part. Making this part is one of those things where if you don't follow the correct machining sequence, it will bite you on the bum. In the part on the left, I machined away both round diameters and tried to hold the part in my 3 jaw while I threaded it. The part slipped in the 3 jaw, smearing all the nice sharp corners of the hex portion. In the second attempt on the right, I left the hex portion full length of the plug, machined and threaded one end, then turned the part around and held it in a special chucking fixture I made and machined and drilled the other end.--Much better results.--Be aware--I have an oddball 5/16" fine thread on this sparkplug, but if you make one, that end can be 1/4"-32 which is a common sparkplug size.


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## Brian Rupnow (Jan 27, 2022)

So, now we have all of the parts for a sparkplug except for the Corian. I will turn the piece of Corian later today or tomorrow.


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## Brian Rupnow (Jan 27, 2022)

This is where we are eventually going.


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## gbritnell (Jan 27, 2022)

Brian,
How are you going to secure the insulator into the body of the plug? I make my plugs the same as Steve Huck. Make the cylindrical boss at the top of the plug body with an .008 wall thickness.  Turn the insulator with a shoulder. Make a die set to crimp the flange over the shoulder of the plug. I have made 3-4 die sets for different sized plugs and it works great.
gbritnell


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## Brian Rupnow (Jan 27, 2022)

I watched a guy on YouTube make this plug, and he was just using an epoxy to stick things together. It wasn't J.B.weld, but something like it. Can you tell me more about the die sets you use for crimping? I made up the Corian insulator this afternoon. It machined beautifully. Then I went to drill the 1/16" hole thru the center for the 1/16" music wire, and everything turned to crap. Will try again tomorrow with a brand new 1/16" drill. I remember having this same trouble 12 or 13 years ago when I built the sparkplug for the Kerzel hit and miss.


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## DKGrimm (Jan 27, 2022)

My experience drilling Corian is that it doesn't like to be pushed.  Slow speed, sharp drill, light pressure, and small increment peck drilling will do it.  You have to keep it from starting to soften and melt at the drill tip.


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## almega (Jan 28, 2022)

Brian - Being new to spark plugs, I find this interesting and could be significantly more cost effective than buying them but I have some questions, if you don't mind. In the sequence of operations for the body, did you first machine the external round dimensions, then cut the threads, followed by milling the hex profile and then finally bore the internal bore and counterbore? Also, for the material used, was it 12L14 or 1018? Do you think fastening the insulator into the body could be done with some of the extreme heat JB Weld, which is supposed to be good for 1000 F degrees? To help secure the insulator would it be necessary to texture the internal of the bore and the OD of the insulator to give the adhesive some purchase (perhaps even thread)? In operation, does the spark jump between the electrode and the body or does there need to be a small bit of something to create a gap similar to an automotive spark plug?


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## pileskis (Jan 28, 2022)

I’d use tungsten for the center electrode too. Why music wire?

Sid


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## stevehuckss396 (Jan 28, 2022)

1/16 inch is awfully large for an electrode. You may be having trouble because the walls of the insulator are getting thin. My largest electrode is .040 and smallest is .015 inch. Also the drill bit needs to be sharp,  like brand new sharp. Corian does not like dull cutters.


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## stevehuckss396 (Jan 28, 2022)

Because music wire is cheap, comes in hundreds of sizes, and will last the life of the model.


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## kquiggle (Jan 28, 2022)

I have yet to make a sparkplug, but it's on my list of things to do. Having read about different types of insulator materials, I am wondering if anyone has tried PEEK or some of the other high temperature plastics? PEEK in particular seems to be a good candidate, and while not cheap seems affordable in the small quantities required.


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## Brian Rupnow (Jan 28, 2022)

Sid--I didn't have any tungsten. Almega--There are no threads on the Corian. To machine the Corian, first turn it to overall diameter and length. Then drill the 1/16" hole. Then insert the center electrode full length into the hole--no epoxy yet. Then machine the area which is turned down to 3/16" diameter. No texturing is necessary. The material was 1018. The spark jumps from the center electrode to the body. Apparently it is okay to use J.B.Weld to "glue" these parts together.  I strongly advise that you buy a finished sparkplug from a supplier.  The hours you spend making your own sparkplug will far outweigh the expense of buying one.


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## Jasonb (Jan 28, 2022)

Surly the metal in the JB Weld is only a problem if you get a big blob of it where you don't want it. A very thin smear against the metal is not going to cause a short as it will be no greater distance to the actual metal.

Having said that where do you get your details that it will cause a short? I know of one supplier of model engines that sell it and suggest it's use as an electrical insulator.


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## almega (Jan 28, 2022)

About the JB Weld and metal content, wouldn't it only be in contact with the metal and and the insulator at the separate bonding surfaces between the body and the electrode and would not bridge across the electrode to the body at any point? At least that is how it appears to me from your drawing. What am I missing? I do understand the time aspect but this is a hobby and consumption of time and gaining experience/knowledge is the point. If I was trying to make a living off my machining capabilities, I would starve to death in a very short time.


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## Jasonb (Jan 28, 2022)

Just looked on JB Weld's website. They say it won't conduct electricity and is considered an insulator which would tend to agree with the supplier of engine kits.


