Lubrication - Oil pumps 

Essential to the well being of every engine is its oil pump and oil. Much in the same way as the heart and blood is deemed relatively important to us humans! It’s also another of those greatly misunderstood and, unfortunately, misrepresented subjects connected to engines. 
An alarming number of so-called A-series engine specialists would have you believe that once parting with a veritable bundle of your hard earned readies on that mega motor, what you really need to keep it in good health is a high-pressure/high-flow oil pump. Seems a sensible consideration - except in the A-series world it’s impossible. Oh, there are pumps that will give better flow rates than others, but high pressure? Absolutely not. 
Oil pressure is regulated by the relief valve in the block of an A-series, not in the oil pump as with say a Ford. Yeah, you know - the one that gets jammed, causing heart failure as you believe your crank bearings have dissolved, and is a pig to get out. Unless you’ve the Cooper S 'ball-and-spring' type fitted. There are other factors that affect ultimate oil pressure such as crank/bearing/rod clearances, cam bearing clearances, oil pump clearances, etc. - but mainly by the relief valve. Funnily enough, if there wasn’t a relief valve, most quality, correctly built A-series pumps could achieve pressures in excess of 200psi! Outstanding! 
So the next person that tries to flog you a ‘high pressure’ oil pump for your A-series - eye him skeptically and check his credentials if purporting to be an ‘expert’, and ask ‘How?’ Should make interesting listening. 
High pressure isn’t important anyway. FLOW is. Folk panicking because they didn’t have 80psi oil pressure in their race engines when hot always filled me with mirth. Even more so as they faffed around sticking packing washers behind the relief valve spring in an attempt to achieve that illusive figure, and having anxiety attacks because they could only muster 75psi!! Very funny. 
OK, let’s have a look at how oil pumps function, what’s good and bad, and how to select what you actually need. 
Rotor and annulus 
Sounds more like some form of medical complaint! Inside the oil pump there are two moving pieces, the rotor and annulus. The rotor is the central component driven by the camshaft, and rotates in an encompassing ring - the annulus. The rotor has protruding lobes, the annulus recesses into which the lobes fit. The pumping action is achieved by having one more recess than lobes, causing 'pockets' as the two are rotated by the camshaft, which in turn creates suction on the inlet side, expelling on the outlet side. 
This system of lobes and recesses causes a certain amount of pulsing. Generally the more lobes/recesses the less pulsing occurs, the better the flow rate and therefore oil supply. So why has the old A-series only got 'four into five' or 'five into six' recess-type pumps where most modern engines have far higher lobe/recess counts? Space. Pumps with higher lobe/recess configurations are quite a bit bigger in diameter by necessity of design to achieve suitable flow rates. 
Flow delivery 
Under close scrutiny, the poor old oil pump has to achieve a conflicting number of goals. At idle it needs to pump sufficient oil at low engine rpm - the oil pump runs at half engine speed as it is driven by the camshaft - yet not over supply at high rpm. It also has to have sufficient capacity to create enough suction to get the oil out of the sump (gearbox) to the main galleries feeding the crank, cam, rocker gear, etc. via the tortuous route through pick-up pipe, gearbox, and engine block. Consisting of varying bore diameters and various ninety- degree bends with real sharp corners in them. 
Careful design is needed to cope with the differing fluid velocities in the supply line from sump to intake port, and maintain sufficient flow at high rpm. Particularly bearing in mind that the design is surprisingly inefficient. Yup, I said inefficient. Don’t panic though as at high rpm something like 30-40% of the oil delivered by the pump is dumped straight back into the gearbox via the pressure relief valve. So there’s more than enough to go round. Literally. 
So what’s this ineffiencey thing? Well, the pump never actually fills each recess 100% each time. The fill tends to be better at low rpm, but as rpm climbs the percentage fill falls. The basic design is the problem - only filling the recess from one side and the limited time for filling as rpm rises. From tests carried out on a dyno, an overall average figure of 75% efficiency was discovered. Better at low rpm, worse higher up. Check out the relevant table for theoretical pumping capacities based on measured capacity, and those allowing for the overall average 75% efficiency. Total filling could be achieved, but only by supplying both sides of the recess... 
