I have Autolite 216 plugs in my Offy heads but they leave 2-3 threads exposed as they don't screw in far enough to cover up the threads. I have read that these exposed threads could be a potential hot spot causing pre-detonation; would extended plugs be the solution to this problem or would counter sinking to the depth of the exposed threads be the best solution?
Extended plugs for Offy 425 heads?
- Thread starter BobM
- Start date
GOSFAST
Well-known member
Not good to leave the threads exposed.
For years we've been "fire-slotting" the spark plug holes on all the aluminum aftermarket castings!
I placed a photo below showing the procedure, you need to look closely to see the mod!
(Add) I've added a seond photo which is a little easier to see the "fire-slots"!
Thanks, Gary in N.Y.
P.S. We feel this is one area that adds slightly to the overall power gains on overhauls. If your "chasing" HP it is a good deal.
Last edited:
Thanks, will get machinist to counter sink spark plug holes, already have "fire paths."Not good to leave the threads exposed.
For years we've been "fire-slotting" the spark plug holes on all the aluminum aftermarket castings!
I placed a photo below showing the procedure, you need to look closely to see the mod!
Thanks, Gary in N.Y.
P.S. We feel this is one area that adds slightly to the overall power gains on overhauls. If your "chasing" HP it is a good deal.
Walt Dupont--Me.
Well-known member
It's easy enough to get a long reach plug with same heat range. Like the 216's I think are 7/16 reach, so get smething like 1/2 in. reach. Walt
Ol' Ron
Well-known member
I guess everybody has a way to do this, here's mine. I counter bore the plug hole with a 45 degree tool I leave enough threads to use a#???? ( Can't remember the number) a 1/'2" reach power tip plug. Then as gary says file a fire slot to the cylinder.
The reason for the added material in this area is for high lift cams, some builders like to have allot of room over the valve, which is commom in blower engines.
PS
Before modifying the plug holes, make sure the heads fit your engine . Piston and valve clearance. Most high lift cams hit the 425 heads.
The reason for the added material in this area is for high lift cams, some builders like to have allot of room over the valve, which is commom in blower engines.
PS
Before modifying the plug holes, make sure the heads fit your engine . Piston and valve clearance. Most high lift cams hit the 425 heads.