DeTomaso Mailing List: November 99, Message #326

What I think he is referring to:

The roller tip on the rocker swings an arc as the rocker is acted on by the
cam action.  The roller tip, therefore, sweeps back and forth over the top
of the valve stem as it opens and closes the valve.  If one does not choose
the correct pushrod length, then it is possible for that contact area to be
quite off center of the valve centerline causing asymmetric loads in the
guides.  This can cause guide wear.  In the extreme case the roller tip can
sweep right over the edge of the valve stem causing catastrophic side loads
to the valve stem.

If I remember correctly, the rule of thumb is the roller should sweep
approximately to the centerline of the valve at maximum valve lift and
approximately 1/3 to the outboard side of the valvestem at zero lift.  I
suppose with extremely high lifts, the contact would have to be allowed to
cross over the center to avoid falling off the outboard side.

A way to choose pushrod length is to purchase an adjustable pushrod
(two-pieces, has screwthread, see Jegs) and install it after the heads are
torqued on.  Then adjust it to provide the appropriate roller tip contact,
measure its length and order the final set of push rods.

Another item to watch for,  especially with high lift cams, is that the
rocker arms are free to travel without interfering with the rocker arm stud
or any other obstruction.  There are a number of ways to screw that up.

-----Original Message-----
From: Eustaquio, Cal J <cal.j.eustaquio@lmco.com>
To: Multiple recipients of list <detomaso@realbig.com>
Date: Friday, November 12, 1999 10:06 AM
Subject: Rocker arm geometry???


>Hi All:
>Just wanted to know about some phenomenon that the local engine rebuilder
>informed me about during a phone conversation I had with him yesterday.
>
>During my talk regarding head work on the engine, he told me that
installing
>and fitting the head to the engine is not a trivial thing. He told me that
>the rocker arm assembly (using roller rockers) required that during it's up
>and down motion, attention must be payed attention to very carefully,
>otherwise, one can wear down the valve guide very quickly. He said that the
>best way to avoid this would be to be judicious about pushrod selection.
>
>I guess I can see his point. We discussed "side forces" of the piston
>against the cylinder (pretty important when considering stresses to the
>cylinder wall). Pretty much the same phenomeonon is occurring should the
>rocker arm travel be to excessive. Is this correct? Anyone care to comment?
>Thanks. Cal. #2428r.
>