BMC GENUINE PUMP 1.568 (7.119) 1.176 (5.339) 
MINI SPARES CENTRE 1.984 (9.007) 1.488 (6.755) 
BMC TURBO PUMP 1.763 (8.004) 1.322 (6.003) 1.234 (5.603) @ 70% MINI SPARES TURBO PUMP 2.204 (10.006) 1.653 (7.505) 1.719 (7.805) @ 78% 
Figures are in UK gallons per minute (liters per minute) 
Good and bad 
I mentioned the fact that most A-series oil pumps have either 4 into 5 or 5 into 6 type set-ups. And from another earlier comment it would seem that the 5 into 6 one would be the one to use. Well this ain’t necessarily so. Much of whether it’s worth it is down to the overall pump design and quality when compared to any 4 into 5 design. 
Current manufacturing trends seem to have had an adverse affect on build quality. One in particular causes pump efficiency problems. Overly large tolerances between the rotor lobe tips and annulus creates drain off problems when the engine is not running and inefficient pumping when the oil’s hot. At start up, it takes a few seconds for the oil to be picked up and pumped around the engine’s arteries when drain down occurs - a situation that worsens the hotter the oil gets. Oil gets pretty thin when hot, large tolerances can see some of the oil being pumped around the internals of the pump rather than round the engine and loss of supply as it is unable to maintain a constant pick-up at higher rpm. 
Then there are the high capacity versions - those designed for the Metro turbo. The extra capacity on the OE pumps was achieved simply by making the rotor and annulus deeper. But from what I’ve already said, was this wise? Marginally, and was done mainly to allow for the extra oil needed by the turbo to maintain good health. Particularly when un-caring humans are added into the equation who refrain from using either decent oil or changing it at regular intervals - but usually both!! As with all things OE - over engineered to last a lifetime. Again, check out the flow capacity table - where you can also see the alternative offered by Mini Spares Centre. This is a far better way of achieving higher flow capacity - the design using a larger diameter rotor and annulus. Tests have shown this to be 78% efficient, where the standard OE type dropped away to 70% at high rpm. Is this volume of delivery necessary, if so - when? 
The bottom line 
Pumping oil absorbs horsepower. Supplying more oil at a higher pressure than is needed consumes excessive power. The higher the rpm, the more power is consumed in any given situation. You only need to use an oil pump that supplies enough oil for the application, and sufficient pressure to maintain supply to all the various components. So to all intents and purposes, a turbo spec pump is a complete waste of time on a normally aspirated engine. You just DO NOT need one. And that includes a vast majority of race engines - even then there has to be a VERY good reason for needing it (i.e. when using piston crown cooling jets). Besides, bearing in mind the test results it’s a pointless exercise, and you’ll save yourselves a few quid too. 
Ignore the old fogies - 70 to 80 psi oil pressure is NOT necessary, and excessive oil pressure can raise the oil temperature. 50psi hot and flat out with 25psi at idle is all you need. I have used as little as 40psi with no problems. Some of this depends on the oil used, more of this in a bit. Adjust this by either cutting one coil at a time off of the pressure relief valve spring until the desired pressure is obtained, or by an adjustable relief valve cap. 
A good quality 4 into 5-type will be superior to a so-so 5 into 6. ‘Good quality’ is suggestive of matched and hand built components - which rules out OE type pumps. Don’t get me wrong, not all OE pumps are no good, but by experience a good many of them are out of tolerance when new - effectively worn out before fitted. As a guide, the absolute maximum gap between the rotor lobe tip and the annulus should be 0.004” when measured in-situ. Incidentally, the 5 into 6-type is the old Holborn Eaton item that has a steel back plate (instead of being all aluminium) that David Vizard acclaims in his books. In comparison to what was available then this was the best one, but not now. The Mini Spares supplied Sureflow pumps are all hand-built to maintain best tolerances. Investigate who does what these days - I believe the choice to be Holborn Eaton (steel backed type), Concentric, Mini Spares Centre, and Kent and Piper also supply oil pumps - but I'd be surprised if they are of their own manufacture. 
