Oil pressure relief valve.

[ian Howell]

I have suspected for some time that a leaky oil pressure relief valve may at least be contributing to the relatively low oil pressures that my 1930 Long Four achieves.

I have stripped and cleaned the valve during my 'overhaul' but although it clearly works I suspected it may have been leaking at all times. I took it all to pieces again today and sure enough, on a 'air' test there was a distinct leak and no detectable pressure at which the valve opened.

I have removed and reseated the valve seat. This is a brass tube that is a tight push fit in the body of the valve assembly. The end face of steel valve piston body closes on to the end of this tube to close the valve up to the pre-set pressure.

WARNING: - There is a hole in the side of the brass sleeve that MUST be aligned with the hole in the valve body when re-assembling).

The actual steel valve has obviously been badly corroded at some time (presumably back in the 60's or earlier - before I got it) so I have made a new one and, testing it on air, there is now a distinct valve operation at 23psi.

BUT, I have not finished yet!

The counterbore for the valve spring is distinctly flat at the bottom and there are four very small holes from the counterbore to the outside of the valve body.

I have reground a drill to give me the desired 'flat bottom' and I am making a jig to drill the small holes, but WHAT ARE THEY FOR?

The valve body is not a 'hydraulic' fit in the valve body casting, so presumably they are not for pressure equalisation, and even with the spring cavity full of oil this would not appear to inhibit the valve operation, so WHAT ARE THE HOLES FOR?!

[george garside]

The Jowett Manual gives 14 - 16psi as correct oil pressure for 1930 models (and 25-30psi for 1931 onwards)

in March '33 The Light car and Cyclecar magazine published an interview with William Jowett who they reported as saying


''The rest of the engine is lubricated under a pressure of 30psi. The crankshaft is drilled and the oil is fed into it from the main bearings. An interesting point mentioned by Mr Jowett was that at normal speeds ample oil would reach the big ends if the pressure was only 1/2lb per square inch'' (yes it does say half a pound per square inch!)

george

[Keith Clements]

Oil flow is more important than Oil pressure. I do not think that oil pressure contributes to maintaining the oil film in the bearings. What it does do is help get the oil to where it is needed. Think of the oil delivery system as a pipe with lots of different sized holes running along it draining the oil from the pipe. The holes at the end of the pipe may be starved of oil if the oil flow is not good enough. This might be caused by a constriction in the pipe which would raise the pressure but reduce the flow. Thus a pressure of just a few psi might be good enough if the flow is still reasonable. It is possible the reporter got it wrong and Mr Jowett actually said 'half that'.

[ian Howell]

Thanks for that George - I will back it off to 14-16psi before I fit it, BUT what are the holes for?

I can only think that IF the valve is a snug fit in the body then they may equalise pressure inside and outside, but so what?

Also, as there are four of them I suppose if the valve rotates at all there must always be one that lines up with the 'spill' hole in the body, but again - so what?

As always . . .

[ian Howell]

I am - at least until I know better - taking the coward's way out and refitting the original valve into the housing, holes and all.

As I said earlier, I have re-seated the sleeve in the valve body and now I have reground the face of the valve itself so there should be a good seal up to the set pressure.

We shall see, but at least now I will know that low oil pressure is due to slack bearings (surprise, surprise!) but the engine is not disastrously noisy, has reasonable compression and does not seem to burn oil, so unless there is a calamity I shall leave the 'engine job' until after I have got the body 'skinned' and interior re-trimmed.