Final Drive Spline Repair

What the heck we talkin' about here?

One part of BMW /5 motorcycles (and other models, too) which wears out after a mere 100k miles or so is the final drive splines. The final drive is the gearbox at the end of drive shaft which converts the longitudinal rotation of the engine, transmission and driveshaft into the lateral rotation that is required by the rear wheel. It also performs a gear reduction duty, so that the engine can run at its optimum number of revolutions per minute, the transmission doesn't have to do as much speed reduction, and the bike can be accelerated and driven at humane speeds. In the final drive unit, the engine, through the transmission and driveshaft, drives a small pinion gear, which drives the larger ring gear, which then drives the rear wheel. The final drive ratio is the ratio of the number of teeth on the ring gear to the number of teeth on the pinion gear, and it's stamped on the final drive unit near the driveshaft oil filler hole. Several different ratios are available (mine is 32/10). The ratio determines the speed reduction in the final drive unit. High ratios provide good acceleration but a lower top speed, while low ratios provide a higher top speed with reduced acceleration.

A spline is a gear-like mechanical construction which allows one shaft to transmit torque to another shaft which is concentric to the driving shaft, much like gears allow one shaft to transmit torque to a parallel shaft (gears in the /5 transmission are like this) or to a shaft at another angle (the gears inside the final drive unit are like this). The spline is composed of interlocking lands on the two shafts which are cut parallel to the shaft. Lands are to splines what teeth are to gears. The way the lands are cut, the two shafts can slide freely on each other longitudinally along their shafts, but rotationally they must rotate together. Radially, the two shafts will also be kept concentric by the spline.

The spline on the final drive is the piece of the ring gear which engages the rear wheel, and a corresponding coupling nut which is riveted to the rear wheel hub. This construction allows the rear wheel to be removed from the final drive by simply removing the rear axle, which is important to the suspension of the motorcycle but is not involved in the transfer of power from the engine to the rear wheel.

For some reason, the lands on the spline which is part of the ring gear is usually the first part to wear down in the driveshaft/final drive/rear wheel complex of power transmission parts at the rear of the motorcycle. Once the wear on the lands begins, I imagine that the rate of wear accelerates. The lands get thinner, until eventually they will get completely torn away as the hapless motorcyclist wishes to accelerate away. At this point, unfortunately, the engine will no longer be connected to the rear wheel at all, so it is rather important to address the final drive splines before they are too worn. But even before the splines are in danger of being stripped, the backlash (the amount one shaft can rotate without rotating the other shaft) will increase, rendering the bike somewhat uncomfortable to ride, especially when starting from a stop. A quick release of the clutch allows the engine to first take up the backlash before suddenly lurching the bike into motion; a slow release of the clutch allows the backlash to be taken up slowly before more smoothly engaging the rear wheel, but it may be too slowly if your aim is to get out of there NOW.

How can I tell if I'm in trouble?

Here I'm going to quote an email sent to me by Kim Dromlewicz:

Date: Fri, 8 Sep 95 15:33:55 PDT
From: kimd@nit.AirTouch.COM (Kim Dromlewicz)
Subject: Re: final drive blues

Roger,  Awhile back I posted (to the BMW list) the decription of my problems
with getting the rear splines reapired through a dealer other than Cal BMW. 
I won't go into the whole story but basically:
1. It is quite common for the rear wheel spline to show less wear than the
   drive spline.  The drive spline can be fairly worn before you need worry.
   If the spline "lands" look as below then:

		      /  \       <----- tooth or "land":  good
		     /    \
		____/      \___

		      |  \       <----- tooth or "land":  viable
		    _/    \
		___/       \___

		         ^      <----- tooth or "land":  bad
		      __/ \
		_____/     \___

2. I will cost you about $250 for a good used rear.  Problem with this is 
   that the used rear will have some wear as does your wheel spline (if you
   look really carefully you will see the wear on the wheel spline).  Mating
   the two non-matching wear patterns will cause very fast wear from that
   point on and you will be back at square one sooner rather than later.

3. To respline the rear drive, assuming you remove the drive gear from the
   rear housing yourself (not too difficult a job) and take that and the
   wheel in to have Cal BMW do it should cost in the range of $350.  This
   way you have a unit as good as new that will last a long time.

 I did this just about three or four months ago and was impressed with the
 less than two week turnaround time from Cal BMW.
The lands on my splines were a good 2/3 gone on my /5 when I got to looking at them (I purchased the bike at about 105k miles), but the real motivation to get inside my final drive unit was to replace the seals within it so that my brakes would stop being goo'd up with oil. Repairing the splines was a classic case of the shade-tree mechanic muttering "well, since I'm in there anyway..."

What can I do to fix them splines?

There are actually two splines to consider getting fixed: the one on the final drive unit ring gear, and the one on the rear wheel coupling. Typically, the ring gear is the most worn; I've been told by fellow riders who are on the 'net that if the ring gear splines are worn the coupling splines will be too, but to a lesser degree, and therefore if the ring gear splines are repaired/replaced with new, the coupling should also be replaced (they aren't repaired), or else the slight wear that already exists will accelerate new wear. The Capitol Cycle '95 catalog states "Drive Spline Repair ... Drive coupling should be replaced with this procedure also." Clymer repair manual rather obtrously states "Inspect the splines of the coupling hub (reference to picture of final drive splines). If the splines are worn or damaged, the coupling hub must be replaced as described under Final Drive Overhaul in this chapter. NOTE: If the splines are worn or damaged, also inspect the splines on the end of the drive shaft for damage as described in this chapter; it may also need to be replaced." I can't seem to find any other reference to the splines on the rear wheel coupling.

