Hi Jim ! If you raise the pedestals then you alter the geometry and your
rockers will not work properly, in particular in you have a higher lift cam
than stock.
When valve is at half lift, then the rocker tip bottom should be horizontal
with the axle centreline, so that your rockers will be working equally both
sides up and down when working the valves, and not only down as it will be
the case if you shim the pedestals:
If you shim the pedestals then the rockers will sit higher and the tip will
be sitting lower than before vs axle centre, and with this you shorten the
rocker ratio (the lower the tip, the closest it is to the axle and the
pedestal) and in addition you put more side pressure on the valve stem and
you get additional wear. It is not much, but you put additional strain here.
So the ticket is to get shorter pushrods and maybe also skim the pedestal
bases if you have a higher lift, to restore the geometry, to have it as
neutral as possible and not biased towards the bottom.
David Vizard explains this very well in "Theory and practice of cylinder
head modification"
If you make a simple schematic it should become clearer than my words!
Cheers!
Alex
-----Message d'origine-----
De : fot-bounces@autox.team.net [mailto:fot-bounces@autox.team.net] De la
part de toodamnfunky@comcast.net
Envoyi : mercredi 4 mai 2011 16:10
@ : fot
Objet : [Fot] Shimming the rocker pedestals ?
FoT
I just built a fresh motor with a different block and discovered my pushrods
are too long.
I had new ( big ) valves put in and started with a prepared from stock
block.
.008 was taken off the block and I canbt remember how much off the head to
give me 12.5:1 comp ratio. I would guess the head was shaved over .120b .
Anyway, while doing the final assembly and prepping to run it in, my
adjusters are run out all the way to max. There is a full 3/16b of thread
projecting from the adjustment screw. I got cylinders 1& 2 to adjust to
.22/.24 lash but when I got to # 3
that's where the adjuster had no effect, that was my first clue.
Our first race of the season is on the 13 th so I need to find a solution in
the next few days. One thing I have always wondered about is why couldnbt
the pedestals just be raised to compensate instead of shortening all the
pushrods.
Thanks again for input,
B
Jim G.
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