# my version of building a spark plug



## Joachim Steinke

Hallo everyone,

here is my version of building a miniature sparkplug. 

Needles to say, there are already various publications of surely practicable solutions existing in the web, some had been shown here on HMEM as well. So my report will not contain lots of revolutionary ideas ore totally unknown techniques.ha ha ha..this is only my variation of the theme.

I intended to use a real small scale plug in my latest engine, so I had bought a single Rimfire Z2 at a model engine exhibition last year. Sadly I damaged it after only a couple of test runs. I must have had too much protein in my breakfast, because I managed to break off the body just above the thread while screwing it in the cylinder head. 

Now I was in need of a new one, but Rimfires are not available in German stores, I had to order them in the Netherlands (ore USA) which would take some time for delivery. But, as mostly, I was impatient and wanted to play with my brand new motor, so I decided to build one by myself.

The basic design of a spark plug is no great deal, but which isolating material could be suitable? I have read of Teflon being used, but I had no PTFE in stock and wanted to come forth. So I decided to use Corian, a synthetic material made of Aluminiumhydroxid and PMMA. It has good machining and isolation properties and is heat resistant up to more then 200deg Celsius. I realized that the temperature limit might become a problem, but having no alternative I gave it a try. 

Here comes my design, which is clearly comparable to Rimfire as it has to fit in the already existing cylinder head.









The Z2 is one of the smallest Rimfire plugs, only 0.87in long and with a tiny NS 10-40 thread. This plan shows all the inner details, all measures are in millimetre.








And here some pictures of the making:

Living in Germany, I dont have a threading die for NS 10-40 in my drawers. So the thread is done with the lead screw on the lathe. For the main body (and the electrode too) I used ETG100, a high strength machining steel.  








For the graduated inner boring I took one of my Ph-Horn Super Mini massive carbide boring bars. With my smallest version I can go down to a starting hole below 0.04in at a boring length of max. 0.4in, thats some real amazing tooling.








The real fiddly point is the thin part (0.055in diameter) of the electrode. In case of need I have had to build me a little travelling by backrest or on second plan had to hold it very carefully with the tailstock on a thicker lost end. But it was not necessary, as I managed to turn it fully free. The trick is to use a very sharp and suiting cutting tool. 

I used a carbide insert CCMT 06, (sharp grinded, highly positive rake angle, radius 0.2mm) and the method is to turn the last passes with the same, relative large infeed. So the final infeed was a reduction of diameter from 0.08in to 0.055in in one pass. And the fine tip of 0.03in was reduced afterwards by holding the part in a small ER11 collet on the 0.08in shaft area.

Sorry, I dont have a picture of that procedure, here its all done and the parting tool is already installed:








Now it was time for the Corian isolator. Nothing much to mention about that, that stuff is easy to cut like butter.ha ha ha..








After milling the hex head and the little arm for the negative electrode out of the main body the three parts are ready. As you can see, my electrode is a little bit thicker as the Rimfire one, which is only 0.02in at the tip.








At last I had to join the parts. The electrode is bonded with 2 components epoxy (Araldit) in the Corian part. There is a long area for bonding, so that should be solid enough. The isolator is also bonded in at first stage, and in addition I pressed the thin flange of the main body around the shaft, using a ball bearing as a spinning tool on the lathe.  








Voila, all done, the original Rimfire in direct comparison to my version:








Okay, the plug is working well, especially for starting and midrange speed. To my impression the ignition becomes a bit instable at full speed. With the Rimfire the engine runs over 7000rpm (even 8000) for longer periods without any problem, with my own it tends to have some ignition misses above the 6000rpm mark. 

But the effect is more temporary and not so serious. It can also depend on some instability of my home made ignition system, which will be the reason for some specific questions I want to ask here in the next days.

Nevertheless, when the new Rimfire where delivered I switched over to the commercial ones again.ha ha ha.

At very last a picture of a Rimfire and my own plug after some time of use.

The Rimfire has definitely seen more than one hour of full speed, my plug has a running time of approximately 30min. Both plugs are not cleaned with abrasive procedures like brushing them with steel wool or so, they are only cleaned with a soft paint brush and some acetone. 








