What size is the alternator belt supposed to be?

[Adnan]

Hi,

Doing R&R on my '91 535 and I found that BMA had sent me a 13x1055 mm belt for the alternator/water pump. The one on the car is a 12.5x1040 mm. I realize that the difference in width is not an issue.

The ETK shows both a 12.5x1055 and a 12.5x1040 but, when you pull the actual drawing, only the 1055 is listed.

Which is correct, or is there enough adjustability in the alternator that either size will work? Amazingly enough, the dealer didn't have these in stock.

Thanks,
Adnan

[George M]

12.5 x 1055. The BMA belt should work fine as it is the proper length...just a bit more stout....will ride a hair higher in the pullies and therefore require slightly longer effective length which is has.
FYI....all belt sizes are in the owner's manual.
HTH,
George

[Brian C.]

It seems that as the membership here has grown, many "oldtimers" like ourselves have been scarce of late. I hope all is well.

Brian C.

[George M]

Hi Brian,
Yeah...board is gettin' overrun with the new bloods....way it should be I guess.
Hope life is treating you kind and you had a happy Thanksgiving. No complains on
my end...everything is going fine...just been busy doing stuff unrelated to BMW's.
Still love to roll the Bimmer out now and then for a run.
Weather is really turning now though...and not for the better :-)
Hope you're catching some of the skins game...gonna be a big payday tommorow for somebody.
Best Regards,
George

[Adnan]

Hi George,

Long time no talk. Hope you've been well.

I did some more digging and it appears that at some point in the M30's history, the belt size dropped from 1055 to 1040. In fact, I tried exactly what you're thinking but when I checked the alternator adjuster, it was almost to the full extension with the 1040. The 1055 won't fit, for certain.

And oddly enough, the A/C belt was an absolute bear to take off. I hope the new one will go on without too much grief. I've done it once before so I know it's possible to install.

Thanks,
Adnan

[George M]

Hi Adnan,
Hmmm...interesting that the alt belt was re-spec'ed...hadn't heard that. Believe I am running the 1055 length belt on my M-30 and am in about the middle of travel on the alt adjustment rack. Keep in mind that a 15mm longer belt only accounts for about 7.5 mm more slide travel or just over a quarter of an inch...may still work with the slightly thicker belt which will use more effective length due to riding higher on the pulley's...not ideal though as you say. The AC belt is infamous for being a bear to install. Most that run the stock AC belt length pull the pinion bolt to get max. pivot on the slide...push the compressor right up against the engine to get the belt on. Believe you know not to use a screwdriver and pop the starter to install the belt :-) If need be...remove the compressor from its pivot hinge/carriage bolt...shouldn't have to though. The enlightened few purchase a slightly longer AC belt to make the whole job a lot easier. BMW missed the mark on the spec for the AC belt.
Cheers,
George