OIL pressure on a 1935 jowett 7hp

[MartinRaine]

Hi I have very low oil pressure when the engine is hot on tick over the gauge moves when you start it up from hot but only goes to 0 but if you look on the gauge it has an area marked on it for idle and the pointer is in this. when running at 30mph the oil pressure is 20-25psi. I have screwed the Prv in but it dose not make any difference. The oil i am using is 20/50 is this the correct oil, or do i need a thicker grade ? is the oil pressure OK ? The gauge is moving so it may need recalibrating ? Can any one help. The car runs spot on with this oil pressure. Ps i am jusr running the engine in. Thanks Martin Raine.

[Forumadmin]

I will let the pre-war guys answer on this specific model; but I would not be too worried. 20/50 may be OK, although a straight 30 maybe the correct oil. Some modern oils do attack bearings in old engines, using an oil specifically formulated for classic engines is something some people recommend.

I did have a similar problem on the Javelin and it was caused by the engine running too hot caused by a blocked rad. Clearly big ends or other oil delivery issues could also be a cause of low oil pressure; but you should start to hear a thumping noise if they have gone. Also drain and look at the oil to see if there is any white metal/ bearing material in it.

[Tony Fearn]

Hello Martin,

I looked at your post last night and hoped that George might get involved because he knows much more than me about the technical side. Nevertheless, here are my thoughts:-

I think that it’s ok when the engine is just ticking over that there’s not much showing on the gauge.

Both my pre-war cars show 25 psi at 30mph in top and according to the book this is ok.

20/50s is the oil I use and haven’t had a problem that I know of.

I don’t know how you would re-calibrate the oil gauge. I wouldn’t think it’s possible.

I’m surprised that the adjustment of the prv makes no difference. I cleaned a couple of them which came from “the spares in the bath” before I handed them over to JCS Ltd.. One of them came apart ok and was easy to clean.



The other one’s piston had stuck in the barrel and so it couldn’t slide backwards or forwards and it was a bit of a pig to remove. If your relief valve piston is stuck then whatever pressure you put on the spring will make no difference as far as I can see.
I’ve attached a little sketch to show you what I mean.

scan0018.jpg

Tony.

[MartinRaine]

Hi thanks for the info. I have found the problem when i calibrated the gauge it was found to be 15psi out. I striped it down to find the linkage in it to be nearly worn through. I had another gauge and made a good one out of the two, i now have oil pressure at tick over. Thanks Martin

[george garside]

Tony, missed this one as away on holiday but you have covered it well .

for what its worth the light car magazine of march 24 1933 reporting on an interview with William & Benjemin Jowett said

'An interesting point mentioned by Mr Jowett was that at normal speeeds ample oil would reach the big ends if the pressure were only 1/2 Lb per square inch' Earlier in the article he had stated the normal oil pressure as 30psi.

It would therefore seem that something in the region of 25psi leaves an extremely good safety margin for climbing in in 3 aclivities or belting round Montlhery track non stop for 72 hours.

george