Larry,
There isn't a problem when the head is skimmed just a thou or two to
get it flat. What does warrant investigation though is the thickness of
the head, since the head might have been skimmed before, or even had
metal ground off for the purpose of increasing compression. It's good
practice to measure the head to be sure, then to address pushrods as
necessary.
In the TD tuning manual, it specifies removal of 1/8 inch to raise
compression from 7.25 to 8.25 or so, and the book said to insert spacers
under the pedestals to compensate for the reduced head thickness. We
know now that that worsens an already-compromised pushrod-rocker-valve
geometry, but that's what MG recommended. Today we would simply shorten
the pushrod and make a better, or at least not worse, geometry.
Actually, shortening a pushrod has the effect of strengthing it, not
weakening. The idea is that a pole or pipe or pushrod, which really is a
pipe, is built to withstand a certain compression load. As its length is
shortened, there is less tendency for the pole to bow out sideways. In
effect, it's stronger.
Try with a piece of spaghetti. Stand on end and push it to get a bow.
Then shorten same piece and try again. Same force begets less bowing.
Then eat it. Satisfactory experiment. This gets important in boat masts
which withstand tremendous compression loads when backstays are cranked
up to tighten headstays, then the boats are sailed hard and the shrouds
are tightened by wind pressure against the rig. The force is that of a
bow&arrow, trying to have the bowstrings (headstay, backstay & shrouds)
push the arrow (mast) through the keel of the boat.
Point of all this is that you can shorten pushrods with impunity, and
it is easy to do.
Bob
On Mon, 22 Mar 2004 14:21:44 -0600 "Larry Daniels" <ldaniels@vbe.com>
writes:
> On another thread, mention was made of shortening pushrods when the
> head is shaved. I would imagine that just taking a few thousandths off
of
> the head to true it wouldn't be a problem. At what point would there
be a
> problem with not shortening the pushrods? How much can a pushrod
reasonably be
> shortened without threatening it's integrity?
>
> Larry Daniels
> 79 B LE
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