Sal---Do pushrods need to be shortened when the head is milled? you
asked. The original head design I'm sure figured in
what the proper push rod length should be to keep the rocker geometry at
the optimum. This means when the face of the rocker pad was doing the
work of opening the valve, against the resistance of the spring(s) there
was as little side load, or scrubbing on the valve stem as possible. As
material is removed from the bottom of the head the valves of course are
lowered, as is the rocker shaft. The pushrod length doesn't change
during all of this. How MUCH is taken from the head is what needs to
be considered. If you're only going to resurface the head, like .010 or
.020. it's hardly necessary to shorten the pushrods. The more that is
removed from the head, the more side loads are placed on the valve stems
and will shorten the service life of the valve guides. I have no idea
of at what point the pushrods SHOULD be shortened, only the incremental
consequences of not doing it.
This is also one of the selling points of Roller Rockers that include a
roller bearing at each valve actuator. No scrubbing. The valve gets
pushed 'straight' down.
>From a cost stand point, RR's are very expensive and should be
considered only if the rocker ratio is also going to be changed, to get
more valve opening. Of course all of this is for those who intend to
build a killer engine, and want it to last for more than one lap.
There is at least one decent book on this subject. Lots of pictures and
easy to understand, written for the shade tree mechanic. It's called:
How to Build and Modify
Cylinder heads, Camshafts,
& Valvetrains.
Published in 1993 by
Motorbooks International Publishers
& Wholesalers, P.O.Box 2
729 Prospect Ave. Osceola, Wi. 54020
The cost was about $20.
Dick T.
'73
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