there has been a lot of comments on this subject, but very few facts
valves transfer about 30% of their heat across the stem to guide interface
into the head casting and eventually into the cooling water .
to achieve this, the clearance must be the absolute minimal possible
engines have slightly different valve stem to guide clearance requirements,
but as an indication for cast iron guides in an OHV engine, .001K to .003K
on intake and, and .002K to .004K for exhaust guides. Wear of about 0.0015"
is usually sufficient cause to reject them.
We often see cast iron guides with a lot more wear than this and the engine
still runs perfectly OK as a shopping cart, but not as a racer. The biggest
common problem with extreme guide wear is the valves no longer seat
reliably.
on the intake, the incoming air-fuel charge slightly cools the valve stem,
so you can run them with slightly lower clearances
on the exhaust, this does not happen, so they need to be "just loose
enough" not to sieze under extreme conditions they meet.
clearly, the objective in both cases is to absolutely minimize the
clearance. NOT maximize it or even worse, "add a bit for good
measure" because of a material specification issue.
In extreme cases, when valves are not shedding their heat, the engine can
be prone to detonation as the heads start glowing. These valves can often
be seen to have tiny cracks in the head
whilst i'm all for making mods that improve engine performance, and spend
most of my workling days doing just this, i really don't believe this is
one of them.
I'm glad another old racer like Kas has taken the same pragmatic view i
have, if it aint broke, don't fix it
Terry
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