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## Brian Rupnow (Jan 28, 2022)

This fellow has a great video on making sparkplugs. Most of what I've shown here is taken directly from the video. The video is a two part series.


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## Brian Rupnow (Jan 28, 2022)

I just looked up the j.b.weld specs, and you're right. It says it is a good insulator, so that is what I will use.


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## almega (Jan 28, 2022)

Here is the stuff I have and looking at the ingredients, there is no metal in it. Mostly silica and resin. Also, it is not a two part material so you don't have to mix anything. I haven't used it yet but others have said it will do the job.


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## gbritnell (Jan 28, 2022)

As with a lot of processes in model making there are many ways to achieve somewhat the same end. This is the process that I use and Steve Huck makes his the same way. 
The first picture show the components needed for crimping the plug flange over the shoulder on the insulator. There is the guide sleeve, the plug thread bushing and the crimping die. 
The next 2 photos are of an old plug body that I take to shows to show people how the plugs are made. In the first picture you can see the thing flange around the top of the plug body.
The next photo shows the plug screwed into the threaded bushing.
The next photo shows the plug/bushing inserted into the guide sleeve. 
The next photo is of the crimping die. I made this from W-1 drill rod and hardened it. I have made them just from drill rod without hardening but after a number of plugs they start to cut a step into the chamfer, so that is why I now harden them. The chamfer is about 30 degrees from the horizontal. The hole in the crimping die is just a little bigger than the insulator diameter (couple of thousands for clearance)
The next photo shows the crimping die inserted into the bushing.
The last photo is one of my 1/4-32 size plugs with the flange crimped in place. No epoxy, no JB Weld and no leaks.


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## gbritnell (Jan 28, 2022)

Oh and by the way I use .025 music wire for my electrodes. Some of my insulators are .700 long and I drill the whole thing with a .025 drill using a fairly fast spindle speed and just pushing the tailstock by hand so I can peck in and out. As the drill get deeper into the Corian you'll need to clean the swarf from the flutes otherwise it will bind up. I just put a dab of spit on the dill as I get deeper into the Corian. This helps the  Corian from sticking to the drill. I have made about 80 plugs this way with never a broken drill. I solder the electrode into the brass top leaving it long.  I then insert it into the plug and keep adjusting the length until I get the proper gap. I have even made some with threaded brass caps (3-48 thread) When the electrode wears I just unscrew it, heat it up, pull the electrode wire out a little bit and readjust the gap.


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## gbritnell (Jan 28, 2022)

Here are a couple of pictures of the plugs for my Ford 300 six that Terry Mayhugh is currently building. You can make them any size you want.


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## Jasonb (Jan 28, 2022)

Almega, thi sis the one we all tend to use






						OriginalEpAdh | JB-Weld
					






					www.jb-weld.co.uk


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## Jasonb (Jan 28, 2022)

George do you silver solder the electrode  into the cap or just use electrical/plumbing solder which is what I have used on the lower number of plugs that I have made. I can't really see the need for silver solder as the Corian will have long since burned away before the plug gets hot enough to melt the solder.


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## almega (Jan 28, 2022)

Jasonb said:


> Almega, thi sis the one we all tend to use
> 
> 
> 
> ...


jasonb - I do use that version quite a bit for other projects where temperature is not an issue, usually like body putty to clean up the lines of whatever I'm working on, before painting. I was concerned about using it in areas of engines where things get pretty hot. Have you had experience with it in hot areas?


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## Brian Rupnow (Jan 28, 2022)

So here we are---a finished sparkplug. If I made these and sold them, I'd have to charge at least $300 each, considering the time it took me to make it. The components are J.B.Welded together. Does it work?--Not sure yet, but after that j.b. weld sets up for an hour I'll find out. Again, I caution you, if you need a sparkplug for your model engine, then buy it from somebody who does this as a business. The last plug I made was about 12 or 13 years ago. The next plug I build may be  12 or 13 years from now. If you want to make one just to see if you can, well now you have the drawings.


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## Brian Rupnow (Jan 28, 2022)

This is the drawing of the Corian insulator. And I must say, Corian is the worst damned stuff I ever worked with. It took me three different attempts before I made one that didn't crack or split. That's okay though, I doubt I will make anymore sparkplugs.


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## Brian Rupnow (Jan 28, 2022)

George--a huge "Thank you" for all the explanation and pictures.---Brian


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## pileskis (Jan 28, 2022)

stevehuckss396 said:


> Because music wire is cheap, comes in hundreds of sizes, and will last the life of the model.


Steve- I yield to your expertise of course. 
I used tungsten welding electrodes.
Brian does have a TIG welder now. Might as well use those electrodes for something!!
I used .040.

Sid


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## Brian Rupnow (Jan 28, 2022)

And here we have the finished sparkplug. It works, just as I figured it would. I'm not quick enough on the trigger to get a single picture of the spark when it occurs. That is why I made the attached video.---Brian


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## Carbuilder (Jan 28, 2022)

kquiggle said:


> I have yet to make a sparkplug, but it's on my list of things to do. Having read about different types of insulator materials, I am wondering if anyone has tried PEEK or some of the other high temperature plastics? PEEK in particular seems to be a good candidate, and while not cheap seems affordable in the small quantities required.



