Technical 1948 Chevrolet Spark plug thread sized.

I can't find the exact thread size for the champion UY6. Says it's 10 mm, but doesn't give the pitch and when I measure it, it Doesn't seem right. Someone had put a helicoil in the spark plug hole and when I took the plug out, it pulled the threads out and I need to get another helicoil . When I measure the outside of the threads it comes up as .3860. Any help would be appreciated.

 

Anybody?

 

try 10 mm x 1.0 mm pitch

(hint: measure the threads on a plug, not the threads in the hole)

 

So I tried a 10 / 1.25, but the thread seemed too tight and then I got a 10 / 1.0, and the thread seemed too big?

 

Did you hold the tap next to the threads on the spark plug, to see if they match? The inserts themselves won't really tell you much.

Or get a 1.0mm pitch bolt and compare thread spacing on the plug, and the tap.

 

It looks like those Helicoils are REALLY expensive! LOL

 

10mm spark plug? That seems mighty small, like a plug for a weed wacker or brush cutter. Chevy 216 should be 14mm x 1.25mm pitch.

 

Per page 42 of the 1948 Chevy specifications they did use a 10mm spark plug. http://www.1948chevy.com/support/48pg1.htm

They did switch to a 14mm plug later on, see page 35 of the 1952 specifications. https://www.oldcarmanualproject.com/manuals/Chevy/Restopak/1952/1952ChevyRestopak/5236.html

 

I encountered one of those 10mm plug heads, long ago, a slightly later engine that had been installed in a 41 sedan.

 

Gonna run around to the part stores before work.

 

Chevrolet used 10mm spark plugs from 1940 (or maybe '41) through '48. Some old timers used to drill the holes out with a 1/2 inch bit and tap them for the more common 14mm plugs.

 

Just measured the spark plug and it is 10 m millimeter by one exactly. Seems that the inserts that came with my thread Kit are the wrong ones, so I'm gonna have to find another kit

 

You know the inserts are bigger, when they're not installed yet....???

If the tap is the wrong pitch, then yeah, you have a problem.

 

I get that They would be a little bigger, but they don't even fit on the installation tool.

 

they're supposed to be a loose fit on the tool. Kind of hard to tell from a few words, if they're right or not. But you can return it and try another kit, eh?

 

Yes, I went to the store this morning and they didn't have another so they're sending me one from the next store over.

 

Flathead Cadillacs had 10 mm plugs, too. I remember going to the junkyard for some parts, opening the hood on an old Cad and saw those teeny spark plugs. I had never seen them before and thought they looked kinda goofy. I could see their use in little european engines, but why did GM use them in Chevrolets and Cadillacs?

 

You don't want the coil type insert for spark plugs, you want a solid version with a collar preventing it from going in too far.
With the coil the spark plug may accidently grab the coil when the plug is half way in, and screwing the coil half way into the combustion chamber as the plug is tightened all the way. At best the exposed coil turns red hot and causes detonation as fuel ignites at the wrong time, at worst it falls off and the piston/head/valves chew on it for a while.

 

Yup I was thinking about getting those solid ones.

 

Packard fell for the 10mm idea as well. Turned out the same for everybody.

 

Got the proper kit installed, worked out good.