One possibility comes to mind from personal experience.
The lower gasket kit comes with two oil pump gaskets. 3 Main engine
and 5 main engine. A common mistake is to use the earlier gasket on the 5
main oil pump. If the engine is assembled upside down the gasket looks fine
on the block. What you don't see is that the 5M oil pump has a larger port
than the 3M pump. The misaligned gasket allows pressurized oil to spray out
of the gap, lowering oil pressure.
See if you have a left over 3 bolt gasket from your set. If you do
have the left over gasket and access to a fax machine, I can fax you a
picture of the correct gasket.
I managed to do this on a 68 MGB rebuild. It took me about 5 hours
to pull the motor back out, switch the gasket and reinstall. That is an
indication of how mad I was at myself.
This last weekend when we started
> it for the first time it only showed 20 lbs of oil pressure and after
> about 10 minutes of running it went down to about 15 lbs. We shut it
> off. At this point I'm not sure how to go about diagnosing it. I
> suggested pulling the engine and start dissecting it to find
> the fault.
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