1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

1971 Thunderbolt low oil pressure when hot

Discussion in 'BSA' started by 1973x75, Jul 27, 2019.

  1. 1973x75


    Nov 15, 2015
    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?
  2. lazyeye6


    Feb 28, 2014
    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.
  3. Onder


    May 11, 2010
    Not to forget end play on the crank with these bikes...
  4. RoadScholar

    RoadScholar VIP MEMBER

    Dec 28, 2008
    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.

  5. 1973x75


    Nov 15, 2015
    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.

Share This Page