I made 1 spark plug for my Little Demon so far and used PEEK. Bought it from McMaster Carr; 1/4" diameter rod is pretty cheap. It machines nicely, is slightly flexible, not brittle at all. No idea how it will work, but I think it should be OK.


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## Carbuilder (Jan 28, 2022)

Nice sparks there Brian!


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## Steamchick (Jan 29, 2022)

Unless you are running "car" distances, there should be no need for a tungsten tipped electrode. Steel is OK: Piano wire is high tensile steel. A side electrode allows the spark to be located at a particular point, and closes the gap where it would otherwise be too large (and cause flash-over on the insulator - not a good place to try and ignite the gas!). A centre earth (like on a car plug) may project too far into the combustion chamber on some models.
It is a combination of geometry and getting the optimum spark location for firing the mixture.
The insulator shape is quite critical on car engines - etc. - for whatever purpose, as the shape determines the heating (to burn off combustibles) and cooling (so it doesn't melt!). Get that wrong and suffer problems of running or a holed piston.
I know a CZ engine (2-stroke motorcycle) that would start with ANY spark plug, but stopped in seconds to minutes, depe3nding on which plug of 6 was fitted -  until the correct heat of plug was fitted, when it ran for thousands of miles.... The owner thought it "should be OK" with a similar plug - just a single grade "cooler running" often called a "hotter" plug, as it runs in "hotter-running" engines!). - lesson - do as the book says... I solved it for him by cleaning the original (rusty) plug - as it was the correct grade according to the book... He then bought a new plug of the correct grade, instead of trying everything else in his junk box!
So get the HEAT RANGE shape correct.
K2


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## Steamchick (Jan 29, 2022)

PEEK may not withstand combustion temperature, at tip temperatures of the insulator that are high enough to burn off any fouling. IE. around 400deg.C...?   Try domestic-fuse-body ceramics?
Pure PTFE may work, as it ablates when it gets hot. I just can't believe it will be durable though. And must be clamped into the steel body, as it does not stick to any adhesive (chemical adhesion). - Only fried egg in the frying pan...
K2


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## Carbuilder (Jan 29, 2022)

Corian, from the specs I can find, is not good for anywhere near 400C (212F seem to be the max recommended temperature), but seems to work just fine. I suppose the heat is being removed from the plug faster than the combustion temperatures can heat it up. PEEK should resist higher temperatures than Corian.


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## Brian Rupnow (Jan 29, 2022)

And, as a fitting end to this post, the engine that this sparkplug was intended for is up and running very well.


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## almega (Jan 29, 2022)

That is spectacular. Just out of curiosity, what prompted you to make the "$300" spark plug for this engine as opposed to purchasing one as you had suggested?


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## ranger (Jan 29, 2022)

Found these in a ’Tin’ I kept ‘glow’ plugs in. The ‘NGK’ plug is obviously a purchased one, the other is one made from the body of a burnt out glow plug, a piece of piano, (music) wire And either a piece of Epoxy ‘Stick’ or ‘JB Weld’. I can’t remember if I made a mould for the JB Weld, or used the epoxy stick and ‘Finger’ formed it. The insulator is flush with the end of the body, and sanded flush after curing. This ‘Surface Discharge’ plug worked ok in a modified two stroke aero model engine (Methanol and 20% Castor oil without fouling). The NGK plug would foul. Another possible benefit of The surface discharge plug in a very small engine is no excess volume inside the plug to affect compression etc. If you know people who are into ‘Proper’ RC aircraft, (not electric), or cars, you shouldn’t have a problem obtaining a regular supply of ’burnt out’ glow plugs, saves making the bodies!


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## Brian Rupnow (Jan 29, 2022)

I did first buy a sparkplug from  gentleman who sells them on the internet. He makes good quality sparkplugs and will make "specials" on request. I could get the engine to fire and run with one of these purchased plugs in it, if I was running a Viton o-ring on the piston, but not with  cast iron rings. After making and installing my cast iron rings, the engine wouldn't fire. I checked everything I could on the engine, and compression was good, ignition timing was good, gas tank was full---everything was perfect---except that the engine wouldn't fire. The only thing left that it could possibly be was the purchased sparkplug.-So---I built a sparkplug. And I still couldn't get the engine to run--and so I decided one more time to reset the valve timing. That fixed it. The engine ran.---Probably if I had checked the valve timing before building the sparkplug, it would have ran with the purchased sparkplug.


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## stewart drummond (Jan 29, 2022)

surface discharge , no separate earth electrode tip , good luck


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## chevell69 (Jan 30, 2022)

Brian Rupnow said:


> So, now we have all of the parts for a sparkplug except for the Corian. I will turn the piece of Corian later today or tomorrow.


do you have to use corian can u use delrin


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## Oldiron (Jan 30, 2022)

If you want tungsten how about a TIG electrode?


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## Brian Rupnow (Jan 30, 2022)

I didn't want to use a tig electrode. I could have, I have a package of them for my tig welder. I also have two 36" lengths of music wire, which is considerably cheaper than tig electrode. As far as using Delrin is concerned, I know two people making and selling sparkplugs, and they use Corian. Why would I stick my neck out and use something that I had never seen anybody else use?


