[s-cars] S-car head repair

[Charlie Smith]

Earlier, Tom Mullane wrote:
>
> Charlie,
>
> I admit that my experience is mostly with 2 valve per cylinder VW/Audi
> engines where valve/piston contact is generally NOT near the edge of the
> piston.  But I would think that any damage that would cause this type of
> ring problem would be obvious and severe.  No?

I agree, the 2 valve engines are probably more prone to this, because
the valves are bigger and stronger.

If the valves hit hard enough to "mark" the top of the piston, if these
marks are of any depth, it's possible to have squished the top edge of
the piston down onto the top ring.  I have not seen a 20V Audi piston.
But, especially if there are already valve cutouts in the piston, there
isn't that much material between the cutout or point where the valve
touched the piston and the inside back edge of the ring groove.  That's
where the ring groove is going to get squished.  That's probably 3/16"
back from the outer edge of the piston.

In a good (unworn) piston the ring to groove clearance isn't very big
at all, probably on the order of 0.001".  (Can't quote off the top of
my head, and can't get to the Bentley).  It doesn't have to get squished
very much to snare the ring tightly.

    - Charlie