jetmech3421
01-13-2006, 01:46 PM
I'd been living with a clunking noise out of my M3's rear end that would occur when I would back out of my parking space here at work, or on rare occation, when I would start out in first gear. I looked over the subframe area, subframe mounts for cracks, upper shock mounts, and RTABs myself and also by a professional BMW mechanic. There were no obvious sources for the sound.
I read a few threads in here regarding the forward mounting bolt for the differential cracking so I also checked that. With the car on jack stands, I put a wrench on the bolt and loostened it while looking at the bolt shank through the hole in the diffo that it goes into (you can see if from behind the diffo). It rotated along with the bolt head so I figured it was all good.....WRONG!
A couple of days later, I heard a lound bang on acceleration in first gear. After that, I could feel something in the rear end banging around on every shift. If I down shifted agressively, I could hear the drive shaft rubbing on the surrounding heat shield. The bolt I though was OK, sheared completely in half leaving the threaded end in the diffo. On removal, I could see that the bolt had been partially cracked for some time before it let loose completely.
After reading about this failure in several threads in here, I though I was looking at having to drop the diffo. Well, I was able to actually, remove the two rear diffo mounting bolts, the driveshaft to diffo nuts, the rear sway bar center mounts, and drop the exhaust off the rear and center mounts (without disconnecting it from the entire exhaust system), while leaving the half shafts still connected to the diffo. This allowed me to pull the diffo back far enough to have the nose of it drop down below the subframe so I could drill out the busted bolt and install a new one.
Bottom line:
1. If you're experiencing a clunking noise from your rear end, do yourself a favour and take a look at the forward diffo mounting bolt. Hell, go ahead and replace it, they're only $1.50. Just make sure you remove it and inspect it, don't just tighten it up and think you're OK.
2. If the bolt does break, you can drop the diffo out far enough to drill out the busted bolt without completely removing it from the car.
Now that I have a solid bolt in place, I no longer feel drive train shock that I had previously thought was normal. Visually inspect that bolt, it's quick and easy to do and could save you from a big problem later!
I read a few threads in here regarding the forward mounting bolt for the differential cracking so I also checked that. With the car on jack stands, I put a wrench on the bolt and loostened it while looking at the bolt shank through the hole in the diffo that it goes into (you can see if from behind the diffo). It rotated along with the bolt head so I figured it was all good.....WRONG!
A couple of days later, I heard a lound bang on acceleration in first gear. After that, I could feel something in the rear end banging around on every shift. If I down shifted agressively, I could hear the drive shaft rubbing on the surrounding heat shield. The bolt I though was OK, sheared completely in half leaving the threaded end in the diffo. On removal, I could see that the bolt had been partially cracked for some time before it let loose completely.
After reading about this failure in several threads in here, I though I was looking at having to drop the diffo. Well, I was able to actually, remove the two rear diffo mounting bolts, the driveshaft to diffo nuts, the rear sway bar center mounts, and drop the exhaust off the rear and center mounts (without disconnecting it from the entire exhaust system), while leaving the half shafts still connected to the diffo. This allowed me to pull the diffo back far enough to have the nose of it drop down below the subframe so I could drill out the busted bolt and install a new one.
Bottom line:
1. If you're experiencing a clunking noise from your rear end, do yourself a favour and take a look at the forward diffo mounting bolt. Hell, go ahead and replace it, they're only $1.50. Just make sure you remove it and inspect it, don't just tighten it up and think you're OK.
2. If the bolt does break, you can drop the diffo out far enough to drill out the busted bolt without completely removing it from the car.
Now that I have a solid bolt in place, I no longer feel drive train shock that I had previously thought was normal. Visually inspect that bolt, it's quick and easy to do and could save you from a big problem later!