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## Brian Rupnow (Jan 30, 2022)

Okay kids and turtles--Here is a strange observation. From the brass cap to the base on this spark plug is 1/2". When the engine is running, it runs good for a while, then begins arcing between the brass cap and the steel body along the outside of the Corian. The gap between the center electrode and the steel body is only 1/16" at the tip, down inside the cylinder. It will arc two or three times in a row, and the engine falters and stumbles when that is happening. Then it will stop arcing and the engine will resume running normally again. I guess the solution to that would be to make another sparkplug 1/4" longer . I doubt that it would arc thru a 3/4" air gap.


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## Gordon (Jan 30, 2022)

Does the Corian become slightly conductive when it gets hot? Regular spark plugs are porcelain or similar.


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## almega (Jan 30, 2022)

Once it did its arcing thing did it repeat that? If not, is it possible there was some contamination on the outside of the insulator that acted as a conductor until it was burned off and then the engine performed as expected?


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## Jasonb (Jan 31, 2022)

If you look at all the other spark plugs pictured in this thread and the other commercial ones like Rimfire none have the brass endcap overhanging the insulator. Spark is less likely to want to jump around a corner. Look at your design again rather than just making it longer.

It is always harder for a spark to jump in the compressed gas than outside the engine plus if the plug is oiling up then the spark will take the easy route. Your straight wire discharging to the plug body will also mean a larger gap than a bent wire or the usual bar on the body so spark has more difficulty jumping the bigger gap in the combustion chamber so more likely to jump outside.


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## stevehuckss396 (Jan 31, 2022)

1/16 is a huge gap. Gap should be about half that or less. Under compression it is very difficult to get a spark to jump a gap. With a gap that big it must become easier to jump the half inch gap. Electricity always takes the path of least resistance.


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## Stefan-K (Jan 31, 2022)

Jasonb i totally agree. That's exactly what happens here. I would also remove the overhanging brasscap. Instead i would drill a small piece of threaded rod, maybe M3 and solder the music wire into it and then glue the whole Thing into the plug. 
Most People forget that you need much more energy to make a spark jump a gap in a compressed Gas than outside in atmospheric pressure. High voltage will always take the easiest way. And half an inch is no Problem for a couple thousand volts. 
Stefan


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## Jasonb (Jan 31, 2022)

Yes the body of the plug would be better off drilled through say 1/8" and the 3/16" hole stopped just short of breaking through which combined with the 1/16" electrode would mean a 30thou gap.

Adjust the 1/8" hole to get the desired gap eg if you want 25thou then use a #33 or 2.9mm drill


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## Rocket Man (Jan 31, 2022)

WHY build your own spark plug they come in 100s of, sizes, shapes, lengths, threads.  You can make adaptors so any spark plugs will screw into any engine.   I buy 1" long sparks plugs from O'Riley auto parts they cost $3 each.  I thought this photo would show the 1" spark plug but its on the back side.  I don't remember part number, I have to walk out to the shop to see what the # is.   Spark plugs also come in smaller sizes 3/4" long and 1/2" long.


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## Carbuilder (Jan 31, 2022)

Rocket Man said:


> WHY build your own spark plug they come in 100s of, sizes, shapes, lengths, threads.



Seems like kind of a strange thing to say on a forum like this. Why make anything when you can buy it? I do it for the challenge. If you just want a part to get a job done, then fine. But the satisfaction of designing and making something that didn't exist before you made it.....as the ads say....priceless!


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## Rocket Man (Jan 31, 2022)

Carbuilder said:


> Seems like kind of a strange thing to say on a forum like this. Why make anything when you can buy it? I do it for the challenge. If you just want a part to get a job done, then fine. But the satisfaction of designing and making something that didn't exist before you made it.....as the ads say....priceless!



Do you also mine your own iron ore, cast & forge your own metal, make your own, nuts, bolts, screws and make your own paint.  I build engines too but I buy several things I need, nuts, bolts, screws, set screws, metal, belts, piston rings, pins, roller bearings, O'Rings, pipes, spark plugs, gasket material, valves, springs, paint, oil, grease.  I design my own engines I don't build kits.   I never would have guessed people make every single part they need.   I worked as Design Engineer for many years in factories we built special machines but never made parts we could buy like, electric motors, gear boxes, bolts, flat washers, nuts, chain, V belts, sprockets, gears, hydraulic pumps, radiators, rubber hoses, electric wire, wire connectors, electric boxes, fuses, switches, relays, etc.   I know what you mean the satisfaction of designing & building something.  For each his own.   Have FUN.   I almost forgot to mention fire clay works good in spark plugs don't add more than 3% water mix very well then hammer it to compact it very tight in the hole with home made tools.


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## Brian Rupnow (Jan 31, 2022)

Something you should know---I'm not Mister sparkplug. Like I said, the last one I built was about 13 years ago. I'm sure that the arcing on the outside of the sparkplug was because of the relatively huge 1/16" gap at the business end. I didn't know it was going to do that, or I would have made the sparkplug 1/2" longer. See--now I know something that I didn't know before. Before I do anything more, I will pull out the sparkplug I made and put the one I purchased in instead. I'm sure it will work fine. If it does or doesn't I will let you know.-------I spent today making new cabinets for tooling. It was very nice not being Mr. Machinist for a day.


