Re: 3 angle valve grinding??

Posted by jdemaris on November 12, 2004 at 19:13:44 from (209.23.28.181):

In Reply to: 3 angle valve grinding?? posted by Lynn Kasdorf on November 12, 2004 at 13:03:22:

I'll try not to be redundant here and repeat what has already been said. I first read about "three seat" valve jobs back in the 60s in a racing magazine and never quite understood what they were talking about. I was wondering if they were actually making three valve seats instead of one - or just came up with new terminology to make standard procedure sound like something new. I still don't know. If you put a new valve seat insert into a head (or block for a flathead)it may be narrow enough where there won't be three angles. But, after grinding - or carbide cutting seats, they get wider - and thusly have to be narrowed back to specs. And, that's for at least two reasons. Number one, already discussed, is to get the correct width for valve cooling. This is especially critical in an air-cooled engine. Second - is to get the correct pressure on the valve seat. The spring on the valve provides closting pressure, but, if the valve seat is too wide - the pounds per square inch is lessened on the valve face and seat.
As far as valve-lapping compound being obsolete, I disagree. A properly machined valve face and seat don't need lapping for proper seal - but a light use of lapping compound works kind of like Prussian Blue as a marker and lets you see exactly where the sealing area is. Sometimes, with a freshly machined seat and valve face, it's hard to see for sure. Lap it a little, and the sealing area gets dark and lets you see for sure. It takes a little extra time, but it sometimes exposes mistakes.

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