Crownwheel roller bearing dead

[arthurrr]

Hi motobrickers!

Last weekend i was riding my '93 k75s and i noticed a repetitive 'grinding' noise coming out of the rear wheel. I visited a local motorcycle mechanic (i was about 600 miles from home), he suspected the bearings in the final drive (crownwheel or pinion) to be worn but riding the 600 miles home wasn't a problem according to him (very little play on the rear wheel). The 600 miles home were horrible. The noise became progressively worse, i could sometimes feel the rear wheel 'vibrating' as if i had a flat tire. I also noticed FD oil leaking from behind the disc when the bike was on the side stand.

Anyway, i made it home, took out the final drive and wasn't surprised to find the crownwheel bearing completely dead. The sealing was damaged (hence the leaking) and the housing was full of metal chunks.

The tapered roller bearing and pinion/crownwheel teeth look okay. What worries me a bit is the position of its race in the FD housing itself: it looks "off center" with some grinding marks at the level of the speed sensor and below the tapered bearing. It looks like there is a hole drilled in the housing at the bottom. Quite some metal is grinded of the inner side of the FD housing at the level of the crownwheel, mainly at one side (compatible with the direction in which the tapered bearing race is off center). I've never seen a FD housing of a k75 and can't find clear pictures on the internet, hence my question: is this okay? The housing certainly suffered A LOT during those 600 miles, should i worry about all the metal scraped off? Difficult to photograph (and explain!) but let's hope i made myself more or less clear..

Second question: i took off the brake disc without remembering its position. In one of the youtube tutorials on final drive rebuilds the guy marked one of bolts. Is this important, what would be the consequence of installing the disc 180 degrees rotated?

Thanks!! :2thumbup:

[Scott_]

I wouldn't worry about the brake disk rotational alignment.
Personally I'd consider looking for a complete replacement final drive at this point. Even if you cant see it with your eyes, there has to be some damage to the ring and pinion gear faces as well as the input pinion bearing.

[Laitch]

I've never seen a FD housing of a k75 and can't find clear pictures on the internet, hence my question: is this okay? 

I don't think it's ok. Compare it with the excellent pictures in this thread.
http://www.motobrick.com/index.php/topic,10249.msg87360.html#msg87360

There's not a lot of difference between the K100 and K75 2V assemblies. I agree with Scott_ that you should start hunting for a replacement. When grinding of an assembly starts, use of that assembly should immediately stop so there's hope of salvaging it.

[Martin]

+1 Any noises like that should be investigated thoroughly before riding. Any chance of rebuilding is pretty well gone south. You are now better off chasing a second hand unit. There are BMW specific wreckers Beemer Boneyard, is just one of many.

Regards Martin.

[arthurrr]

Thanks gentlemen for the advise. I agree a final drive replacement is probably the best option. As in my region second hand parts are difficult to find, a good donor bike is probably the only option.
I don't have the funds currently to purchase a donor bike so i'll replace the bearing and inspect the magnetic plug regularly. While i agree that this is a less favorable option, other than premature wear of the bearings in the final drive unit i guess the risks are relatively low?

[kris]

The inside of my FD is all silver in colour....not sure what's up with the black. I would disassemble the unit completely and take it to a machinist/ mechanic for an opinion. He/she may go over the unit with a micrometer to discover whether damage has been done. It is clear that (superficial) damage has been done, given all the crap that was flying around in there. Get the opinion first before spending money on the big bearing (not sure if it's expensive in Europe but in Canada it's $180 plus tax). The tapered bearing is considerably less expensive. I picked up an NTN bearing 4T - 30205 (check that number) for $27 plus tax. Both bearings are easy to install using heat and cold (freezing). You should be able to extract the tapered bearing race by heating the seat of the bearing on the exterior with MAP gas and then using a pick to pull it out. Worked for me. Otherwise, you'll need to track down the appropriate extractor - may be able to rent or borrow the tool. Check around.


You may have gotten lucky with this and not done damage.


Sorry for your troubles! And good luck.

[arthurrr]

... in Canada it's $180 plus tax...

Wow that's a lot of money! I bought mine here in Europe for less than 90 usd..
Quick update for those who are interested:
I've replaced the big bearing. Taking it off was a bit of a hassle (chisel & hammer work), installing it was easy, indeed. I heated the bearing to 85°C (185°F), cooled the crown wheel shaft using liquid nitrogen and the bearing easily dropped on with that lovely 'clingg' sound.
I didn't replace the tapered bearing, i was afraid not to be able to extract and reinstall it properly. I might replace it next time i'm in there using kris' method, good idea, didn't think about that actually!


I thoroughly cleaned the housing, reassembled everything and did a short ride and was surprised..sooo smooth! I was probably riding with a worn bearing for a long time without knowing it - difficult to recognize the sound if you don't know it. Anyway, there is no play whatsoever in the rear wheel, i'll regularly check the magnetic plug and let's hope i'm good for some time now..

[Laitch]

 :clap: