Joe
google Rocker Geometry by Jim Miller, It is wealth of Information. It
explains were the rocker arm should be in relation to the valve and the rocker
shaft at half lift. It is a lot of reading , but after you go through it a
couple of times, it will click , and you think, "OK" I get it.
John Wagner
Just the wrench
PS Have a Marry Christmas.
On Thursday, December 19, 2013
10:45 PM, Greg And Alison Blake <ablake2@austin.rr.com> wrote:
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@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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>>
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>>
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