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## Brian Rupnow (Feb 9, 2022)

The tip of my new sparkplug electrode is almost totally eaten away by what I assume is spark erosion. Does a tungsten electrode resist spark erosion better than steel?


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## stevehuckss396 (Feb 9, 2022)

Pull the gap back to about .020 to .025 inches. A gap that is too large will stress the s#!* out of the ignition system. The coil has to work way too hard to jump the gap. Excessive volts means excessive energy that is not needed to ignite the fuel. That creates stress on the coil, plugs, and the CDI or points. 

The Gap on my V8 is only .012 inch and works just fine. 1/16 inch is just way too much.


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## Brian Rupnow (Feb 9, 2022)

Steve--I'm in the process of a redesign to my sparkplug right now, and the new design reduces the spark gap to 0.030"  I ask about tungsten withstanding spark erosion because I have 1/16" tungsten electrodes for my tig welder that I can use..


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## stevehuckss396 (Feb 9, 2022)

If you get the gap down to 20 to 25 there will be way less wasted energy and the music wire will be sufficient. Be much easier on the coil too.


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## Brian Rupnow (Feb 9, 2022)

Okay---still in design phase here. Will set up for a 0.025" gap.


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## Brian Rupnow (Feb 9, 2022)

Perhaps the large gap of 1/16" accounted for the rapid spark erosion of the center electrode.


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## trackmaggot (Feb 9, 2022)

If you decide to investigate Tungsten electrodes, you may be interested in micro torch electrodes. You can get them in .5, 1.0 and 1.5mm diameters.  They are 66mm (2.6 inches) in length, and come in packs of 15 for 45USD.
Link below for reference









						Orion Micro TIG Welding Electrodes
					

Orion Tungsten Electrodes for Orion Pulse Arc Micro Welders.




					sunstonewelders.com


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## gilbycoath (Feb 9, 2022)

i have not made a spark plug, I have under construction a Westbury Seagull.
So my suggestion maybe out of order. but I have a collection of Nichrome wire for the plugs, my idea of what might be a good material.
Haven't seen it mentioned before


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## TonyM (Feb 10, 2022)

Nichrome has high resistivity and is used for heating elements, Whether it is enough to make a difference between that and tungsten or carbon steel in a small spark plug I do not know.


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## Jasonb (Feb 10, 2022)

Brian, I have used tig electrodes for spark plugs and the contacts on ignitors and no signs of errosion

Plug gap was smaller than your 1/16" but if you make the body like I showed it won't be a problem as you can set the gap by the hole dia at the end.


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## Steamchick (Feb 10, 2022)

Brian Rupnow said:


> The tip of my new sparkplug electrode is almost totally eaten away by what I assume is spark erosion. Does a tungsten electrode resist spark erosion better than steel?


Yes!
But if simple points ignition, try reversing the polarity?  Or take an old car spark-plug and use the core electrode and side electrode as the material to make your own. (BE VERY CAREFUL when breaking off the ceramic! Wear proper eye protection!) Often these are odd alloys?
Nichrome is OK as the resistance won't affect the spark.
K2


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## Jasonb (Feb 10, 2022)

I'd rather change the electrode than the polarity, Your spark plug body has a lot more work invested in it than a length of music wire and being a lower carbon steel than the wire will probably erode even faster.


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## Steamchick (Feb 10, 2022)

Hi Jason, not knowing much about spark erosion, why does more carbon (easy to burn off in a spark!) make the iron molecules erode slower (or quicker?).
I can cope with a more massive molecule - like tungsten - needing more arc energy to erode a molecule, but iron is iron, carbon is carbon, and I know it changes the crystalline structure to change the mix, but I am without knowledge of the arc resistance of steels.
K2


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## Jasonb (Feb 10, 2022)

Just an assumption that the unhardened body of the plug would likely be eroded quicker than the hardened electrode, even if it were no slower I'd still not want to have the body get eroded by swapping polarity as it would have taken a lot more time to machine than just cutting the electrode.


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## Steamchick (Feb 10, 2022)

Fair comment Jason. But sometimes life is a compromise. The tiny bit of central electrode may be too small for sensible running lifetimes, but the body metal may be tens of times more, and thus more durable. But that only works if the polarity reversal makes a difference!
K2


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## stevehuckss396 (Feb 10, 2022)

There has to be more to it than that. I have had the same plugs in my Peewee since first start 2010. No electrode erosion of any kind. They still work very well. My theory is that the gap is huge and the voltage has to build up to a very high voltage to jump the gap damaging the electrode. Another theory is, if an automotive coil (old school) is being used it may be delivering way more energy (*Joules) *than the little CDI coil which is all volts and no amps. Or theory number 3, Both!


