Shop built sparkplugs for model engines

[Steamchick]

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

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

 

[Brian Rupnow]

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.

 

[almega]

It definitely looks pretty.

 

[johwen]

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

 

[Brian Rupnow]

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.

 

[Steamchick]

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

 

[Steamchick]

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

 

[minh-thanh]

Hi All !
Another option

 

[Brian Rupnow]

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

 

[Jasonb]

Nice to know my suggestions for the redesign worked

 

[Brian Rupnow]

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

 

[almega]

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?

 

[Brian Rupnow]

Yes

 

[Steamchick]

Well done Brian!
K2

 

[Brian Rupnow]

Reviving an old thread here--Some people have suggested using a "machinable ceramic" for the insulator portion of this sparkplug. I have lots of Corian, but I find it is a pig to machine. McMaster-Carr has 1/4" diameter x 3" long "glass-mica machinable ceramic" for $27.55 American. The outside diameter is correct as purchased and that length is enough to make two sparkplugs, as my insulator length is 1.438" finished dimension. They seem to have a bunch of "machinable ceramic" products, so I'm looking for confirmation that the "glass-mica machinable ceramic" is the one I want.---Thanks---Brian

 

[mfrick]

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

McMaster Carr sells a machinable ceramic material that works well.
MF

 

[DKGrimm]

McMaster Carr sells a machinable ceramic material that works well.
MF

I've had friends recommend this material to me, but I have no idea how rough it is on tools. Carbide? If so, what do you use to drill the center electrode hole?

 

[L98fiero]

I've had friends recommend this material to me, but I have no idea how rough it is on tools. Carbide? If so, what do you use to drill the center electrode hole?

If it's Macor, I'm pretty sure it is, it can be machined with HSS, it does wear the tool but not so bad as to be unworkable. Here is machining information for Macor. Macor Machining Guide | Precision Ceramics USA

 

[peterl95124]

Reviving an old thread here--Some people have suggested using a "machinable ceramic" for the insulator portion of this sparkplug. I have lots of Corian, but I find it is a pig to machine. McMaster-Carr has 1/4" diameter x 3" long "glass-mica machinable ceramic" for $27.55 American. The outside diameter is correct as purchased and that length is enough to make two sparkplugs, as my insulator length is 1.438" finished dimension. They seem to have a bunch of "machinable ceramic" products, so I'm looking for confirmation that the "glass-mica machinable ceramic" is the one I want.---Thanks---Brian

I use "Macor" brand "machinable glass mica ceramic", I haven't tried McMaster-Carr version but assume its the same. I've had great success, it machines well with plain old HSS and I've looked at the drills afterwards under high magnification and can't tell they've been used at all (unlike with Alumina which *destroyed* every drill I tried including carbide). The extra slow running engine I showed you with the extra large flywheel has a Macor insulator. Its all I will use going forward, and might even replace the Corian in some of my older engines (the real problem with Corian is its limited temperature range, I won't let a Corian spark plug run for longer than a minute or so).

 

[Mike Ginn]

FYI Macor 6mm dia is approx £40 ($50?) in the UK for 100mm length - ouch!