Oil pressure on a KH500

[eadriclloyd]

I had my KH500 off the road since the summer whilst sorting out gearbox problems.
In the meantime I removed the oil tank and flushed it out as well as removing the feed and return steel oil pipes, for flushing, to the oil pump.
I drained the sump of oil as well.
Two days ago I filled the oil tank with straight 30 oil and kicked the engine over, with the spark plugs out, until the oil pressure gauge started to give approx 15 lb pressure rise.
Today I went for a 4 mile ride and initially had oil pressure of 90 PSI.
After a mile and a half the pressure had dropped to 50 PSI and by the end of the ride was about 30 PSI, all pressures at 40 mph in 4th gear. The oil tank was slightly warm.
This surprised me because last time I used the bike, earlier in the summer, the oil pressure would stay high, 60 to 70 PSI, for many miles until the oil was up to temperature.
Is there any point in the oil system that I need to bleed air from?
I am puzzled as to how you could bleed the air from the section of oil pipe between the oil pressure gauge, located on the fuel tank, and the rocker supply union.
Thanks, Eddie Lloyd

[allan.walker]

Hi Eddie,
I have a Square Four and a Norton Commando. I've fitted oil pressure gauges to each one and at the time of fitting, I wondered about the question of bleeding the pipes to the gauges.
I couldn't find any information online about it, so I just connected them and they work correctly.
Perhaps the column of air in the oil feed line to the gauge 'transmits' the pressure to the gauge. I can't see any way that the gauges would self-bleed.

It would be simple to disconnect the pipe from the gauge and turn the engine over to move the oil up the pipe before reconnecting the gauge, I suppose.

From my experience, I'm happy to leave this as a mystery - if it isn't broken, I don't look for a way to fix it!

Regarding the difference in pressure readings you have experienced, did you you use the same brand and grade of oil pre and post-flush? Could some sludge in the oil pipes have been causing a higher reading?

Cheers,
Allan.

[simon.holyfield]

Have you changed the viscosity of the oil you are using Eddie?

[paul.jameson]

Your current pressure readings are very similar indeed to those on my KH, although I run SAE 40 in mine. I have found that the greatest impact on oil pressure is the degree of wear on the rockers and shafts, which is logical enough. So I run mine with the feed to the rockers from the return now which has led to me getting oil pressures as high as yours.

All in all, it sounds fine to me.

[eadriclloyd]

Alan, Simon & Paul, Thanks for your constructive replies.
I did change from SAE 40 to SAE 30 oil, as the handbook suggests for winter running. This has not made much difference to the oil pressure in previous years.
I rode the bike again today and the oil pressure stayed high, 60 to 70 PSI, for the same 4 mile trip that I did last time.
It would seem to me that when I rode the bike last week that there was air in the oil system which has subsequently risen out of its own accord.
Eddie

[Paul Slootheer]

Eddie, as a former owner of two KH’s, one 1952 model and a later one, I once also did experience sudden oil pressure dropping due to a small particle which was sticking between the ball and the seat of the pressure relieve valve! when I did clean this out all was ok! 4 miles is way to short for such dropping of pressure. When cold my KH could rise up to 90-95 psi (on 20W-50…) after a proper warming up it stayed, at cruising speed, at aprox at 70 psi. At idle it could drop untill 15-20psi when hot. As Paul says it is a good idea to take a bleed from the return to feed the rockers! I did the same, it is more then enough to lubricate them and you get 100% oil pressure on the bigends!

[David Smith]

Sounds about the same as mine. I run the rocker feed from the return side line before an oil filter I have fitted immediately before the return to the tank. I run SAE40 Detergent oil, and have a Brie Tie anti wet sumping valve in the feed line from the tank. My guess is the timing side and big end clearances get bigger when it is warm and drop the pressure a bit as the oil temperature increase is only small when the pressure drops to 30 psi or so.