Question for the hit & miss crowd

[stevehuckss396]

I have never built a H&M engine so I was wondering what the popular spark plug sizes are. Having purchased Dale Detrich's stock of plugs and parts last year it's time for me to get going on the endevor. I have 1/4-32 and 8-40 plugs that dale designed as well as my own 8-40 design from the Demon project. I also plan to make my own 1/4-32 and 10-40 plugs for sale. For the hit and miss crowd I was tossing around the idea of making copies of the X3 plugs in 1/4-32. I have done some reading and it appears that that was a popular plug in the full scale H&M engines way back when. With there being so many different engines and sizes i would like some help on where to start.

Would the X3 be a good choice or is there a different plug that would be more appropriate for a H&M engine?
Of the models out there would 1/4-32 be a good place to start or is 10-40 more popular a thread size on the H&M's

Any information or suggestions for future designs are very much welcome.

 

[Brian Rupnow]

Steve--With a 1" diameter piston and 3/8" diameter valves, there is room for a 10 mm sparkplug which can be purchased at an auto parts store.. Anything smaller than 1" and I use 1/4" sparkplugs. I've never used a plug smaller than 1/4".---Brian

 

[stevehuckss396]

Hello Brian.

Let me ask you this. If you spent the time to buy a casting kit and build a model that looks a great deal like the real thing. Would you spend a couple extra bucks for a correct looking spark plug?

The standard plugs are 12 bucks. Would you spend 20 for a plug that was a miniature of what was in it originally? Just wondering. I think I would but Im curious how others feel.

 

[Cogsy]

Down here we mostly have to stick to metric so I usually make my plugs M6 (6mm x 1). In the scenario you describe above, I probably would spend the extra on a scale plug, if I was already buying plugs, but then again I don't make scale models of actual engines (yet anyway).

 

[Brian Rupnow]

Steve--I don't have a good answer for you, because I don't buy casting kits. I have bought a number of 1/4" sparkplugs from S & S engineering, not because they look right, but because I couldn't make a 10mm sparkplug fit.

 

[kuhncw]

Steve,

I have three spark plug fired engines. My 0.3 scale Deere (1.00 dia bore) has a 1/4/32 plug which looks about right. The 0.75 bore tank cooled (modified Little Brother) has a 10-40 Rim Fire plug which looks scale to me. My Silver Bullet (0.75 bore) has 10-40 Rim Fire plugs. As designed, the Bullet had 1/4 -32 plugs, but I felt the smaller plugs looked more to scale. All the multi-cylinder engines Doug Kelley designed have 10-40 plugs. Doug's engines are 0.900 to 1.000 bore. So to me, plug size depends on the look and size/scale of the engine. I'd think the 10-40 and the 1/4-32 would cover the big majority of the models.

I would pay the $20 to get a plug that looks in scale with the model.

Good luck with your spark plug endeavor. Looking forward to seeing you at NAMES.

Regards,

Chuck

 

[TonyM]

If you spent the time to buy a casting kit and build a model that looks a great deal like the real thing. Would you spend a couple extra bucks for a correct looking spark plug?
The standard plugs are 12 bucks. Would you spend 20 for a plug that was a miniature of what was in it originally? Just wondering. I think I would but Im curious how others feel.[/QUOTE]

My old engines mainly used Champion 6 COM style which are 1" AF plugs therefore real 1/4 scale would mean 1/4 or 6mm AF and something like 4mm thread. Probably not practical for the average user but
I would certainly use a larger plug that looked more like the original which is short and fat as opposed to the modern style.

 

[stevehuckss396]

Hello Tony!

I just found one on EBay and it's on the way. I will look into replicating it in 1/3rd and 1/4 and see if I can do it at a reasonable price.

 

[TonyM]

There were a few variations of plugs used in those days. The two main sizes 3/4 (18mm) with 1inch AF hex and 7/8 (22mm) with 1 1/8 AF hex
I think a good compromise for 1/4 scale would be a 6mm thread with 8mm AF hex and for 1/3 scale 8mm thread and 10mm hex
I for one would love to see what you come up with.

 

[popnrattle]

"full scale" spark voltage, of course, is still needed and is the reason why using 1/4-32 miniature plug allows spark to jump intermittently to valve spring when using buzz coil in the 1/4 scale Associated. Fuel tubing seems to help, but authenticity suffers a bit. Using Aotolite 4303(M10x1 thread) for my "bar stock" creations though. Later, RT.

Make your own "buzz coil" link

 

[stevehuckss396]

There were a few variations of plugs used in those days. The two main sizes 3/4 (18mm) with 1inch AF hex and 7/8 (22mm) with 1 1/8 AF hex
I think a good compromise for 1/4 scale would be a 6mm thread with 8mm AF hex and for 1/3 scale 8mm thread and 10mm hex
I for one would love to see what you come up with.

I have one of each. For the Champion com6 in 1/4 scale I think the #10-40 is just a few thousands larger and the 1/4 inch hex is right on. The cap will need to be made larger than scale or spark plug clips will not click on. Also the cap and body will only be about 1/4 inch from each other. Will need to make one and see if it arcs between them. Of course the threads will not be to scale but they wont be seen.

As soon as I get done making some 8-40's I have to make some 10-40's. After that I may make a few Com6 and do some testing.

 

[TonyM]

I was wondering if anyone has successfully tried surface discharge plugs in small scale. They would certainly be easier to make in small scale than traditional electrode type.

 

[stevehuckss396]

Yes. The Dale Detrich plugs are a type of surface discharge plug where the electrode fires right to the body of the spark plug instead of a ground strap/tab. The electrode is bent so it only fires in one spot and not in a 360 degree pattern. I would guess the electrode can be increased in size to recreate the 360 firing plug.