Drive Shaft / final Drive

[swbell]

So my splines have wear on them, not totally shot, but would I trust them for a 2k mile trip? NO.

Found a drive shaft / final drive but cannot figure out for sure if it will fit my '91K100LT. the drive shaft / final drive is off an july '86 K100. I don't know if it is an RS or RT, does that matter? Is chanceing it worth laying out the cash? worst case, I would have to resell them if it doesn't work, right?

If I replace both drive shaft/ final drive will the front of the drive shaft go into place with out a problem?

suggestions/considerations would be greatful.

thanks,
-steve

 

[98lee]

BMW changed the final drive end of the drive shaft and the input shaft of the final drive from 16 splines to 20 splines in Feb. of 1986. The transmission end remained the same before and after. The parts should interchange without any problems.

The K100 used three different final drive ratios. They are stamped on the top of the final drive. You probably have an 11/32 (# of pinion gear teeth/ # of ring gear teeth) which gives you a rearend ratio of 2.91:1.

The other two ratios that they used were 11/31 (2.81:1) and 11/33 (3.00:1).
All these ratios are very close to each other.

The 3.00 would yield slightly better acceleration at the cost of slightly worse gas milage and top speed, while the 2.81 would give slightly better gas milage and top speed at the expense of acceleration and throttle response.

With the torque of the K100 motor, I don't know how noticeable it would actually be. But I would at least check your ratio vs the new one before deciding.

 

[swbell]

Is there a way to check the ratio without taking it apart?

 

[rocketman]

See the post above yours for the three possible sets of numbers, it is stamped on the outside top of the drive where the drive shaft connects.

RM

 

[swbell]

Thanks for the help on finding the # stamp, didn't know that was there.

I Looked at my final drive this morning and the # stamped on the outside is 31/11. It makes me wonder if the one I'm looking at will work, it is a 32/11. Would it work if I am replaceing both the final drive & the drive shaft?

 

[98lee]

The drive shafts are virtually identical (other than the universal diameter is probably a few thousandths larger and the driveshaft tube is probably rolled at the end around the rubber on yours). They are absolutely interchangeable.

They final drives are also totally interchangeable (other than the possibility of housing color). The one from '86 will be silver. I don't know if yours is silver or black.

The functional difference is the internal gear ratio of the final drive. Yours is 2.81:1 vs the new one is 2.91:1. The new ratio is approx. 3.5% lower.

This MIGHT lower your gas milage by 1.5 mpg (it might not lower it that much except at steady cruise on the slab).

It WILL lower your top speed by 4.7 mph from 130+mph (who cares?).

It WILL also slightly improve your acceleration at full throttle (helps with passing cars on two lane roads.)

 

[swbell]

Lee, thanks for your insight. I did make the purchase tonight and expect the parts to show up next week when i'm out-of-town. I'll keep you posted on how it goes.