Message text written by "Jim Muller"
>Not to be pedantic or anything (not that that ever stopped me from
being silly), but we are assuming in most of this discussion that the
back sides of the cam lobes are indeed round. In a perfect world
they would be, and I can't think of anything systematic (except
possibly excessive valve bounce) that would cause them to wear into
non-roundness. Still, it is possible, I suppose, that uneven metal
hardness or poor grinding or some other unpredictabilty might cause
such. If so, then "valve clearance" is a poorly-defined concept!
Perhaps Randall Y or Joe C or Andy M might have seen it. Given that
you'd rather have valves too loose than too tight, this possibility
suggests checking each valve at several places in the rotation.
Lotta' extra work!
I thought the same thing but didn't feel like opening that can of worms.
But since it is now open...
The purpose of the clearance is to accomodate differences in thermal
expansion coefficients and to accomdate valve seat errosion. That said,
the clearance is important throughout the entire portion of the
circumference when the lifter is off-lobe. Therefore I would think that
the radius over this portion of the cam would be as closely controlled as
are the lobes themselves meaning that taking a clearance reading at any
point considered off-lobe would suffice. At anyrate, all the camshafts I
have seen have polished surfaces along the whole circumference.
Therefore it doesn't matter whether you measure "on balance," with the rule
of 9 (or 13) or set the intake when the exhaust is just opening and the
intake when the exhaust is just closing (assuming you are turning the
engine in a forward direction) just so long as you are a comfortable
distance from the lobe.
But this is just speculation.
57 TR3 (rule of 9)
71 TR6 (rule of 13)
80 TR8 (hydraulic lifters take care of all that - when they work)