Bob---I have found that even with snug fitting cam followers, rockers
and shaft and stem to guide clearances, too much lash produces clatter.
Maybe better description for this sound would be like hearing
typewriter keys striking its roller. (Remember those)?
I can sort of go along with the heat sink idea. When a valve is on its
seat it will shed some heat. How much more heat it will lose because of
more lash would seem miniscule. The valve (stem) contact with its guide
absorbs much more of the heat, and so products like sodium filled guides
are now found. (I have no idea if they're any better)
My thoughts about how valves "burn" is more towards combustion gasses
getting past the edges when the valve should be completely closed, and
isn't All that should be necessary for a valve to be closed is to have
no contact with the rocker. If this is true, then it should make no
difference if the lash is .001 or .018. Only a confidence window should
be necessary. Ths does not mean to disregard the cam designer's
recommendations, as there are other reasons for following the card.
I DO like this type of discussion, and I hope others will join in!
Dick.
From: lang@isis.mit.edu(Robert M. Lang) (PST+3) To:
Lizirbydavis@cs.com Cc: tr6taylor@webtv.net
How are the valve stem/rocker arm clearances established? Why does the
clearance need to be increased with higher ratio rocker arms and higher
lift cams? I have a GP2 cam and 1.65:1 roller rockers. The valve gear
clatter with the recommended clearances is much more that I expected.
What harm will come from closing up the clearances? (not that I would!)
Joe Davis
Bob wrote:
More cold clearance means more time "on the valve seat" where the heat
transfer takes place. Bigger cams mean more heat - to get the heat
transfer you need more time on the seat.
Valve clatter does not come from valve lash, per se. Your valve clatter
is more likely slop in the rocker shaft, loose valve to stem clearance,
loose lifters, etc.
If you tighten up the valve clearances, you WILL burn valves. If you
want to play with this, you can tighten the intakes, but NEVER tighten
the exhaust clearances unless you like doing valve jobs often.
rml
Bob Lang
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