Spark Plug Thumb Nuts Thread Size

[Buccaneer]

The Martin I’m working on has the spark plug thumb nuts with the
groove for the "fork" type wire terminal.
I was going to replace the old J-8 Champion plugs with J8C plugs,
but of course, the thumb nuts from the old plugs don’t fit the
new plugs. I had some new brass nuts but they only fit the old plugs
as well.

So…….., I thought I’d make some new brass nuts to fit the new plugs,
but am having trouble trying to decide what size metric tap I need.
The threads on the new Champion plugs have a ".8mm" thread,
and the OD of the threads measure at .150 inch.

The closest I can find is a "M4x0.7" tap, which says the maximum thread
OD is .157 inch.

Anyone ever make the thumb nuts for modern plugs and know what size
tap one needs for sure?
Thanks1

Prepare to be boarded!

[amuller]

Maybe take the plug to a good hardware store and experiment?

[green-thumbs]

NOS J8 would be easy way out, probably in same price range as a metric tap you will only use once.
Fellow at Constantine had a variety of NOS spark plugs for sale.
As the saying goes.."they are out there" so ask around.
Louis

[wagemaker]

Who knew there were so many?
http://www.magnetoparts.com/thumb_nuts.htm

[wagemaker]

These should work. Less than the price of a tap and drill.

https://www.ebay.com/itm/one-PAIR-of-Vi … 9006!US!-1

[Buccaneer]

Thanks for the replies. Guess I better "rethink" it being metric after all.
A 32 tpi would be equal to a .8 mm thread like I thought it was.
.03125 inch = 0.79375 mm
I tried all my little US taps but didn’t find one that worked.
Will look it over again tomorrow.
Just thought it would be a fun diversion to play on the lathe a
little while!

Prepare to be boarded!

[david-bartlett]

Brillman Company has them also.

https://brillman.com/

[Buccaneer]

I believe Brillman is where I got the thumb nuts I have.
It turns out those nuts were an 8-32 thread, and the
new Champion plugs probably are too, but they’re
a "crappy" turned thread. Anyway, I found an 8-32
die in my box and cleaned up the threads on the
spark plugs and the Brillman nuts work great now.
If the plugs weren’t an 8-32 thread, they are now. 😎

Prepare to be boarded!

[Author]

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