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## johwen (Feb 10, 2022)

Hi Brian,
I make my own spark Plugs very simple to make see attached sketch. No fancy materials needed Body 3/8 inch hex length to suit engine 4mm ceramic tube with 2mm bore. Nice square ends, protrude though the end of body .020 inches for spark gap. Centre electrode mild steel turned down to 2mm from 5mm OD rod. Length to suit length of insulator. Insulator length say .750" above body. When inserted into insulator and Loctite in place you will end up with a full circle .020" gap. Make a brass terminal nut to suit. These plugs work extremely well for me, easy to clean and gap is permanent and look scale in the engine.
See attached sketch, I made a mistake and said 5/16" hex when it should have been 3/8" John


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## rutzen (Feb 11, 2022)

Hi, I made the spark plug for my Rumely tractor, I used Corian for the insulator and I used nickel silver 1/16 rod for the electrode and there is no sign of any erosion so far.  I seem to remember from way back that conducting the heat away from the central electrode is more important than the hardness of the material. I just happened to have the nickel silver, I would have used brass otherwise.  It's easy to solder the connection the top as well.


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## awake (Feb 11, 2022)

I'm wondering if hardness of the material is a red herring. EDM (spark erosion) seems to work equally well regardless of soft or hard material ... right?


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## almega (Feb 11, 2022)

johwen - I am curious as to where you are able to purchase your ceramic tubing.


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## johwen (Feb 11, 2022)

You will find several listed on EBAY try your local area it is used in High temperature insulation in many areas such as oven manufacturers.

I was lucky here in Melbourne Australia to be able to buy same from a manufacturer about 6 kilometers from where I live. he even cut them to length for me.

So do a search on the net you will find some one who can help. Johwen


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## Bentwings (Feb 13, 2022)

johwen said:


> Hi Brian,
> I make my own spark Plugs very simple to make see attached sketch. No fancy materials needed Body 3/8 inch hex length to suit engine 4mm ceramic tube with 2mm bore. Nice square ends, protrude though the end of body .020 inches for spark gap. Centre electrode mild steel turned down to 2mm from 5mm OD rod. Length to suit length of insulator. Insulator length say .750" above body. When inserted into insulator and Loctite in place you will end up with a full circle .020" gap. Make a brass terminal nut to suit. These plugs work extremely well for me, easy to clean and gap is permanent and look scale in the engine.
> See attached sketch, I made a mistake and said 5/16" hex when it should have been 3/8" John


Very innovative actually I think model spark plugs are available I think they us 1/4 32 to  I don’t ave the supplier name handy most modern two and fou strokes now use automotive standard plus even my model plane engines use standard over the counter plugs


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## almega (Feb 13, 2022)

I am thinking it is an interesting challenge to build your own spark plugs but with them available at $5.99 each, why bother. That would be like making your own nuts and bolts. Check out this link.









						WELCOME TO RCGF STORE GAS ENGINE
					

RCGF Stinger  CM6 SPARK PLUG,This spark plugs will fit RCGF Stinger 15cc RE,20cc RE,26cc SE,26cc RE,35cc SE,35cc RE ,70cc twin ,125cc twin engines.




					www.rcgfusa.com


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## Vietti (Feb 13, 2022)

Or go to your auto parts store and pay $4 +/-.  Google them, good deals on ebay too.


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## Ghosty (Feb 13, 2022)

almega said:


> I am thinking it is an interesting challenge to build your own spark plugs but with them available at $5.99 each, why bother. That would be like making your own nuts and bolts. Check out this link.
> 
> 
> 
> ...


They are M10x1 threaded plug, no good for 1/4" plug application


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## stevehuckss396 (Feb 13, 2022)

"I am thinking it is an interesting challenge to build your own spark plugs but with them available at $5.99 each, why bother. That would be like making your own nuts and bolts. Check out this link."


Because I dont have a single engine that that giant spark plug will fit in. I don't think any of the sparkplug makers care much about making #8-40 plugs. Either I make them or I don't have them. If you find 8-40 plugs for sale, send me that link.


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## almega (Feb 13, 2022)

stevehuckss396 said:


> Because I dont have a single engine that that giant spark plug will fit in. I don't think any of the sparkplug makers care much about making #8-40 plugs. Either I make them or I don't have them. If you find 8-40 plugs for sale, send me that link



Best I could find was 10-40 plugs here:  World Famous Rimfire Spark Plugs Distributed by Roland Morrison

BTW, this link started with Brian making a 5/16 plug so perhaps in your instance it would be necessary to make your own 8-40 plugs.


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## Brian Rupnow (Feb 13, 2022)

I think that Steve Huck makes and sells great sparkplugs. I have ran into some weird issues with ignition on this engine I am currently playing with, but it is certainly no bad reflection on Steve's sparkplugs. I'm having some health issues right now, so I'm not forging ahead with this engine as I generally do.---Brian


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## stevehuckss396 (Feb 13, 2022)

"BTW, this link started with Brian making a 5/16 plug so perhaps in your instance it would be necessary to make your own 8-40 plugs."

Yes I know. I made the original purchased plug for him. I'm not too sure you can find a 5/16 plug either.  He also had call for an extended length plug a few months back. It's easy to say just buy one but finding something that works for your design can sometimes be impossible. I guess the point is that he made one because he couldn't just buy one. The idea that he should just buy one when they are only 6 bucks made me laugh. He had to make what didn't exist. That's why you make one.


