Contitech 4PK592 alternator belt won't mount up onto pulleys

[duegi]

Folks, need input please.

I've got a 2005 R1100S BCR with 20K miles. I'm doing a Winter tune up including replacing the alt belt which has some fraying on the belt edge. This BCR has the Denso 40 amp alternator.

I took off the Contitech belt 4PK592 (bmw part # 12 31 7 681 841). I bought the same belt (4PK592) and it is 2cm shorter (outside diameter measurement) than the original and will not mount even with the alternator bolts loosened and the alternator is in it's lowest position. I've also heated up the belt in hot water and it still won't stretch mount. I believe the old Contitech 4PK592 is an elastic belt but stretching an additional 2cm over it's life seems excessive. I've also tried turning over the motor with a wrench to thread the belt on the pulleys but just not enough new belt length to do so.

I'm worried about forcing this belt onto the pulleys and tearing up the alternator bearings after. For reference I've been riding BMWs since the early 80's and know my way around the motos with a wrench. I've never had this issue with any other R1100, R1150, or R1200s I've ever owned. Chatted with a long time BMW tech who echo's my thoughts on why the original belt would stretch 2cm.

[Swamper]

I've had the same problem with my r1150r 04. Although I had bought the same number belt replacement it was to small. Found out they had two belt sizes for the r1150r so I bought the other one which was just a tad bigger for the earlier models and it worked perfect. I think the 1100 is the same as the 1150's so you might try to get the bigger belt, it's listed for the earlier 1150 models. You probably have belt 4pk592, the longer one is 4pk611. Not sure but those last three numbers might be the length.

[JimMoore]

Bmw switched to a smaller pulley and shorter belt at one point. Then they ran out of belts, or pulleys, or something and switched back. Then the dealer was supposed to re-switch the belt and pulley during maintenance. You have the old pulley, so the new belt won't fit. You want the one that ends in 779 (BMW) or the 611 (61.1 cm) from Contitech.

[duegi]

Thank you gentlemen, I’ve got the longer Dayco belt on the way. Chatted with another long time BMW tech who said the same about mixing pulleys and belts on the R1100S. He said get the longer belt and it should be fine.

[duegi]

Mounted the Dayco 5040240 (4PK610) alternator belt, it seems to be a touch long. I only have 1-2mm of adjustment to deal with any belt stretch that may occur. I've now ordered the in between belt, Dayco 5040238 (4PK605) so see how it will fit.

I remember having an issue with a R1150RT I had a long time ago and trying to get the correct belt. Seems that the problematic period was the transition from the 1100 to 1150 series. Per Jim Moore's post above it seems the production line would run out of one pulley and put on the other. A tech friend thought that the BMW production line may have had motors with alternator pulleys already attached and possibly not changed out to match the alternator that was called for on model. Another old time tech mentioned the same and that during the build the folks would put on whatever belt would fit and call it a day.

[duegi]

Mounted Dayco 5040238 (4PK605) today, snug fit onto the pulleys rotating the engine pulley by hand (spark plugs out). Tightened up fine and some room for adjustment if any stretch although I don't think the Dayco belts stretch. Fired up the engine and all good, no squeal or unusual noises.

This gets me to wondering if folks that had alternator bearing failures might not have had this outcome if the proper belt was used initially.

[sgoodwin]

What people would that be? There's never been a dearth of bad alternators in any threads I remember. Just problems like yours having trouble finding the right belt because BMW decided to use different pulleys & belts.

[Chris Canning]

It’s classic BMW, I’ve currently got 3 and they just change parts/spec when they feel like it finding out is the trick, i’ve Had an X/R from new in 16 see the official BM front fork oil(Non ESA)which was 7.5wt has now gone to 11.5wt....