As you can see, there is a brown ring around the electrode of my plug. I believe this is some epoxy material coming out of the boring under the influence of heat. Perhaps I was too generous with the glue, it sure had been sufficient to use it on the upper shaft region only. The Corian seems to have no problem with the temperature, in any case I cant detect some visual changes on the outer surface. The isolation seems to by all right too, the sparks are coming precise and without any misfiring. So to my opinion the material seems to be quite practical for a spark plug isolator. 

On the other hand, I dont think it makes much sense to make your own plugs at all. A lot of work and there is still the remaining uncertainty of a final and serious function in your engine. The ignition system is a very important end sensitive component, many builders with running problems dont realize that and believe the reasons lay on the carburator side or they depend on a false valve timing etc. Its wise to ensure the proper function of the ignition at first priority, might avoid a lot of trouble. 

And last but not least it depends on the specific purpose. A slow engine, which works from time to time for some minutes, might by happy with a not so optimal spark plug, but a fast and hot running motor has higher demands for a perfect and reliable function. 

Achim


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## cfellows

Nice writeup. Thanks for all the good information. By the way, you do some pretty amazing work!

Chuck


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## JustMe

Hi Joachim; 

My big clumsy fingers did it again, I have unintentionally blocked your beautiful pictures. Would you tell me what photo hosting service you use to post photo's here? I think I could unblock them if I knew which hosting service you use.

Thank you.

Jim


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## dsquire

JustMe  said:
			
		

> Hi Joachim;
> 
> My big clumsy fingers did it again, I have unintentionally blocked your beautiful pictures. Would you tell me what photo hosting service you use to post photo's here? I think I could unblock them if I knew which hosting service you use.
> 
> Thank you.
> 
> Jim



Jim

Here is the link to the first picture. Possibly this will help you.

http://pl-hi.de/JST/SLVE/SLVE_VER1_1001.JPG

This was taken from the source code for this page. Just do "Ctrl+U" and it will bring the source code up if you are using FireFox. In Windows I believe it is "View" then "Source" .

Hope this helps. :bow:

Cheers 

Don


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## T70MkIII

Thanks for this very useful and detailed information, Joachim.


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## putputman

Thanks for the post Achim. It is very detailed and will be easy to follow.

I have one questioned that maybe someone can answer on this post.

What is the purpose of the gap between the insulator and the metal body down at the electrode end of the plug. ??? ??? If the insulating material is a good insulator, I don't see the purpose of an air gap. I realize all commercial plugs are built this way. Just curious.

I have only built a couple plugs and didn't provide a gap. They seemed to work OK.


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## radfordc

The gap around the "nose" of the plug helps determine the heat range of the plug.

Some basic structural factors affecting the heat range of a spark plug are:

Surface area and/or length of the insulator nose 
Thermal conductivity of the insulator, center electrode, etc. 
Structure of the center electrode such as a copper core, etc. 
Relative position of the insulator tip to the end of the shell (projection) 

The major structural difference affecting the heat rating is the length of the insulator nose. A hot type spark plug has a longer insulator nose. The insulator nose of a hotter spark plug has a longer distance between the firing tip of the insulator, and the point where insulator meets the metal shell. Therefore, the path for the dissipation of heat from the insulator nose to the cylinder head is longer and the firing end stays hotter. The insulator nose of a hotter spark plug also has a greater surface area that is exposed to more of the ignited gases and is easily heated to higher temperatures. A colder spark plug functions in an opposite manner.


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## rudydubya

Nicely done Achim, and excellent illustrations and photographs. I have enjoyed reading and viewing all your posts. Looking forward to more.

Regards,
Rudy


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## putputman

Thank you radfordc. I always knew there were differant heat ranges in spark plugs, but never knew how they achieved it. Some things you just take for granted and never give them another thought.


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## JustMe

Thanks dsquire;

I looked at the source code, but most of it is Greek to me, and I didn't find any sort of reference to a photo hosting service. I'm pretty sure I blocked the hosting service, it happened some time ago, I was looking at Mr. Steinkke's posts on the drill grinder design thread on Mad Modder when I made an unintentional right click, I did a left click without really looking, just to clear the window which the RC had brought up, and all the pictures disappeared. Since then I haven't been able to see any of Mr. Steinke's pictures on MM or HMEM.

I use Ubuntu with firefox and have it set to allow images from both the above sites. I can see pictures on other peoples threads, just can't see those posted by Mr. Steinke.