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## almega (Feb 13, 2022)

Brian - I hope you are able to get past the health issues soon. It's not fun to be down and out.

stevehuckss396 - Not trying to be contentious here and certainly no animosity toward anyone. Given the limitations of products available in the sizes we are working with, I understand the need sometimes to make our own widgets or sometimes we just want to take on the challenge to make something that may be available. This whole discussion has been instructional for me as I was not aware such small spark plugs could be purchased at the outset. Now that I am aware they are available at such modest prices, I doubt I would make one and I will design the head(s) of my engines to accommodate what I am able to purchase. That is just my chosen path, not a path that I am proposing anyone else follow if they want to go another way. As I have learned on this forum, there are many ways to approach any problem that arises. Peace to all.


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## johwen (Feb 14, 2022)

stevehuckss396 said:


> "I am thinking it is an interesting challenge to build your own spark plugs but with them available at $5.99 each, why bother. That would be like making your own nuts and bolts. Check out this link."
> 
> 
> Because I dont have a single engine that that giant spark plug will fit in. I don't think any of the sparkplug makers care much about making #8-40 plugs. Either I make them or I don't have them. If you find 8-40 plugs for sale, send me that link.


Hi Steve,
Model engineering is all about making stuff. Some guys make the tooling to make the stuff making spark plug is a synch. With the materials at hand takes me around an hour. all it takes is about an hour and 90% of the time is turning the body and centre electrode the Loctite together and you've created something yourself. johwen


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## stevehuckss396 (Feb 14, 2022)

For me it's mostly necessity. I build small and that means having to make stuff that isn't available. Piston rings is another thing. I make them because 5/8 inch rings just aren't out there. I have even made nuts and screws because what I needed wasn't available and I wasn't willing to compromise. Believe it or not it happens.


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## simonbirt (Feb 14, 2022)

Has anybody experience of using home made spark plugs with Corian or PTFE insulators in high revving engines? The reason I ask is I made some a while back with PTFE which has a slightly higher temperature tolerance than Corian. These work fine in my Farm Boy, in fact it is still running on the original plug. Hit and miss engines run cool. I am now building a 4 cylinder Seal which may be a more demanding application, PTFE doesn’t melt as such, it vaporises at arround 360 deg C or 620 F. Corian  I believe has a lower melting point. I imagine it is the cylinder head temperature that is important as the combustion temperature is transient and obviously much higher than the melting point of most of the materials we use to contain It. 

By the way, the drawings for my plugs are on another Spark Plug thread. The PTFE insulator is held in by swaging, which works well.


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## mayhugh1 (Feb 14, 2022)

stevehuckss396 said:


> There has to be more to it than that. I have had the same plugs in my Peewee since first start 2010. No electrode erosion of any kind. They still work very well. My theory is that the gap is huge and the voltage has to build up to a very high voltage to jump the gap damaging the electrode. Another theory is, if an automotive coil (old school) is being used it may be delivering way more energy (*Joules) *than the little CDI coil which is all volts and no amps. Or theory number 3, Both!


Something to keep in mind with a points-type or simple transistor ignition is that energy is being built in the coil during dwell - that's the time the points are closed or the transistor is ON. In a multi-cylinder engine the dwell is necessarily limited to tens of crankshaft degrees since the coil has to service other plugs. The resistance in series with the coil is often minimized in order to get enough current through the coil to build enough energy in the time allotted for each plug.

In a single or dual cylinder engine, the dwell is huge. The points spend most of their time closed, and if an automotive coil with insufficient series resistance is being used, the energy built is much more than needed in a model engine. When the points open, and all this energy is dumped into a single cylinder model engine plug, you're going to greatly shorten the life of the plug. If you also run a really wide plug gap, that excess energy will show up as some terrifically high voltages during compression that can result in a rash of arc-over problems that a typical plug isn't designed to handle. (i.e. it doesn't have a 3 inch long insulator).

The solution for a single cylinder model engine isn't to design a beefier plug or to use exotic metal electrodes. The solution is use a coil designed for model engines, or reduce the coil voltage, or add resistance in series with the coil, or to use a capacitive discharge ignition that works independent of dwell.

When testing a model engine ignition by firing a plug in open air you're looking for a thin blue spark - not a fat yellow-red flame. A flame is telling you that something is burning. - Terry


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## Bentwings (Feb 15, 2022)

almega said:


> I am thinking it is an interesting challenge to build your own spark plugs but with them available at $5.99 each, why bother. That would be like making your own nuts and bolts. Check out this link.
> 
> 
> 
> ...


ha ha I like that yes there are small model spark plugs. I thing most builders are going to automotive plugs as they are readily available . All my Rc engines have them I haven’t used a small model spark plug in years. I know room is usually the issue but if you want to make them and they work fine just another project 
Byron


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## Steamchick (Feb 16, 2022)

