M42 Belt tensioner, lever arm and pulley failure and replacement

[m60power]

The belt tensioner for the main accessory belt failed and was rattling badly on my 1995 318i. I pulled the old tensioner off and it was destroyed - both halves of the body had come apart and the lower bushing had ovaled out so much that the bolt and washer that attaches it to the pulley deflection lever arm had cut a nice deep groove into the arm. Anyway I ordered a new tensioner, lever arm, and associated bolts/washers.

I went to install it today and ran into a big problem. The pulley that is mounted to the deflection arm is what tensions the belt via the force acting on it from the tensioner. To loosen the belt (per the bentley manual) you use a socket on the bolt that attaches the pulley to the arm and rotate it counter clockwise. I was worried this would just loosen the bolt, but the bolt was tight enough that it worked perfectly to move the tensioner and arm and thus relieve the tension on the belt.

I put all the new components on, transferred the pulley to the new lever arm but when I went to rotate the tensioner to install the new belt it just loosened the pulley bolt (since you have to turn it counter clockwise). So I torqued the pulley bolt more and tried again. The torque to overcome the tensioner spring was still more than the bolt was torqued and it broke the bolt loose again. So I torqued it more, same damn thing. Then torqued it more. Well eventually I sheared the pulley bolt off trying to tighten it enough to so that it wouldn't loosen when installing the belt.

Since the old bolt was rusted the torque required to loosen the tensioner wasn't enough torque to actually loosen the bolt (I had to really crank on it to get that pulley bolt loose), but with a new bolt and clean threads there wasn't enough friction to overcome the torque required to move the tensioner

So now I have to order a new lever and hardware and try again How the hell do I keep the bolt from loosening while moving the tensioner? Is there something I'm missing here? On my m60 (540, 740) the belt tension is relieved by turning clockwise, so the bolt on the pulley won't loosen when you pull the belt off. The tensioner design on the m42 seems like a terrible idea lol.

[m60power]

So I'm an idiot. I should have checked another source besides the Bentley manual before resorting to excessive force. I checked the BMW TIS and sure enough it shows a different (correct) method to loosen the m42 belt tensioner. There is a rectangular "horn" on the deflection arm that you put an open-ended wrench over to rotate the tensioner/pulley assembly counter clockwise. Can't believe I didn't realize this. In the meantime I was browsing forums and whatnot trying to find more info on the tensioner replacement procedure and I didn't realize there were so many different belt and pulley configurations that came on the m42 over its production run. It also seems like there are no less than 3 totally different tensioner designs depending on production date and configuration. I also found some threads that claim the bolt that mounts the pulley to the tensioner is reverse threaded, which would have been great if that was the case for my car...

[Eric93se]

Yeah, I was thinking that you must be doing something wrong. I did some looking but just came across a site with some basic info on how to apply tension before locking it down. Glad you figured it out!

http://zoso.no-ip.org/318i-belts.html

[m60power]

Yeah, I was thinking that you must be doing something wrong. I did some looking but just came across a site with some basic info on how to apply tension before locking it down. Glad you figured it out!

http://zoso.no-ip.org/318i-belts.html

That webpage has good info, although it doesn't apply to my m42. That's a totally different belt tensioner design.