At 05:16 PM 6/17/99 +1000, Tim wrote:
>....
>Jeez mate, we haven't had 97 in this country for about 15 years!!!!!
>....
>Our leaded fuel is lucky if it gets to 96, more typically 94; unleaded
premium is supposedly 96 - 98 (and 5c/litre more), if you'd like to spring
me for an unleaded head. Standard unleaded is pushing 92.....
Well, under the circumstances, it looks like you're in for pulling the
head. Maybe you'll get lucky and find lots of mineral deposits in there
that you can scrape off and lower the compression. Leaded gas is known to
do that over a long periiod of time. Also bad valve guides will allow a
little oil into the chambers which can also build up as carbon deposits.
Sometimes a good cleaning of the head and the tops of the pistons will work
wonders.
Otherwise, I suspect that you will find flat top pistons, or that the head
has been shaved down nearly 1/8 inch. The cure for the flat top pistons is
obviously a new set of dished top pistons. Cure for a thin head is either
a different head or a buildup of gaskets and/or shims under the head.
Doubling up on the head gasket can lower the compression ration a few
points, but getting the compression readings down from 170 psi to around
140 would likely take two head gaskets and a steel shim.
If you're a little creative you can measure the head thickness without
removing it. Remove the valve cover and lay a straight edge across the top
of the head. Then measure the distance vertically from the top of the head
gasket to the underside of the straight edge. B-series heads were
originally 3-3/16" thick (3.187 inches). A clean up cut on the underside
usually only takes off 0.010" to 0.015", so if it's down more than about
0.030" it would have been intentionally cut thinner to increase the
compression ratio. So go do a measurement and let us know.
Cheers,
Barney Gaylord
1958 MGA with an attitude
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