I have been following the recent posts about oil pressure with
interest. I have been chasing an oil pressure problem recently with my
948 that is at the 2500 mile mark after it's rebuild. From a fresh
rebuild the oil pressure would be a little over 60 psi cold and 20 psi
at idle. After warm up it used to drop to 50 psi or less and about 15
psi at idle. About 2 weeks ago I noticed the oil pressure cold was
still 60/20 psi but after it warmed up it would drop to just a little
over 40 psi and 10 psi at idle! Peter C. on the list sent me a correct
spring for the oil pressure relief valve and I put it in this weekend.
The cold oil pressure rose to 70/40 psi ! After warming up the engine
with about 20 minutes of driving around town the oil pressure dropped to
50/15 psi again. After another 15 minutes or so of driving the oil
pressure dropped further to 45/10 psi. Over the weekend I tried to
chase this pressure loss down and Plastigaged the middle main bearing
and found it to be perfect at .0015 running clearance. I observed the
rocker assembly with the valve cover off and could run the motor for a
few minutes before the area below the rockers filled with oil. The oil
was migrating away from the rocker bushed slowly, so that looked fine.
There are only a few places in the engine to build oil pressure. The
main and rod bearings, the rocker bushes and the oil filter bypass
relief valve. Why is my pressure so good at cold and drop so
dramatically when warmed up. As part of my effort to improve oil
pressure I changed the 20/50 oil with a straight Castrol 40 wt as
recommended to me by my friend that has rebuilt Healey motors for over
30 years. He says he runs straight 40 wt in all his Healeys, big and
small. What's going on here?
Mike MacLean Supercharged 60 Sprite
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