Hello all,
Anyone know what difference the cooper S fan belt (AEJ 101) has to the standard belt? - I'm sure the water pump pulley is a larger diameter, so the belt would also logically be a larger diameter as well??

The difference will be measured by length.

I think you'll find all fan belts are pretty much the same ...

There was (is?) a modified (larger diameter) pump pulley intended for cars running at constant high revs (i.e. rally/race cars) to slow the pump down.. Sorry that I don't know whether the larger pulley requires a larger belt ... or the standard one allows enough adjustment.

However most cars for normal operation will use the standard pulley.

Cheers, Ian

Is this any help.

If your running the large pulley then you need around an 850mm belt. If running the smaller generator pulley then something around 815mm is more appropriate. You can get away with +/- 5mm or 10mm each way. I usually try and replace the belt with the same size one that comes off the car. Over the years people change the pulley sizes on the generator and the fan so the original belts may not fit. I think an Alternator car takes a 825mm belt.

Unless modded for racing the S had a standard water pump pulley.
I use an 815 belt, with an alternator, because the Clubby 15ACR alternator was touching the S cooler hose end with the 825 belt.
815 belt JUST fits, if you take the 2 top bolts out first.

_________________
DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

The MK 1 S generator/dynamo pulley has a larger diameter pulley to slow the RPM down but the water pump pulley is the same size as other Minis.
It has a ball bearing at the pulley end but only a bush at the commutator end. It isn't designed to run continuously at high speed.
Fan belt needs to be longer because of the generator pulley, not the water pump pulley.
The MK 2 S uses an alternator which needs to run at a higher RPM and doesn't need a larger pulley to slow the RPM down.
Modern alternators can run at 2 - 3 times crankshaft speed and need special ball bearings with non contact labyrinth type seals.

There is very little difference electrically between a dynamo and an alternator. They both produce an internal AC output which in the case of the dynamo is rectified by a commutator, which is a rotating mechanical rectifier, and in the case of the alternator the output is rectified by a solid state rectifier/diode pack. The dynamo is a single phase generator and the alternator is usually a 3 phase generator.
The alternator may have either an external or internal voltage/load regulator.

I hope this helps.

RonR

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68 build Mk1 Cooper S
72 Clubman GT project. Total restoration required

Thanks for the good advice everyone, - It seems I need a belt of 815 dia. but I'm not keen on removing the old one to check just yet!

you might be able to read size on old belt

Is the length measurement taken around the inside diameter of the belt? Run a string around the current one and adjust as appropriate to get an approximation.

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

I think it's outside measurement.
There are 2 standard fan belt widths 10 and 11 mm.
Minis use an 11A815 or 11A825.

_________________
DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.