spark plug size poll

[stevehuckss396]

Yeah i'm at it again. Trying to get an idea of what the popular size plugs are out there. So a couple quick questions if you have a minute to help.

What size plug/thread in 1/4 scale

What size plug/thread in 1/3 scale


I'm asking because in the case of an older 7/8 threaded plug a true 1/4 scale falls between a 1/4 and #10 and a true 1/3 scale falls between 1/4 and 5/16

1/4 scale 5.5mm
1/3 scale 7.4mm

Also interested to hear about other scales and configurations.

[Jasonb]

As the thread does not actually show on the plug when it is in place could you just not make everything above the washer to scale and have a smaller thread eg 1/4 x 32 for the 1/3rd scale and #10 for the 1/4 scale then most people will have the taps already.

Alternative would be to use 7/32 and 9/32 ME threads in 32tpi if you want closer to diameter though not a spopular your side of the pond as here. M7 x 0.75 would be another option for the 1/3rd scale plug as that is used on gas fittings so a few people may have the taps. In US I expect #12 and #18 will win the day.

[Jo]

This is going to sound a bit stupid  ..... How about selling spark plugs with no thread and let people cut their own threads 


There are a bunch of standard plug sizes, if you start producing scale plugs then whoever picks up an engine second hand is going  it up when they try fitting a standard plug.

Jo

[stevehuckss396]

As the thread does not actually show on the plug when it is in place could you just not make everything above the washer to scale and have a smaller thread eg 1/4 x 32 for the 1/3rd scale and #10 for the 1/4 scale then most people will have the taps already.

That is what I would do ideally. Drop down to the next common thread and keep the lower half modern. I guess I would like to know if builders have moved down or did they go up to the next common size. Many of the hit miss engines I see at shows were built many years ago back when spark plug options were limited.

[stevehuckss396]

This is going to sound a bit stupid  ..... How about selling spark plugs with no thread and let people cut their own threads 


There are a bunch of standard plug sizes, if you start producing scale plugs then whoever picks up an engine second hand is going  it up when they try fitting a standard plug.

Jo

They would be scale plugs on top with standard, modern lower end. I guess what I want to know is, is a 1/4 inch thread more common in a 1/4 scale engine or a #10?

If someone has to cut there own thread then they would also have to machine the ground strap at the bottom. I'm not sure anyone would be interested in them.

I just dont want to make a plug that nobody wants because it wont fit or needs an adapter.

[scc]

From my experience many years ago with customers cars and motorbikes making plugs with no thread is a great idea They would be a perfect fit on all those cylinder heads stripped by "do it yourselfers".