Is it worth it to rebuild a final drive?

[SweetT]

My buddy had the big ball bearing that holds the wheel in place go out on his K1200RS final drive. he bought a used one and swapped them out. I kept he old one thinking I could rebuild it and have myself a spare. Considering that a good used one costs about $400 would it be worth it to take the time to rebuild this one? Could I do it with normal hand tools?

[roy]

Depends:
Did the bearing failure cause any damage to any of the gears or housing?
Did the bearing failure cause damage to the pinion bearings?

Answer yes to either of the above and I would guess it would not be pratical.

IF all that is damaged is the Main bearing and seal they can be rebuilt. To do it properly you will need some fixturing and a accurate measuring device capable of measuring .001" or better. Preload on the above mentioned bearing is critical and you can NOT just replace the bearing. You MUST check preload and shim accordingly.

Roy

[twins4life]

I have replaced two. Didnt need to reshim the crown gear, I guess you could if you want to. Bearings are made to exacting specs and I have had <500 millionths difference between old and new bearings, so the OEM shim is just fine. Just mic your old bearing and compare the width to the new bearing width. 500millionths is nothing to worry about.

I know the engineers say to reshim each time but, given that we are constantly fixing engineering mistakes.....

I get the bearing by number from the local bearing supplier. About $40.

Take the 7mm screws out and tap out the bearing retainer. Take the new bearing and the bearing retainer off of the FD to a machine shop to have them press it apart and then press the new bearing back in.

It is so simple the local shop has pressed two apart / back together and never charged me anything. It takes 5 minutes.

(No offence to Roy intended but the shim primarily makes up for maching differences in the aluminum cases and to set pinion depth). If you take out a bearing that is 1" wide and replace it with a bearing that is 1" wide, how could the shim(s) change?

Again, no offence intended to Roy, I'm just posting another opinion and you know the old saying about opinions :whistle

Just caution the shop that there is a shim under the bearing that needs to be put back. I'd ask them to mic the width of the old bearing and then mic the new bearing. They'll probably laugh though as bearings are manufactured to precise deminsions.

Edit: I just realized I am in the wrong forum, I have two R bikes and I'm not sure how different the final drives are between the R & K bikes.

[cjack]

(No offence to Roy intended but the shim primarily makes up for maching differences in the aluminum cases and to set pinion depth). If you take out a bearing that is 1" wide and replace it with a bearing that is 1" wide, how could the shim(s) change?

Again, no offence intended to Roy, I'm just posting another opinion and you know the old saying about opinions :whistle

Just caution the shop that there is a shim under the bearing that needs to be put back. I'd ask them to mic the width of the old bearing and then mic the new bearing. They'll probably laugh though as bearings are manufactured to precise deminsions.



So you are assuming that the old bearing (the one that failed...) was shimmed properly.

[bmw_ken]

So you are assuming that the old bearing (the one that failed...) was shimmed properly.

I was thinking the same thing. When Tom Cutter rebuilt my final drive this winter, he found that it had been shimmed incorrectly from the factory. Hopefully that was the cause of my problem.

[twins4life]

So you are assuming that the old bearing (the one that failed...) was shimmed properly.

Yes. What I have seen is improperly shimmed ball bearings will damage the taper needle bearing on the other end. [based on experience with similar NON BMW assemblies]

If the taper needle bearing is ok, then I *assume* the big bearing is shimmed right. Take the O ring out and see how it seats up. If to bearing retainer "sits"high and doesnt seat, then it is shimmed too thick.

Stick two bolts in the center section. Wiggle the bolts and notice the play in the bearing before you bolt them together. Then as you seat the bearing retainer (minus the O ring) keep wiggling and checking the play in the main ball bearing. As the covers touch, the two bolts should lose all play as the taper bearing seats. If there is still play, the shims are too thin.

This is crude of course, but pressing the bearing on and off and on and off the shaft (it usually takes 1-3 cycles) to get the preload perfect has a price to be paid too.

Remember this ball bearing as specd by BMW is a .C3
C1 = tight
C2= what ever they call it
C3 = loose
For reasons known only to BMW they chose the loosest of the possible bearing tolerances.

You can take a brand new bearing and rock the inner race a bit.

I think the bearing failures are due to the fact the crown gear is spinning and approx 1/4" away from the stationary bearing retainer. Not as easy path for oil to flow back in at 70MPH.

The transmissions we work on have input bearing failures for basically the same reason. The factory fix is to drill a hole in the top of the bearing retainer and the bearing has a small hole for the oil to drain into the races. The holes must be lined up of course.

We dont see many failures from shimming errors. Almost all are lubrication (lack of) related, usually from a low oil level but sometimes, as I just mentioned, the oil cant flow back to a spinning bearing when a rotating mass is fractions of an inch away from a stationary part and the oil must flow through the gap.

On the differentials (trucks) , the factory shimming is mediocre at best and the majority of the bearing failures are left side carrier bearings and left side wheel bearings. Want to know why? Go set a golf ball on any straight road. It will roll to the right as the center (left side of the axle) is cambered for water drain. Which drains the oil away from the left side bearings nicely. Our solution? Over-fill the differentials. Guess what? Bearing failures almost went away and are now random positions.

Want to know what the engineers said to our solution of slight overfilling? You can guess I'm sure. :dance Needless to say the diffs are still overfilled and the sky didnt fall. But the bearing failures sure went down. Care to guess if I over-fill my FD's a little? I also run 75W-90 synthetic oil, the thinnest gear oil I can find, on the belief that thin oil can make it's way to the bearing better than a thicker oil. JMO.

[roy]

I have replaced two. Didnt need to reshim the crown gear, I guess you could if you want to. Bearings are made to exacting specs and I have had <500 millionths difference between old and new bearings, so the OEM shim is just fine. Just mic your old bearing and compare the width to the new bearing width. 500millionths is nothing to worry about.

I know the engineers say to reshim each time but, given that we are constantly fixing engineering mistakes.....

I get the bearing by number from the local bearing supplier. About $40.

Take the 7mm screws out and tap out the bearing retainer. Take the new bearing and the bearing retainer off of the FD to a machine shop to have them press it apart and then press the new bearing back in.

It is so simple the local shop has pressed two apart / back together and never charged me anything. It takes 5 minutes.

(No offence to Roy intended but the shim primarily makes up for maching differences in the aluminum cases and to set pinion depth). If you take out a bearing that is 1" wide and replace it with a bearing that is 1" wide, how could the shim(s) change?

Again, no offence intended to Roy, I'm just posting another opinion and you know the old saying about opinions :whistle

Just caution the shop that there is a shim under the bearing that needs to be put back. I'd ask them to mic the width of the old bearing and then mic the new bearing. They'll probably laugh though as bearings are manufactured to precise deminsions.

Edit: I just realized I am in the wrong forum, I have two R bikes and I'm not sure how different the final drives are between the R & K bikes.

Twins:
problem with bearings is that the races are the same width but the axial play can be different. If you pull the spec sheet on a C3 bearing bearing the axial play tolerance is several times greater than the preload which is .002 -.004 in the final drive. (NOTE: I do not remember if the axial play is .012 or .018 for the bearing)

When these bearings are installed the outer race is shimmed to preload the balls against the Inner race which also loads the tapered bearing. Just throwing in a new bearing may or may not have the same axial free play so it may or may not give the same preload.

Don't want to start a war.. but also don't want to intentionally mislead some one either.

Roy

[twins4life]

No war started! We're not competing, just discussing various viewpoints.

It's like "what brand of MC do you ride?" Who cares, as long as you RIDE!

:brad