John, and others---This subject came up before, and created enough
controvery to prompt me to write a review of it in an earlier 6PACK
Newsletter. (Titled, "Raising the Compression")
In short, one can mill as much as .100 from the later heads without
shortening the stock pushrods. Probably less on the already thinner
early heads. There is one thing that can be done to stay close to the
stock geometry.
That being to shim up the rocker pedestals the same amount that was
milled from the head. (Note to keep open the oil passage at #6 pedestal)
Others feel that so long as there is enough threaded adjustment left in
the adjustment ball studs, it shouldn't matter.
However, we know that lowering the head thru milling also lowers the
whole valve train, EXCEPT the pushrods. This WILL impose a different
angle on the valve stems, where the rockers make contact, thru creating
more side load. This could conceivably wear out the guides a little
sooner than if everything were correct.
Part of my research here brought me to ask Kastner about his thoughts
here. He said, tho I'm now paraphrasing, that "It probably doesn't make
a tinker's damn if you just mill off a .060" or so. Most of us just shim
up the pedestals and be done with it. When we get serious and take off a
LOT. we shorten the pushrods. Otherwise, I can go thru a set of valve
guides in about 20 (racing) minutes".
"A LOT" to Kas was found to be more than .125 and using high lift cams
with the necessary high rate valve springs.
Dick
|