Drive time 
There are now eight different options for pump fittings on all A-series engines. Small-bore engines (everything that isn’t 1275cc and up) are retained with three bolts, big-bore engines use either four or two bolts EXCEPT Cooper S’s which use three bolts. The oil pumps are driven by either a pin in the end of the cam (pin drive), a separate drive coupling with three lobes on the outside and splined internally (star drive), or by a slot in the end of the cam (slot drive). Pin drive was used on all small-bore pre-A+ engines and Cooper S’. Star drive on pre-A+ 1275s. Slot drive on all A+ engines. All small-bore engine pump drives are flush with the pump body, big-bore engines have a shoulder of some 3/16” between the pump body and the drive. 
OE pump manufacturers steadfastedly stuck to specific pumps for their relevant fitments. Mini Spares, however, have had their pumps produced with all mounting bolt hole configurations on their whole pump range. There are a number of bonuses by doing this. You can fit a slot drive cam in a ‘S’ block simply by using their 1275 A+ oil pump. Or a star drive cam in a small-bore engine - the spacer and drive are still necessary, but no drilling and tapping holes in the block. Or a pin drive cam in a 1275 block by using their Cooper S pump - no drilling, etc. required. Dead clever. 
And finally 
When fitting the oil pump of your choice to whichever engine, kindly observe a few points. Make sure the bolts you are using are not too long. Countless folk have come to me with oil pressure/supply problems after a re-build only to find the bolts they’ve used are too long. They bottom out in the threaded block holes when tightened, so are not holding the pump against the block securely thus causing an air leak. Conversely, DO NOT over-tighten them either. This can easily split the gasket - air leak again. They only need to be done up to 6 - 8 lb ft (BL workshop manual figures), that’s a REAL gentle ‘nip’. And get the gasket on the right way round - large round hole goes on the INTAKE side. That’s the right hand port when looking at the pump drive and semi circular cut out in pump circumference at the bottom. 
Make it the last component you fit to the engine before fitting the transfer gears and cover. Dismantle it and clean it thoroughly before doing so. Especially do not fit the pump until the cam timing has been done. Always remove it if the timing gears are to be removed. I’d like a pound for every pump I’ve seen destroyed by folk bashing the cam back through the cam gear to remove it. This punches the rotor boss back out through the pump lid. Prime it with at least 20W50 oil, better still is engine build lube (which you used when doing the build, didn’t you?). Some folk recommend Vaseline, I’m not so sure but apparently it works OK. 
To avoid a sticking bullet-type relief valve, drill three holes in the sides of it. This will allow any possible ‘jamming’ fragments to drop through out of harms way. And always remember to fit the 'O'-ring seal between gearbox and block round the pick up transfer port! Using a little grease on the seal will help hold it in place whilst during assembly. 
Useful part numbers: 
AEA526 S relief valve spring 
BLS916 S relief valve ball 
6K853 Bullet relief valve spring 
12H865 Bullet relief valve 
HPS4 Oil pressure regulator kit (adjustable relief valve cap) 
Available in chrome, black, red or blue. 
HPS1 22lb oil pressure warning light 
HPS3 15-60lb adjustable oil pressure warning light 
C-AEG410 S pin drive Sureflow oil pump 
C-AEG411 Star drive Sureflow oil pump 
C-AEG412 Slot drive large-bore Sureflow oil pump 
C-AEG413 Slot drive small-bore Sureflow oil pump 
C-AEG414 Pin drive small-bore oil pump 
GLP110MS Slot drive MSC Turbo oil pump 
CHM108 Star drive coupling