Disappearing Oil Pressure!

[alanp]

when you say the gauge tapping is after the pump,I'm assuming its before the PRV?

Yes John, it's tapped into the oilway in the crankcase leading to the PRV.
Thinking about it further, maybe, just maybe, the oil seal in the timing case cover which allows oil pressure to feed into the end of the crank journal (SRM conversion) was somehow misplaced (?) and did a pressure relieving job of its own?? The seal looked ok when I did my check. I'm thinking about getting a new one from SRM just in case.
Ref. the pressure gauge, it's virtually impossible to bleed all of the air out of the instrument anyhow but this is of no concern because pressure is pressure if it's measuring air or oil, but air pockets will soften the gauges reaction to rapid pressure changes a tad methinks.
Cheers
Alan

[kommando]

Make sure the seal in branded 'pioneer', they seem to be the only garter seal you can rely on to stand the pressure on startup. They are UK made and available from Eriks.

http://www.eriks.co.uk/

[chaterlea25]

Hi Bill and all,
its a hard one to wrap your brain around 
if oil or any liquid is displaced by a positive displacement pump against a closed circuit the result is the same as a mechanical block of the pump, a dead stop 
in our engines oil pressure leaks off through the bearing clearances and of course the pressure relief valve
if it didnt the pump would lock and probably the drive fail !!
Take Bills hydraulic ram, the ram will move under hydraulic pressure, if theres air in the cylinder this will be pressurised up to whats needed to lift or move whats loading the ram, Air operated rams are also used in industry
On most hydraulic control systems a pressure vessel is included in the circuit to smoothen the actuation.
There are a couple of ways to dampen the movement of a pressure gauge, some are filled with oil or glycerine
or a needle valve is put before the gauge, restricting the flow, this dampens out the fluctuations in readings
(carb balancing gauges or gauges on piston compressor heads)

HTH
John O R

[bsa-bill]

Cheers John, I live and learn thanks to you lot.
Hadn't thought about air rams but right enough we've had stuff with them, most of our Jonh Deere's came fitted with compressors, probably not cost effective in the long run but saved the day more than a few times (we carried a line and tyre adaptor in the cab)

[beezermacc]

Sounds like a partially blocked tank breather to me, or a collapsing feed pipe. On tickover there isn't sufficient suction to cause a problem. As the engine revs increase the suction on the feed pipe can be so great it causes the pipe to collapse in on itself. A similar effect would be caused if the tank breather is blocked then the tank won't allow oil down the feed pipe. Try taking the oil tank cap off and/or temporarily replace the feed pipe with an original old one.

[alanp]

Sounds like a partially blocked tank breather to me, or a collapsing feed pipe. On tickover there isn't sufficient suction to cause a problem. As the engine revs increase the suction on the feed pipe can be so great it causes the pipe to collapse in on itself. A similar effect would be caused if the tank breather is blocked then the tank won't allow oil down the feed pipe. Try taking the oil tank cap off and/or temporarily replace the feed pipe with an original old one.

Thanks for that. I'm still on it's case but have been diverted recently with electrical work on my Honda XR250 enduro. I couldn't understand why the new LED indicators didn't work until I realised it's system didn't convert the alternator's output from Vac to Vdc and the lights ran on Vac. The LED flasher unit only 'likes' Vdc. Now sorted but the garage is too cold to approach the BSA!!!
Alan