I have a great new alternator from 928 Motorsports, and I'm intending to install it on my 1982 US spec.

Unfortunately, the 6-groove pulley that comes with it won't work with the 4-groove crank pulley.

Does anyone have any idea where I can get a 4-groove pulley that's a flush-fit on the alternator bolt, with a spacer kind of thing at the bottom? The alternator I took off (Paris-Rhone model) is a 4-groove pulley but has a key-fit and won't work over the new alternator bolt thing.


Pictures:

 

I went to http://store.alternatorparts.com and got this for the 1982 4-groove model I have. The outstanding concern is will the 7mm tall nut that came with the new alternator and pulley be too tall for the newest pulley?

It appears that the bolt on the alternator has about 20mm of clearance, and from the diagram the pulley will consume 14.6mm of that. The nut is 7mm, so the bolt will be about 1.5mm short of clearing the nut. I figure if it's torqued on enough, it should be fine. Hopefully.

 

Looks reasonable, but, can't 928 Motorsports swap you for what you need?

 

This might some too simple, unless there are alignment issues that can't be overcome with using shims etc. But there is no reason you can't use a 6 rib alternator pulley with a 4 rib crank pulley AND a 4-rib belt... Those 2 empty ribs on the alternator pulley shouldn't bitch too much about doing nothing:-)

 

it's worth a shot with the 6, the 4 was $25 shipped so i'm not that worried if the 6 doesn't pan out. i was worried that there might be slippage, but if it's tensioned enough it seems reasonable to assume that won't be a problem.

 

Nice solution by DR. You should be able to eyeball it to check for interference.

 

Even if for example you start with the belt in the 4 middle ribs, if it is not aligned correctly it will automatically jump to one side or the other. Beyond that it will require shimming. But all you gotta do is put a straight edge across the crank pulley in line with the alternator pulley and use a dial caliper or ruler and do the math as to which 4 ribs line up...or not. If not, you will need to shim or machine the backside of the pulley.

BTW, the tension of a 4 rib belt on a 6-rib pulley will be no different that a 4 rib belt on a 4-rib pulley. Unless you go from a system orginally designed for a 6 rib belt down to a 4-rib belt, which could be the case with this alternator on your 4-rib system.

I would check with Carl and see what specs his 6 rib alternator requires and ask for the differential compared to your original 4-rib system for proper function and belt life.

Ironically on a stock 928 the 6-rib alternator belt tension spec is much higher than the 4-rib alternator specs of the earlier version. Of course Pulley diameter and/or ratio changes (alternator or crank) comes into play in any of these situations.

Just a bit of trivia for you concerning how "prototype" our 928s really are, I have never seen a 32V alternator pulley that is perfectly "square" with the crank pulley. Not a real problem with the short distance of the stock alternator to crank pulley distance, but requires special tricks when you run that with a longer SC belt.