UK Car Repair: Compression, oil pressure gauge, mechanical oil pressure gauge

Question
Hi, this is a re-send because I was having PC problems and I'm not sure the original got through.  If so please ignore.

I have a 1977 MGB with the installed engine having very low compression.   I have a spare engine with 95-110 PSI.  The question is will the spare work acceptably or am I waisting my time to install it?  And should I just bite the bullet and have one rebuilt.   Thanks for any recommendations / opinions you can offer.

Answer
Hi Brian,
If you held the throttle OPEN when you did the compression tests and let it turn over 4 or 5 revs at least, then neither engine is any good. An engine that has close to 100 PSI on a correct compression test will just about not run. If the engines have been setting up for a long time you need to put about 4 or 5 squirts of eng oil in the plug hole and spin the engine for a couple of minutes with all of the plugs out, (checking the starter to be sure it does not get too hot)then with the throttle OPEN pull another test and be sure to spin the eng at least 4 or 5 revolutions with the gauge and a good battery. You should get from 135 to 175 PSI and not more than about 10% difference between any of the 4 cylinders. If any cylinder is low, pull the valve cover and make sure the valve clearance is OK (0.012") If the valve train is dry and rusty spray all of the valve spring area with something like WD-40 and compress the valve springs by hand many times. (Set all the pistons at about half way in the cylinder so you don't confuse hitting a piston with with a sticking valve.)
I like to see about 140 to 150 PSI compression and with a mechanical oil pressure gauge about 30 to 35 PSI oil pressure using the starter. Anything less than that I figure I need to go inside now.
HMF