Good explanation Mayhugh. I would add a note: Inadequate coil voltage can confuse, as you may get a nice blue spark across the desired gap "in air at atmospheric pressure" but not "inside the engine in a fuel-air mix at compression pressures". A small puff of "easy-start" can sometimes tell if the spark is happening inside the engine. turned slowly towards compression it should kick when the spark fires. (My Moto Guzzi will do it when I am checking the timing and turning the engine by hand, if there is petrol-air mix in the engine! - The "chuff" always makes me jump!).
K2


johwen said:


> Hi Steve,
> Model engineering is all about making stuff. Some guys make the tooling to make the stuff making spark plug is a synch. With the materials at hand takes me around an hour.  johwen


Fair comment, but each to his own philosophy....
Mine is: Model Engineering is the re-engineering of a full-sized "something" - to a smaller (or sometimes larger!) size in order to demonstrate particular features of the original Engineering.
E.g. even a cardboard cut-out viaduct on a double-O gauge railway, that carries a small train across a modelled valley is Model Engineering. But a stick of wood that carries the train, with a cut-out picture in front, doesn't count! ( I have seen both!). It doesn't have to be "shiny-bright metal whirring bits".
K2


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## Brian Rupnow (Feb 16, 2022)

So, here we have "spark-plug Mark 2". Also in picture is "sparkplug mark 1". The newest sparkplug is about 3/8" longer to avoid sparks jumping  to the plug body from the brass terminal on the outside of the plug. The newest sparkplug has been modified so that the spark only has to jump about 0.025" instead of the 0.062" gap on the first plug. Also, on the new plug, the brass top ends at the Corian insulator and doesn't run down the side to "cup" the Corian like the first sparkplug did. Will it be better than the first plug?--I will try it in the engine and find out tomorrow.


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## almega (Feb 16, 2022)

It definitely looks pretty.


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## johwen (Feb 16, 2022)

Brian Rupnow said:


> So, here we have "spark-plug Mark 2". Also in picture is "sparkplug mark 1". The newest sparkplug is about 3/8" longer to avoid sparks jumping  to the plug body from the brass terminal on the outside of the plug. The newest sparkplug has been modified so that the spark only has to jump about 0.025" instead of the 0.062" gap on the first plug. Also, on the new plug, the brass top ends at the Corian insulator and doesn't run down the side to "cup" the Corian like the first sparkplug did. Will it be better than the first plug?--I will try it in the engine and find out tomorrow.


Brian problem with that configuration is the spark is hidden from the fuel as it can jump at the point adjacent to the insulator for good ignition the spark needs to occur in the open that's why normal plugs are made with the spark gap out in the open so the fuel mix can easily reach the spark gap. Johwen


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## Brian Rupnow (Feb 16, 2022)

Just tested the new sparkplug out on my desk top. It sparks--not as ferociously as plug with the 1/16" gap, but it does spark.


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## Steamchick (Feb 17, 2022)

Hi Brian, Having worked with HV insulators.... 
1) the wriggly surface of insulators is to make the surface length (for flashover prevention) much longer than the air gap from top terminal (Live HV) to body metal (earth). So you can add a series of curvy grooves in the outer diameter of the insulator - like a car spark plug - to help prevent flashover.
2) the metal of the body is a simple flat surface joining the outside of the cylindrical part... so can you make a nice big radius here? - It will help reduce the electric field strength locally, and help prevent flashover. a .030" radius is MUCH better than a 0.10" radius here! (Assuming insulator 1/4" dia and steel body 5/16" dia body).

Every little helps.
K2


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## Steamchick (Feb 17, 2022)

Oh... while you are making changes, the cap that connects the HT cable to the top of the plug MUST NOT have any sharp edges that could initiate a flashover spark! - Thereby is the undoing of all the work on the improved insulator and body.
K2


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## minh-thanh (Feb 17, 2022)

Hi All !
 Another option


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## Brian Rupnow (Feb 17, 2022)

This video, made 17-Feb 2022 shows my most recently designed and built single cylinder horizontal engine running with a home made sparkplug. I had issues with the cylinder head, mainly because I messed up the  1/4"-32 threads for a standard sparkplug, so had to redrill and tap to a 5/16" fine thread. I had Steve Huck make me a "special" sparkplug with a 5/16" fine thread, but could never get it to work consistently. Then I made a sparkplug, but my design was a bit fishy, with a 0.060" spark gap. It would spark to get the engine running, but as soon as it warmed up, the spark would jump across the outside of the sparkplug to ground and the engine would stall. Finally, a redesign of the sparkplug with an 0.025" spark gap worked successfully, and the fact that the sparkplug is 3/8" longer than the first home made sparkplug keeps it from arcing to ground on the outside of the sparkplug. I am happy, the engine runs fine, and now I can put this project to bed. If you want to buy a complete set of plans to build this engine, contact me at [email protected]---Brian


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## Jasonb (Feb 17, 2022)

Nice to know my suggestions for the redesign worked


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## Brian Rupnow (Feb 17, 2022)

Jason--It did work, and thank you.---Brian


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## almega (Feb 17, 2022)

Brian - You noted with your first iteration that after a short period of running the engine, the electrode had eroded away. Will you run this one for a similar time to determine the amount of erosion for the new plug design?


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## Brian Rupnow (Feb 17, 2022)

Yes


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## Steamchick (Feb 18, 2022)

Well done Brian!
K2


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