I helped a friend with his recently aquired 1950 TD today. Nice re-paint,
new interior, ODO says 8500 miles, and we figure it lies. It was fogging
mosquitoes like crazy, diagnosis is definitely very loose valve guides.
Before pulling the head we did a compression check, 170-175 psi on all four
cylinders, somewhat more than I expected, figured either a lot of carbon
inside or high compression pistons. Also did a blow-down test on all four
cylinders, just a minor hiss through the crankcase (normal) and absolutely no
leakage to the intake or exhaust. I've never known an MG engine to be that
airtight.
After pulling the head we found a generous dose of soft and wet burnt oil
deposits, but not enough to materially affect the compression. Also found
flat top pistons labeled "std bore" looking fairly new, extremely small
ridges in the bores, and the valve heads in generally good condition. We
also found shim plates under the rocker shaft pedestals, about 1/16" thick,
same shape as the pedestal profile, and #4 with the proper oil hole in it.
It looks like the engine was re-worked recently but the DPO didn't fix the
worn valve guides.
Now the questions:
a.) Moss catalog shows only one part number for standard pistons, the
picture looks like flat tops. Were the original pistons flat tops?
b.) What should the compression numbers be for a 1950 TD engine in good
condition?
c.) Were the rocker pedestal shim plates original?
d.) What is the original overall thickness for the cylinder head?
I'm inclined to think that we have high compression pistons or that the head
may have been milled down quite a lot and the rocker pedestal shims added to
get the valve adjustment to come out right. Oh come all ye wisdom of the
net.
Barney Gaylord -- 1958 MGA
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