Oil Pressure/Leak

[Over Steer]

Hi all;

I have rebuilt a twincam engine using a 711M block. On startup the oil pressure goes up to about 60 psi but after a few minutes it starts to go down to something like 10 psi. This is not at idle but at about 2000rpm. The bearings and oil pump are new. There is obviously a massive leak somewhere but I cannot imagine where. Any ideas?

Thanks

[rgh0]

Sounds like the pressure relief valve in the pump is sticking and not opening or closing smoothly.

Initially pressure high until it is finally forced fully open after a while, then as oil warms up and pressure starts to drop the valve should close but because its sticking it does not and pressure goes low.

I would pull the pump and investigate.

cheers
Rohan

[Over Steer]

The pressure relief valve was the first thing which came to mind and I checked it. It was OK but I put in a new oil pump just to be sure. No change. I suspected the drilling which sprays the crankshaft sprocket, but the hole diameter on the block is more or less the same size as on the aluminium front cover plate. So even if it were leaking at the blockface, there should not be much difference.

[denicholls2]

Not on a Twink, but I've twice experienced the relief plunger jamming open on an engine at initial startup after a rebuild. There are two likely causes of this problem:

1. Startup wear. On the engines I rebuilt, the oil filter was after the relief mechanism, so dirty oil got this far.

2. Too much assembly lube gumming up the works.

In any event, whatever the cause I vote relief plunger. You don't want your new bottom end to have spun a brand new bearing, and that's pretty much the alternative.

[Over Steer]

I've taken the engine out again and am going to measure the crank. will post the as soon as i get them...

till then

[Over Steer]

Sorry for the delay in getting back to this, I had to stop work on the car for some time.

I have found out the reason for the low oil pressure and I haven't stopped kicking myself. The only different thing which was used in this rebuild was the engine block. So that was the place to look. Sure enough the mains had been linebored to 0.015" oversize and I had not checked this out. This is very rare on 711M blocks and it never crossed my mind.

Anyway no damage done and everything is now just fine. Thanks for your suggestions just the same.