<<Yes, when the head is milled, the pushrod hits the rocker screw at
a higher point, but by adjusting the screw to compensate, the point of
attack at the rocker arm itself is unchanged. If that is true, then the
valve train geometry is unchanged, with the granted exception of the
point at which the pushrod bears on the adjustment screw.>>
When you shave the head .065", the point at which the lash is taken up and the
pushrod begins to actuate the rocker is .065" higher than it is on an unshaved
head.
Believe it or not, this small difference can wear the already wear prone
rocker gear and guides at fairly modest mileages. The only way to minimize the
sideways moment on the valve stem tip is to make sure that the rocker is at
the right height. Even then, there is obviously a sideways component the
minute (or rather fraction of a second) that the rocker pushes on the valve
stem in anything other than a straight up and down direction.
You can minimize this sort of wear by using tubular pushrods with pressed in
tips. By using a parting tool on a lathe of exactly the same size as the slice
taken off the head, you can shorten the pushrod and then heat it and press the
tip deeper into the rod tube.
I grant you that many racers neglect this sort of nicety, but in a real sense
it is more important for a street car than a race car. The racer will no doubt
be torn down (probably many times) before wear becomes significant. A street
car will see many miles and you don't want to be doing head jobs every 20,000
or so just because you didn't bother to do the pushrods properly.
The best setup for the MG is of course to have no pushrods, a la Twincam.
There is zero sideways moment on the valve stem with the overhead cams,
although there is a sideways moment imparted by the cam to the cam follower.
Because of it's much larger size and bearing area, it wears much more slowly
than the pushrod engne will, and always pushes straight down on the valve even
as it slowly wears.
Bill S.
(a real MG has 2 cams!)
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