[Fot] Push Rod Length

[Greg And Alison Blake]

Don't forget that if you are using a stock cam that has a race grind, you have
changed the bace circle of the cam. That too will affect push rod length.

Surely Larry Young will chime in.

Greg

> On Dec 19, 2013, at 7:24 PM, Duncan Charlton <duncan.charlton54 at gmail.com>
wrote:
>
> 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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