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6 cyl Bristol cars Type 400 to 406 - restoration, repair, maintenance etc

Worringly high oil pressure

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Old 16-03-21, 04:19 PM
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Default Worringly high oil pressure

Hello, all.
I recently ran some flushing oil through the engine of my 403, which had had little use for many years, though it's been run quite often in the last year or so, and refilled with 20/50. The oil pressure has always been fine, in the 50-70 range, and maybe 30 on tickover, but on starting since the oil change, it goes to the end of the scale if revved, and 70 on on tickover. I haven't warmed it up thoroughly yet, because I was a bit worried by this.
The pressure relief valve obviously needs adjusting, but I'm worried that what I'm seeing is a blockage somewhere, maybe cause by debris stirred up by the flushing, which is stopping the oil circulating properly.
Any suggestions about what might be happening? Would the oil pressure normally be that high when cold, if the relief valve was stuck?
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Old 16-03-21, 06:18 PM
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That sounds exactly as though the pressure relief valve is stuck closed. I am sorry that I know nothing about Bristol 6-cylinder engines, but relief valves are often cylindrical plungers in a close-fitting bore, so it doesn't take much in the way of a spec of crusty soot or something, to jam the cylinder in the bore.
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Old 28-04-21, 06:57 PM
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Mine goes off the scale when cold (rebuilt engine) over 1,500 revs until the water has reached 75 - 80 degrees for a while when it starts dropping back to normal. Running on Valvoline 20/50 synthetic.

On another (subject hijacking but related) note I was replacing dash board lights, which is a notoriously fiddly job, and couldn't get to the oil pressure guage bulb. I undid the two screws but, with the pipe attached, it hardly moved so I refitted it.

My problem is that it now reads 20 psi. If I move the pipe the pressure changes but returns to 20. Is there any simple way to get it back to zero?
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