This discussion of how to set valves is all well and good, but it
calls to mind a different problem that has always worried me. One
supposes that the non-lift part of the camshaft's rotation is round,
but supposin' it isn't? If it is round, then the cam needn't be
exactly opposite from maximum lift for the valve you are setting;
anywhere will do. But if it is slightly out of round, you'll get a
different adjustment depending on where it is in the rotation when
you set it. Never having measured a cam's roundness, I wouldn't know
whether this ever happens in reality. But it seems possible. I've
known at least one cam so worn (not becoming so while I was the
owner, but before I bought it) that had only half of the prescribed
lift. So could not the non-lift areas wear strangely, seein' as how
they are subject to all sorts of inconstant stress and bouncing?
A simple experiment will do. Set or measure the clearance at
different places in the rotation, see how much it changes. I'm too
lazy to do this directly, usually being in a hurry to finish the job
and close the incision back up again. But I've noticed the quality
of the job I've done being different from session to session, and
suspected that the world, er, I mean the cam, isn't as round as it is
made out to be. Either that or my feeler gauges aren't always the
same thickness. Or the world actually changes temperature. It
certainly couldn't be my technique!
--
Jim Muller
jimmuller@rcn.com
'80 Spitfire, '70 GT6+
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