By Greg Rollin
Setting the stem heights is a standard procedure
with a correct valve job, but surprisingly enough, is quite frequently
overlooked by many machine shops. It is important that the valve stem heights
are set or at least equalized. As this will have an effect lifter preload,
rocker arm geometry, valve lift and valve train longevity. This is an
especially important procedure when dealing with stock type non adjustable
rocker arms.
Stem heights are increased when the valve seat is
ground and when the face of the valve is surfaced. As this
puts the tip of the valve stem higher in the head. This will also effect
the valve spring installed height, but more on that in another article. The
additional stem height must be compensated for by grinding the tip of the
valve. You can check for proper valve stem heights by either of the methods
shown below.
When using a factory tool (#BT6428), the minimum
clearance between the valve stem tip and the tool must be .035" for 66-67
330's and 68-72 350's, .005" for 66-72 400's and 455's.
When setting the valve stem height with a dial
indicator (above), the maximum height from the (stock) spring seat to the tip
of the valve stem should be: 1.860" for 1966 and up 330 and 350’s without
rotators and 2.015” for 350’s with rotators. 1966 and up 400 and all 455’s
without rotators; 1.880" and 2.035” for 455’s with rotators.
Caution, the stem tip must not be less than
.005" taller than the retainer/rotator. If it is, the seat and/or valve has been ground too far. If a new valve does not correct the
problem, a new seat will need to be installed to obtain proper stem height.
Valve stem height (as well as pushrod length,
surfaced heads/blocks and cams with different base circle diameters) will effect rocker arm geometry. For proper geometry, the rocker
arm contact pattern across the valve stem tip should travel an equal distance
from the edge of the valve stem tip in the non lift position (above) as it does
to the opposite edge of the valve stem tip in the full lift position (below).
At half valve lift, the rocker arm contact with the valve stem tip should be in
the center of the valve stem tip.
NOTE: Since some of you have wondered, the valve
"spring" in the above photos is a test spring used for the purpose of
checking setups.
As noted, valve stem height will be effected by the amount removed from the seat and face. In
addition, by equalizing the valve depth on all cylinders, this will help keep
the volume of the combustion chambers equal. Obviously, the deeper the valves
are sunk, the more you will increase the combustion chamber volume.
We hope you have found this information helpful.
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Rollin/Supercars Unlimited. Any reproduction of the text and/or images strictly prohibited.
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