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## dsquire

JustMe  said:
			
		

> Thanks dsquire;
> 
> I looked at the source code, but most of it is Greek to me, and I didn't find any sort of reference to a photo hosting service. I'm pretty sure I blocked the hosting service, it happened some time ago, I was looking at Mr. Steinkke's posts on the drill grinder design thread on Mad Modder when I made an unintentional right click, I did a left click without really looking, just to clear the window which the RC had brought up, and all the pictures disappeared. Since then I haven't been able to see any of Mr. Steinke's pictures on MM or HMEM.
> 
> I use Ubuntu with firefox and have it set to allow images from both the above sites. I can see pictures on other peoples threads, just can't see those posted by Mr. Steinke.




JustMe 

I have found this bit of information that may help you with your unblocking of the images.

(start quote)
"You may have accidentally clicked the 'Block Images' item in the right click context menu while trying to save an image.
Check the image exceptions: Tools > Options > Content: Load Images: Exceptions - See: Options window - Content panel

A way to see which images are blocked is to click the Site Identification icon (favicon/image on the left end of the URL/location bar) and click the "More Information" button:
-The Security tab of the Page Info window (also accessible via Tools > Page Info) will open
-Go to the Media tab of that Page Info window.
-Select the first image and scroll down though the list with the Down arrow.
-If an image in the list is grayed and there is a check mark in the box "Block Images from..." then remove the check mark to unblock the images from that domain.

Also see:
Images or animations do not show
Images or animations do not load (Mozillazine KB article)"
(end quote)


The above information was taken from this link. If the above does not help then go here and maybe you can figure out how to unblock the images.

http://support.mozilla.com/en-US/forum/1/691488

Hope that this help sort the problem. :bow:

Cheers 

Don


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## JustMe

Thanks Don. I thought I had replied yesterday but guess I didn't get it right. I followed your directions on the id icon, and found what I had blocked. Removed the block and all is well, I can see the pictures on both sites now.

Jim


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## dsquire

Jim

Glad you got it sorted out. I had just been reading about that problem someplace else so that is why I was able to help you out so quick. It's not that I'm good, I was just lucky on this one.

Cheers 

Don


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## ANTONIODOMIT

I have designe a new type of rotary engine.
I almos have the prototype ready and need the spark plugs.
I have not founf the wright size in the market so I will need to make them my self. Can you help me.
I live in USA.
Thank you
Antonio Domit


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## John Rus

Hi there and welcome to the forum for those who love metal and engines!

I won't be of any help about spark plugs but someone else will be sure to chime in.

We would love to see the progress of your engine in WIP thread....th_wwp

Cheers,
John.


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## djalal

hello sir can you told me about white metal what is it ! and can i change this metal with another one ! thnx​


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## Cogsy

djalal said:


> hello sir can you told me about white metal what is it ! and can i change this metal with another one ! thnx​


 
I think you're talking about the insulator material which in this case is a material called Corian, which is a type of acrylic used a lot in kitchen benchtops. I also use it for spark plug insulators and it can be found on ebay relatively easily if you search for "corian pen turning blank" or similar. Otherwise, try contacting local kitchen counter top fitters/suppliers. They often use the stuff and have masses of scrap material that you can probably get very cheap or even free.

Another alternative material is PTFE (teflon) but I've never used it. Good luck with your build.


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## minh-thanh

Thanks Joachim Steinke ! Useful information for me


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## minh-thanh

Cogsy said:


> I think you're talking about the insulator material which in this case is a material called Corian, which is a type of acrylic used a lot in kitchen benchtops. I also use it for spark plug insulators and it can be found on ebay relatively easily if you search for "corian pen turning blank" or similar. Otherwise, try contacting local kitchen counter top fitters/suppliers. They often use the stuff and have masses of scrap material that you can probably get very cheap or even free.
> 
> Another alternative material is PTFE (teflon) but I've never used it. Good luck with your build.



 Hi Cogsy !
Why do not you use teflon ?


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## Cogsy

Corian is easier for me to get and I've heard Teflon is a little more difficult to machine, but I've never tried it. So far the corian has worked nicely for me but I might try teflon in the future.


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## djalal

thnx sir you help me too much ​


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