1971 Thunderbolt low oil pressure when hot

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Nov 15, 2015
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New con rod bearings, new timing side bushing, new rings and valve guides, honed the cylinders, assembled with care. Oil pump seemed good, so reinstalled. Tested the switch and it activates at about seven psi. 20/50 conventional oil with Lucas zinc additive for break in. After about 15 miles, the oil pressure light comes on at progressively higher rpms as it gets hotter. Should I be looking at the oil pressure relief valve?
 
Yes! That's something very quick and easy to do. There is a piston and a spring inside the valve. The spring can weaken, and
the piston can become corroded and begin hanging up. There are also, I believe, shimming washers in the body which are used
to increase/decrease the point at which this valve opens. Oil pressure switch itself may prove to activate at 7# cold, but may
be affected by the oil becoming hotter. It is quite easy to remove the switch and install an oil pressure gauge too prove what
your pressure is. If all of this checks out you need to look at the oil pump. It is not right for the oil pressure light to come on
when hot at more than around 1000 RPMs. Beyond all of that I can't help but wonder if you installed the correct sized con rod
bearings or if the journals needed a re-grind.
 
Not to forget end play on the crank with these bikes...
 
You said the oil pump seemed good. What criteria did you apply to come to this conclusion? I agree with checking/cleaning/replacing the OPRV first, but consider what the oil pump pulls/pushes through itself. Blow-by particles are largely composed of carbon, which is very near diamond in hardness. Oil pumps do wear and virtually all engine re-builders change them out without a second thought.

Re-bushing the timing side is a good practice, but did you measure the crank and bushing to determine the cold clearance?

Anyway, start with the OPRV and the OP warning sender, hope that solves the symptom.

Best
 
It was the oil pump after all. It spun easily by hand, which led me to believe it was OK. Got a new SRM and all is fine.
 
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