I've always assumed that it depends on the amount of lift provided by the
camshaft. Any extra lift moves the middle point of the valve opening range.
I figure lift, then assume the rocker arm needs to be at 90 degrees to the
rocker shaft when the valve is halfway open. That way you even out the wear
on the guide because the stem is sees the same amount of side force in either
direction left/right (as seen from the front or rear of the engine).
If you cut the head and shim the pedestal, you're going the wrong way and at
the mid-point of lift the rocker arm will be pointed too far down. Best
solution would be to cut the pedestals and use shorter pushrods once you work
out the geometry. I took measurements from my Morgan's TR head (because the
PO had installed shims under the pedestals) and worked out the amount to cut
off the pedestals. There would then be a mathematical ratio between how much
you cut off the pedestal and how much shorter to make the pushrods based on
the length of the rocker arm (ie: from where it touches the valve stem to the
center of the adjusting ball) and where along that axis the centerline of the
rocker arm is. Put overly simply, in other words if the rocker arm was 3"
long and the rocker shaft was 2" from the valve stem, and then you needed to
remove 0.100" from the pedestal, you'd need to shorten the pushrod by 0.150".
Somebody check my math, please!
Duncan
On Dec 19, 2013, at 6:10 PM, Joe Alexander wrote:
> FOT
>
> What is the current conventions on pushrods and length changes?
>
> For 15 or so years we ran with shims under the rocker arm pedestals.
>
> Jack Drews got us a couple sets of shortened push rods, which we have used
> since.
>
> Now I am wondering if there is some formula that is used.
>
> Perhaps 1:1 shortening of the pushrods with milling of the heads?
>
> Presently, I am presuming that there is a fair amount of tolerance in the
> length selection.
>
> I am unaware of any real in depth discussion on this....so perhaps it is
due.
>
> Thanks....
>
> Joe Alexander
> 645 1st Street
> Jesup, IA 50648
> The-vintage-racer.com
> Gasketinnovations.com
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