MG Car Repair: MGB burning oil, oil filler cap, oversize pistons

Question
The car is a 73 MGB USA model that is sending quite a bit of bluish smoke into the cockpit through the oil filler cap when under an uphill load. I've checked the oil pressure relief valve, which slides freely. The spring length is 2 7/8 inches, not the full three inches as described in the specs. Cylinder compression
is not great either, at around 118 to 98 psi. On the flat stretches driven gently there is no smoke inside, just a hot oil
smell, althogh temperature and oil pressure are both fine.

Thanks in advance for your ideas.
John6

Answer
Hi John.  
I think you have a worn engine.  The compressions should all be the same, within 10 psi.  

Also, the breather in the oil filler cap is pumping out oil fumes.  This is known as ring blow-by.  The crankcase is getting pressurised by gases getting past the rings into the bottom of the engine.  It is just possible that the two breather tubes to the carburetters are blocked with a white emulsion, but the low compression figures lead me to think that your problem is rings.

I would take the head off and inspect the bores.  If one or more bores is badly scored, then you need a rebore and oversize pistons.  If the bores look OK and the ridge at the top is not too pronounced, then you may get away with new rings.  To do this, you would need to drop the sump pan, undo the con rods and extract them upwards through the bores.  While they are out, inspect the con rod shell bearings.  If they look worn but not scored then just replace them.  Remove the old piston rings and re-ring them.  Tape up the exposed journals on the crankshaft to protect them from dust.  Hone all four bores.   Sears used to sell a nice hone that you use with a hand-held electric drill.  I think Moss Motors also sells them.

Get back to me if this is not clear.