Unfortunately, in my opinion as an experienced owner, and fairly competent amateur mechanic, changing the belt is kinda difficult without an assistant. And may damage the belt during mounting. I say this because BMW sells a special $120 tool for putting the new belt on. Removing the old one is fairly easy. More on this later.
For tools and materials, you'll need:
- Socket wrench or socket handle
- T-20 & T-25 sockets
- BMW alternator belt, part no. 12317681841 (labelled "4PK592")
My bike has Touratech crashbars, which I had to remove before attempting this job.
The first step is to remove the (3) lower T-25 bolts and the (2) upper T-20 screws from the alternator cover. I couldn't get my cover off until I pulled the foam insert out (shown by the red arrow above). I wasn't careful and patient enough in loosening the insert before pulling it out and I ripped off one of its "ears". It was glued in place, which I wasn't expecting.
I believe the purpose of the foam insert is to keep dirt and water out, or perhaps to quiet the engine a bit. I know that some remove it and discard it. I cleaned mine and reinstalled it after changing the belt.
Here you can see the foam "ear" that I tore off (shown by red arrow). Fearing that it wouldn't stay in place anymore, I removed it and discarded that small piece. I cleaned all the glue off the case, and wiped the dirt from all the nooks and crannies. You can even see a small rock in the lower left depression below the lower belt pully. If a rock like that got under the belt as it went over the pully, I would think it could cause the belt to fail prematurely.
The BMW shop manual says to just pry the old belt off, but I used a better technique suggested to me by Jim Bade in his excellent DVD maintenace video. I cut an 8x1" piece of plastic out of one of my wife's kitchen chopping pads (red arrow above). I rolled the plastic under the belt by rotating the rear wheel in 6th gear. I removed a sparkplug on each side to make it easier to turn the engine.
Then I put a pair of vice-grips on a screwdriver with an angled tip (a short 90 bend) and used it to pull the belt across the yellow plastic and off the upper pully. It came right off!
Now the hard part — getting the new belt on! I looped the new belt over the top pully, and got it started on the bottom pully. Then from the left side of the bike, I tried to rotate the rear wheel in the normal direction of travel, which results in the pully turning clockwise. But the new belt was so tight I couldn't budge it.
So I got my wife to put on a pair of my gloves, kneel behind the bike, and turn the rear wheel for me. Then the belt started slipping, so I got a hammer and used the handle backwards to push the belt against the pully while my wife rotated the wheel. We couldn't do it, it was too tight.
My wife then suggested putting the new belt in a bowl of hot water for awhile to make it more flexible. We tried that and it did help. With MUCH grunting and pushing we got the new belt on.
The edge of the lower pully is fairly sharp, and the belt gets stretched against it pretty hard. The BMW tool keeps the belt from touching the edge of the pully. I'm worried that I might have damaged one or more of the internal cords in the belt given the way I mounted it.
I put the old belt (which looked fine) and the bit of yellow plastic into my saddlebag and will carry it with me in case this new belt fails prematurely. As a result of my experience, my advice to others would be to pay their BMW shop to do the job, or buy the special BMW tool.
Copyright © 2007, by H. Marc Lewis. All rights reserved.