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vettedog72
11-19-2005, 07:34 PM
I have been informed that the old HC head push rod length was 206mm
While the newer LC head had push rods with the length of 210mm.

This is about 160 thousands of difference (0.1574803") and would account for the recommended shave of up to 160 thousandths to get the 9.+ compression ratio on the later heads. (I understand some people go 190 thousandths)

But now, the question of how the ratio of the rocker is calculated comes back to mind. I suspect the factory ratio and geometry assumes a certain depth of the adjustment screw but how much in inches is an unanswerable question. It appears the rocker geometry to the valve stem reminds the same after a head shave. The rocker to valve stem would change with valve grind, valve seat alteration or valve stem length changes. After a head shave the only change in geometry is the working radius at the contact point of the rocker adjustment screw and the push rod (and I guess the working radius of the push rod at the lifter). It is not that I don’t believe shorter push rods are needed after the shave; it is that I don’t understand why the pivotal radius is that important. If some one would shine some light on this “black box”, it would be appreciated.

piman
11-20-2005, 08:00 AM
Hello Vd,

the only reason to fit shorter push rods when the head is machined is to recover the adjustment range. You may run out of adjustment unless shorter push rods are used. I don't believe that the radius you refer to is really measureable if longer push rods do work (i.e., you can achieve the correct vlve clearances) and certainly not from a power point of view. It really is very marginal, all else being equal.

Alec