Exposed spark plug threads

B

BobM

Senior Member
Took my Offy heads to my machinist and he said he wasn't set up to machine the threads out the way Ol'Ron described in one of his posts and suggested I could do it with a die grinder; I'm not sure if that's a good idea. Any opinions would be appreciated.
 
Chuck a chamfering bit in your hand drill and do some "trial-and-error" chamfering of the theads in the combustion chamber. Check frequently with your sparkplugs to determine the required depth until you establish a pattern.
 
Take an old plug, the same as your using, grind the end flat and put it in the spark plug hole. Now use your die grinder to cut the excess threads away down to the end of the plug. Worked great for me.
Butch
 
JWL said:
Chuck a chamfering bit in your hand drill and do some "trial-and-error" chamfering of the theads in the combustion chamber. Check frequently with your sparkplugs to determine the required depth until you establish a pattern.
Click to expand...
The more I read the less I know. In Mike Davidson's book he states that exposed threads aren't generally a problem and that by removing material you lower the compression ratio. But to my way of thinking what good is having the ideal compression ratio and then you discover you have a detonation problem after installing the engine in the car. By the way, what would be the ideal compression ratio for a modified street engine? I'm around 8.2 to 1 I believe. While on the subject of plugs, he mentions that indexing the plugs might be worthwhile. I know the "go fast" overhead guys do this, would it be of any benefit for a flathead?
 
Your C.R. is real close to optimum for regular gasoline. The "IDEAL" ratio will vary with several factors as outlined in my book.

The primary reason for indexing plugs is to achieve mechanical clearance.
 
JWL said:
Your C.R. is real close to optimum for regular gasoline. The "IDEAL" ratio will vary with several factors as outlined in my book.

The primary reason for indexing plugs is to achieve mechanical clearance.
Click to expand...
Re-calculated my compression ratio and is probably closer to 8 to 1. Milled the heads .020 and increased valve pocket depth to .445 ended up with 70cc in the head. Sure seems like it cost me a lot in compression. I've heard of people milling .050" but maybe they weren't Offy heads; at .020" am within .040" of piston edge hitting head. I'll be glad when your book gets here.
 
You must log in or register to reply here.
Back
Top