There's another thing that often gets missed with the issue of warped heads.
Although the head will measure "straight" if you machine the top and bottom of
the head, you will still have some problems. If warped much at all, the head
will have to be straightened first and then skimmed top and bottom. If you don't
do this the chamber shapes/sizes will be altered unevenly. If the head is bowed
up in the middle, then the center two chambers will end up being larger when
they shave the bottom of the head, than the outer two cylinders. And if it is
bowed down in the middle, the center two cylinders will end up being smaller.
Additionally, the valves, guides, and seats will no longer be parallel to one
another. Instead they will end up in a slightly fanned orientation, following
the curvature that the head had prior to skimming. If you straighten the head
prior to skimming, you will minimize these problems as well as reducing the
total amount of material you have to cut off the head.
John F Sandhoff wrote:
>
> It was asked:
> > What doesn't make sense to me is that if a head is warped and you shave
> > the bottom, wouldn't the cam alingment and the top be warped also?
>
> YES.
>
> This has been discussed as the reason for cams mysteriously snapping
> in half. BOTH sides of the U20 head should always be checked for
> flatness, and during assembly the cam should be checked to be sure it
> turns smoothly.
>
> And after shaving, tower shims should be used to return the cam to the
> engineered height, to keep the chain properly tensioned and the valve
> timing correct. Of course, then the wipe pattern on the rockers should
> be checked to be sure the valve geometry hasn't shifted too far....
>
> -- John
> John F Sandhoff sandhoff@csus.edu Sacramento, CA
>
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