Can anyone explain why the bolts that hold in the RSFBs are smaller than the holes?

[feeshta]

Hello all. So yesterday I did the M3 rear subframe bushing swap. I rented the tool, and it went relatively smoothly with the exception of the bolts for the rear exhaust connection stripping out. No matter what I tried, I could not get those bolts to thread back into the captive nut cleanly, and one of them ended up jamming up 3/4 of the way through and snapping off rather than coming back out. I've got to try to get the captive nuts out today.

My question is actually about something else though. During this job, I noticed something rather strange. The bolts that pass through the center of the rear subframe bushings and hold them to the vehicle are much smaller than the actual holes. This is true in both cases, but the M3 rear bushings actually have an even larger hole than the original 135i parts. I don't understand why this would be. Wouldn't that allow possible slop in the entire rear end of the car? I'll post some pictures below to illustrate what I mean.

Here is the original bushing with the bolt sitting in it to illustrate the size.

And the M3 Bushings for comparison



and finally, how the bolts from the exhaust ended up looking. I would swear thes things had damaged threads or something, they came out hard, and absoltely refused to go back in cleanly even after cleaning up both sets of threads with a tap and die set after the initial failure.

[Solo_M_Tech]

Drill it out and use a regular bolt and nut. Best way. I had to do it.

[E82MSport]

The holes in the bushings, as far as I can estimate, are larger for ease of assembly at the factory. The entire rear suspension is loaded as a module into the rear of the vehicle.

If your concerns are alignment related I would not worry, as the forward bushings are located by alignment dowels as shown below. These two dowels squarely locate the subframe to the chassis.

Photo borrowed from forum member Freon

[feeshta]

Originally Posted by ApexResistance

Drill it out and use a regular bolt and nut. Best way. I had to do it.

Yeah, that is the plan. I picked up some hardware over my lunch break today, and will give it a try. Right now the exhaust is only secured on one side, and it alternates between normal operation and full on Harley Davidson mode. RealOEM says those bolts are M10, so that is what I picked up, but I must say these appear larger than the factory one from a cursory glance. Hopefully they will work.

Originally Posted by E82MSport

The holes in the bushings, as far as I can estimate, are larger for ease of assembly at the factory. The entire rear suspension is loaded as a module into the rear of the vehicle.

If your concerns are alignment related I would not worry, as the forward bushings are located by alignment dowels as shown below. These two dowels squarely locate the subframe to the chassis.

Photo borrowed from forum member Freon

Yeah, I saw those, but they also concerned me a little bit as the shape of the socket in the M3 bushings are different than the standard bushings. They are square, where the factory ones are oval.

In any case, I am noticing a marked improvement in the car, and all I have been able to do is drive it to the store and back. The wandering feeling is completely gone as far as I can tell, and it takes bumps better. A large portion or the weird secondary vibrations the car had when you hit something like a bridge seam are now gone, and it's now much more of a "one bump" type of situation, where before there was an odd reverberating feeling that was uncomfortable and annoying.

[MightyMouseTech]

The M3 bushings are different because the front ones are pressed in from the top, while the stock bushings were pressed in from underneath.

[andrey_gta]

I wonder if aftermarket Poly or Derlin bushes reduce the hole size around the bolt

[feeshta]

Originally Posted by MightyMouseTech

The M3 bushings are different because the front ones are pressed in from the top, while the stock bushings were pressed in from underneath.

Yeah, that was a bit of a pain in the rear. It's easy to do everything else in that job, but it was a bit difficult to get those front bushings aligned properly. They always seem to want to pull unevenly.

Even still, the job of getting them in only took about 3 hours, including time spent on the phone verifying that I did indeed have the correct parts, etc. I spent nearly as much time wrestling with those damned bolts on the exhaust, and had time left over to replace the transmission mounts and install a modified CDV.

[E82MSport]

Originally Posted by feeshta

Yeah, that was a bit of a pain in the rear. It's easy to do everything else in that job, but it was a bit difficult to get those front bushings aligned properly. They always seem to want to pull unevenly.

Imagine if the hole tolerance was even smaller, it would make it even harder to install.

I applaud you on how quick you got them installed.

[MightyMouseTech]

Funny. I had the exact same issue with one of my exhaust bolts. One of them was overtightened from the factory and the threads were all pulled when I removed it. Had to repair the threads on the center pipe and got a new exhaust bolt.

[feeshta]

Originally Posted by E82MSport

Imagine if the hole tolerance was even smaller, it would make it even harder to install.

I applaud you on how quick you got them installed.

I have the advantage of access to a full lift, and I've done the job before on my 540i so I knew basically what to expect. the only real difference was the tool and the fact that the new ones go in from the top in front. The holes in the bushings on my 540i were exactly the same as the size of the bolts though, which is why I was a little confused.