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MG MGB Technical - MGB oil pressure relief valve

The oil gauge readings on my '72 Roadster had always been a little erratic and maybe a bit lower than I expected from a 20k miles recon unit, and I suspected (hoped) that perhaps the relief valve might be sticking or some such. So, when I started to strip the car, I invested in a new relief valve head, spring and packing washer. Today I removed the valve to find that the old spring is a full turn shorter than the new one. It is 17 full turns between the flat ends whilst the new one is 18! Weird? To make things worse as far as the relief pressure is concerned, there was no packing washer, another 1.5mm 'length' missing. Overall the new assembly, uncompressed, is some 8mm longer than the old. I also noticed some scoring on one side of the valve head so maybe it was both sticking and opening early because the spring tension was lower. It explains a lot wrt the gauge readings.
Anyone know where can this spring can have come from?
Richard Coombs

I've just done a quick look at other oil relief springs and found that the free length of a midget spring is almost exactly that of my old one. No idea of the number of tuurns etc but it looks suspiciously like an incorrect parts bin was used during the reconditioning!
Richard Coombs

Richard-
Number of turns, free length, packings, and such don't mean a thing. All that matters is load at installed length, and no bind at maximum actual compression. All else being equal, like wire size, coil diameter, and preload, more turns makes a "softer" or lower rate but less highly stressed spring. Within reasonable limits, and assuming the installed load is correct, the softer rate makes no difference either, except in the total travel of the valve, if it has no upper stop.

Changing spring rates might change the reaction to some pressure pulsations in the system, thereby affecting the rather notorious gauge pulsation that these cars have at certain speeds. It never bothered me enough to investigate.

"Erratic" is another story. Bad fit of the valve from wear, or dirt in the bore, or a worn valve or seat surface might cause it. The valves should be lapped in at assembly, but hardly anyone does it, with no ill effects resulting - usually. If the oil system is in good shape the valve will only be fully closed at low RPM, so any erratic behavior will go away at higher speeds.

What were your pressures? Too much oil pressure is a common desire of amateur tinkerers. More is not better, past a certain point.

FRM
FR Millmore

Hi Folks:

Most of the engine disassemblies that members of our MGCC-LIC club result in that the valve head is usually missing the packing piece. It seems that very few people pay much attention to this packing piece, which is difficult to source. Once you know about it, you can substitute a washer with the same thickness/diameter. I have seen oil pressure increases as much as 10 lbs+.

Good luck: Rich Boris 67B roadster
Rich Boris

The packing washer is NOT normally fitted - it is a competition part intended to raise the pressure at high rpm for competition work.

The spring free length should be 3 inches with a load of 15.5 to 16.5 lbs at a fitted length of 2 5/32nds.

Lapping the valve in is not a good idea - it is difficult to clean out all the grinding paste from the block unless the block it completely stripped. Better to simply tap the valve onto its seat using a suitable drift.

Oil pressure a between 50 & 80 psi is fine - anything over that puts excess load on the cam and oil pump drive and wastes power.
Chris at Octarine Services

Thanks Chris. That at least explains the missing washer. I'm satisfied that the fitted spring was the wrong part so I'll just proceed with replacing it and see what the result is, although that will be some time later as I 've still to get the body back from the sprayer. Plenty more to do anyway.
Richard Coombs

Chris-
Thanks for the real ionfo/numbers.
I agree on the difficulty of getting it clean post lapping - I don't do it either. I do examine the seat for obvious flaws.
Vizard says that on A series the seat is actually a separate steel piece, pressed in. Do you know if this is true on B series?

FRM
FR Millmore

FRM,

I have never hacked a block apart to find out!

Chris
Chris at Octarine Services

Chris -
Me Too!

FRM
FR Millmore

Hi Folks:

Getting back to the oil valve packing piece (OEM P/N AEH 798) issue; I bought my 1966 MGB brand new in 1966 and when I disassembled the engine for an overhaul, and removed the oil valve the packing piece was install. The Moss UK catalog lists this piece as part of the standard engine assembly. I was lucky a decade ago and found a few OEM pieces and I am presently using one on my 67B roadster.

Good luck: Rich Boris
Rich Boris

Rich,

Yes it is shown in the catalogues but was not a standard fitment - on your car it may have been added at the factory to address a weak spring or low oil pressure.

The Special Tuning booklet refers to packing pieces of .1, .2, .3 & .4 ins thick by 31/64 ins diameter.

Standard M6 flat washers are the correct diameter and two of these measure about .1 ins thick. I use these on my fast road engines to give an oil pressure of 70 psi.
Chris at Octarine Services

I've just measured the part supplied by Moss (AEH798) and it's 1.5mm thick or 60 thou, somewhat less than the .1" lowest value you quoted. Do you think we're still talking about the same washer Chris, or is this now a standard fit? In any event, do you think I should fit it or not?
Richard Coombs

It will do no harm to fit it.

I have some original spacers and they are 100 thou. I guess that Moss have found a similar sized item that is thinner than the original - possibly from a different application.
Chris at Octarine Services

This thread was discussed between 18/09/2009 and 20/09/2009

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