L36 valve seat replacement.....

[Greg L.]

I need to replace an exhaust valve seat in my 69 L36 and was wondering if there is anything that I need to be aware of or ask while at the machine shop? I know it's probably routine but any advice here would be appreciated because once they cut the seat out, I can't go back...they gotta do it right the first time.

FYI, I was curious to see how the seats would hold up after about 15k miles or so on unleaded and they all look very nice with no noticable wear except for one which for some reason decided to wear to the point where the valve is almost flush. Why would just this one wear out?

Thanks,

Greg Linton
#45455

[David B.]

Re: L36 valve seat replacement.....

Greg,

re: "Why would one...?"

This is from an article on the subject:
"A 1970 engine operated at 70 mph conditions exhibited an average
1.5mm of seat recession in 12,000km. The difference between cylinders has
been attributed to different rates of valve rotation, and experiments have
confirmed that more rotation does increase the recession rate.
The mechanism of valve seat wear is a mixture of two major mechanisms. Iron
oxide from the combustion chamber surfaces adheres to the valve face and
becomes embedded. These hard particles then allow the valve act as a grinding
wheel and cut into the valve seat. The significance of valve seat
recession is that should it occur to the extent that the valve does not seat,
serious engine damage can result from the localised hot spot."

David B.

[Duke W.]

Re: L36 valve seat replacement.....

Valve rotators were not added to Corvette engines until the seventies.

Valve seat recession is usually not an issue on vintage engines and unleaded fuel unless the engine operates at high load - like racing or trailer towing.

The fact that only one valve suffered excess seat recession is an indication that something was wrong with that specifice valve/seat - like the mating surfaces were not concentric, which caused slight leakage. This will allow the valve to run hot by leakage of hot combustion gas during the combustion stroke. Then during the inlet stroke, fresh charge leaks. Combine excessive valve head temperature with oxygen and it will oxidize, which manifests as seat erosion.

GM ground the valves/seats on production engines with a one degree interference angle, so during early operation the valve "made it's own seat" so to speak. This is not a very elegant way to form a valve seat, but it works. For engine restoration I recommend grinding both valve and seat at a 45 degree angle, then lapping them thoroughly to form a good seat. By using dykem blue on the valve and seat you can clearly see the seat geometry - concentricty and seat width. Once you have a good seat, then you can narrow them to your target spec by taking an appropriate angle top and bottom cut on the seat and top cut on the valve. This is a "three-angle" valve job" which improves flow. For a good high performance street engine and good valve seat longevity I recommend .040" seat width on the inlet side and .060" on the exhaust side.

What is the source of the article you quoted from?

Duke

[David B.]

Re: L36 valve seat replacement.....

Duke,

Thanks for the clarification on the valve rotation changeover. The article excerpt is from:

http://www.faqs.org/faqs/autos/gasoline-faq/part4/preamble.html

Regards,
David B.