Could someone explain to me the difference between the Cooper S oil pressure relief spring, and the stock? I thought I had the answer, but I'm not sure myself right now.

I understand the stock was 60 lbs, and Cooper S 65 lbs? Does that sound right? Were there other springs as well?

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

The last of the round-nose parts book (PUB1011) quotes the following:

1275 Cooper S - part AEA536
997 Cooper the same

All the other engines quote part AYH171 (previously 6K853).

I don't have any pressure-related specs, just dimensions and material in the drawing for AYH171. I wonder if there's something in the workshop manual?

eightfifty may know more.

Edit: Minisport and Somerford both show AEA536 for the Cooper S and 6K853 for all other Models, but no mention of pressure.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

Good Morning Mick
The standard mini was 60psi with a 73mm spring.
The Cooper (not S) was 70psi with a 66mm spring
the Cooper S was 60psi but no mention of spring length

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Cheers Dave.

Dave - where did that data come from?

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

Drawings show all three springs produce a force of 13.5 lb at their fitted length of 24.6mm (same fitted length for each). The free length of AEA536 listed for Cooper and Cooper S is 66.3 mm, while the free length of 6K853 abd AYH171 is 73 mm. However, the wire dia of AEA536 is larger hence its ability to produce 13.5 lb for a shorter compression.

Whether or not these force values in lb result in a different blow off pressure depends on the diameter of the plunger - I haven't check to see if they are the same in each case.

It is helpful to know something about these part numbers. The Mini range was in a local content plan and so UK produced parts were often reproduced locally and given a new part number, even if the actual part was unchanged in design. 6K853 is a UK produced spring. AYH171 is an Australian-produced spring. There is no practical different between them. Cooper and Cooper S engines were imported completely built up, and so they contain UK parts (with the possible exception of some ancillaries which were added locally). Thus, an internal part like this spring will have a UK part number AEA536. AEA536 is a Morris Motors part. 6K853 is an Austin part.

As I understand what has been said so far:- all Mini engines ran the same oil pressure but used different parts to get there.

Oil pressure specs from the TP838C Workshop manual


Looking at the UK AKD4935 workshop manual

848cc, 998cc had 60psi


997cc, 998cc Cooper had 70psi


970cc, 1071cc, 1275cc Cooper S had 60psi


12H 1275cc GT had 50psi

Well this has certainly turned into an intersting topic and thank you to the original poster for instigating a most interesting topic. I've collated the data presented by Brad above and also looked in parts books as Doug had done earlier, and also obtained data from drawings, and the picture below shows the results. There is some confusion about the data coming from the workshop manuals, and it is not even clear which spring is fitted to a 1275 Cooper S.

Basically there are two springs, long and short. According to the drawings, both provide the same load at the same fitted length but have different wire diameters. I've checked the actual valve plunger and also the seat in the block, and there are no substantial differences for these parts for all these models (there was a change in part number resulting from a change in material, but dimensions remain the same).

According to the workshop manual data, pressures range from 50 to 70 psi. I don't see how this can possibly be so, even with the same spring. For example, AYH171 fitted to a 1275 cc GT is supposed to give 50 psi, but the same spring in 850 and 998 gives 60 psi. With the Cooper S 1275, I can't work out which spring is fitted, but the part number in the parts book shows AEA536 which, according to the data for 997 Cooper and 998 Cooper, should result in 70 psi - but the workshop manual says 60 psi.

Perhaps the best thing to so would be for someone to measure the pressure on the same engine, and replace one spring with another, to see if there is any difference.

Thank you, I appreciate your comments and data. As a complicator, I wonder how the felt filters played into this? We all use spin-on or paper cartridge, but I think I remember that felt filters (For Cooper engines) were a thing as well. Being felt, they were thick and woolly, so I suppose they reduced flow (and created back-pressure). This is really the second half of my thoughts, as the relief valve and the filter might have had an interplaying relationship.

Are my recollections on felt filters close?

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.