R75/5 1973 Final Drive Fluid Inspection conundrum

[schwarze]

I bought this LWB Toaster tank 900 miles ago. It had just under 15,000 miles on the odometer which did not appear unusual based on the condition of the bike. My first task was to check all of the fluids then drain and replace. At that time I noted that when I loosened the inspection port on the final drive unit, the gear oil literally gushed out. What was not lost in the gushing was measured at 400cc. The unit holds 250cc which I replaced. At that time, I also noted that I could not get a reading on the shaft drive fluid level. I drained that fluid and replaced with the measured amount of fluid as indicated in my Clymer's guide for these models.

900 miles later I endeavored to check the fluids once again. Tranny oil is fine but, as before, when the final drive unit's inspection/fill port was opened, a gush of gear oil ensued that forced me to reinsert the cover. Also, as was my prior experience I could not get a reading on gear oil in the shaft drive. As before I put on my thinking cap.

What is different now than then is that I have joined the BMWOA and have access to the collective brains/experience of its members. One working theory was that gear oil was migrating from the shaft drive into the final drive unit but I could not a seal in the parts diagrams that would be the migration path. Another theory was that the bike was sitting too far rearward on the centerstand thereby allowing the spill. This theory is pretty lame even in my mind.

Does anyone have any thoughts on the subject or approach to diagnosis.

Thanks, Schwarze

 

[GTRider]

Section 3 on this page references oil migrating from the driveshaft housing to the final drive: https://bmwmotorcycletech.info/rearwhlsplines.htm
A friend bought an early 80s R100S with 12k on the clock, and that pinion nut needed to be tightened, so it can happen at low mileage.

Best,
DeVern

 

[kinterridge]

Looks as though the seal on the final drive input has failed. Draining the shaft into the final drive. New seal number 33 12 1 231 542 is I think your culprit.

 

[schwarze]

Thank you so much. You guys are da best!
Schwarze

 

[schwarze]

This looks to involve some special tools. Is there a jobber from whom these tools are available?

Schwarze

 

[clmedlin]

Cycle works fabricates air head tools. He is a machinist and makes quality products - he is just not a very good communicator so don’t expect any responses if you ask questions IMO

 

[schwarze]

Thanks for the reference. I will look up contact info. I have been reading my Clymers manual and watching YouTube University re the disassembly of the final drive unit to get to the seal. I would hope that I could get to the seal and replace it without removing the pinion gear and/or a total tear-down of the entire unit. That looks do-able with only the two special tools to hold the "clutch hub" in place and to remove the "threaded ring" holding the seal.

Schwarze

 

[AntonLargiader]

That pinion seal is probably rock-hard at this point. Seen it many times. If there's a BMW shop near you (an independent would probably be a better bet) it's a trivial job to replace that seal.

 

[pauls1150]

If you can tell us about where you're located, we could probably point you to a decent independent shop, or look here (old incomplete list) - https://ibmwr.org/index.php/independent-bmw-mechanics/

 

[RPGR90s]

I had this exact same scenario on a R90s I had in my shop. Less than 300 cc's in the gearbox, almost no oil in the driveshaft and oil was gushing out of the final drive when drained. I replaced all the appropriate seals in the driveline from the trans output shaft back and so far, so good.

 

[schwarze]

I want to report that last week, my brother and I spent the week tearing down my R75/5 L.W.B. so as to replace the oil seal in the threaded ring around the pinion gear. It was like we were kids again breaking things down and putting them together again

We removed the rear wheel and (with tools purchased from Boxer2Valve) removed the Final Drive Unit (FDU), the Clutch Hub (CH) and Threaded Ring (TR). The seal looked good but we noticed that the nut over the CH was barely finger tight. We also noted that there was no sealant on the threads of the TR.

We had a local machinist replace the oil seal and reinstalled the TR with permatex over the threads before torquing to spec. The nut over the CH was secured with Red Loctite before torquing to spec. A problem was noted with the drain plug for the drive shaft which required heli-coiling the threads with the aid of the said machinist. That done, the rear brakes and hub were inspected and cleaned, and the wheel reinstalled to torque.

Our attention was then directed to the front wheel and telescopic fork fluid. A problem was encountered with one Decorative Cap. It had "fused" with the fork top spring retainer. Once again, the local machinist worked a miracle to separate the two. The fluid was drained and replaced, the front brakes inspected and cleaned, and the wheel replaced to torque spec.

While apart, the chrome and alloy parts were buffed and shined with Simichrome and the bike looks great. I have now gone over the bike that was purchased in June 2024 and replaced all fluids and filters. Next step is to tune-up and time. Oooo-Yeah.

Moral of the story: These bikes were designed to survive and be worked on by their owners but...stay friends with your wrench-friendly siblings and make a good friend of a skilled machinist. Kudos to this chat room--I will return for the next project.

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