I took my ring gear and my entire rear wheel to a dealer (Andy Pelc) with the intention of having him replace the coupling. He inspected my rear wheel coupling, using a sharp probe to feel for wear, as well as checking the backlash with a new ring gear he happened to have. He said my coupling was fine, and therefore I've opted to not to get it replaced. I figure this is relatively authoritative, as he has a stake in selling me parts and labor to fix the coupling. I also cannot feel any wear on the coupling, and now that my ring gear has returned, I cannot feel any backlash either.

As for the ring gear, they aren't sold separately, but in pairs with pinion gears. They cost $675.95 a pair in Capitol Cycle's '95 catalog, but that doesn't include the cost of buying all the new shims which would be needed if the gears were replaced, nor the headache involved in shiming the new gears into the old housing.

As Kim sez: you've got two options: replace the entire final drive unit or fix the splines. I don't know if final drive units are still available OEM from BMW through its dealerships or the usual mail order places, but used ones are available from various motorcycle parts recyclers, such as Eurotech Motor Sports or Re-Psycle BMW Parts. I've got a phone list of these places. Of course, used ones will have some, perhaps significant, wear on them already.

The other option is to get the splines repaired. I went this route. Rumor has it that this can be done in two possible ways: new material is welded onto the old ring gear splines and machined down to form new lands, but Jim Stein says that he got his repaired at Freeman Cycles (508 922 6668) for $250 in which the old splines were cut off and a new spline unit is welded on to what's left of the ring gear. My ring gear was repaired via the first technique.

A purported advantage of repair rather than replace is that the new repaired lands are made from a more durable steel alloy than the original. How true this is, I don't know. I've only heard it on the 'net, and I'm not a metalurgical engineer.

Where can I get the ring gear splines repaired?

My impression is that each and every dealer knows where to send it to get repaired, but mine, at least, isn't telling. Capitol Cycle catalog, in the section "Rear Drives", sez "Drive Spline Repair: Ring gear out of housing $175.00, in housing, $200.00 plus parts..." and in the section "Mail Order Repair Service", sez "Spline Repair: $175.00 for machining. We will remove, machine, and reinstall a ring gear for $200.00 (Parts extra)" implying that they have a machine shop and do it in house. A local BMW riding MSF master instructor told me that Judson Cycles in Lake Crystal MN did good work; I didn't follow thru on that advice to find out if this BMW dealer did the machining or simply sent it out, too. Go ahead and call Judson and let me know what they say: (507) 947-3852. He also mentioned Blue Moon and Bob's, which are big BMW mail order places. Kim Dromlewicz told me that he was having problems with some dealers until he took it to Cal BMW.

[Around the end of July 1999, this question was posted to the /5 email list. Larry G. replied:

If you want the welding and remachining process done to your ring gear,
remove the ring gear from the final drive (or maybe you can send the
assembled final drive -- I removed mine) and send it to Milton Hall,  1605
Bragg Road, Fredericksburg, VA  22407. Give him a call before you send it:
540-786-2994. The cost, removed from the housing, is about $125. You also
need to have your spline replaced in the hub. The cost of the spline dog is
92 BMW dollars and I know of no alternatives. Anyone? With seals and
shipping and all, the total cost is about $300.
and Doug J. said "Hansen's BMW in Medford, OR does many of the splines in the country." (and now that I think about it, maybe that's what that MSF instructor said many moons ago..."Judson"..."Hansen"...I will never be able to keep them distinct in my mind)]. In Aug 1999, Mike L. provided the following contact information for Hansen's BMW:
Hansen's BMW
3598 S. Pacific Hwy
Medford, OR  97501

(541) 535-3342

[In Feb 2001, Steve C. dropped this little tidbit on the /5 email group:

Apparently Joe Groeger in Redwood City, CA has been rebuilding a few
/5 style final drives by completely cutting the spline off the ring
gear and welding on a custom fabricated, heat treated spline made by
a local gear machine shop.  Joe says the factory spline wasn't heat
treated and his replacement lasts much longer.
Kim D. claims to have had contact with someone whose welded-on spline broke at the weld. I did not verify who did that spline repair, nor if the unfortunate event ever occurred.]

How much does the repair cost?

Andy Pelc charged me $200 (in 1996). He told me that it actually cost $175 to remachine the splines and the rest was shipping and handling, and his profit margin. He also told me when I came to pick up the repaired ring gear that the place that did it just upped its prices, so if I were to take it to him now it would've cost me $225.

The Capitol Cycle '95 Catalog quotes $175 for an out-of-housing spline repair.

Hansen's webpage lists prices that depend upon how desperate you are.

What else do I need?

  • Some metric sockets, with both a socket driver and a torque wrench.
  • A new cardan gasket and cardan seal. A new gasket for the drive shaft housing to final drive unit mating surfaces. A new oil seal for the other side of the ring gear.
  • New crush washers for the fill and drain plugs of both the final drive unit and the drive shaft housing.
  • A flat punch, at most a quarter inch in diameter at the business end, and a hammer.
  • A pop can, cut in half.
  • An oven, which you don't mind getting oily fumes in. Also, heavy duty gloves or oven mitts which you don't mind getting oily.
  • Two M5x30 bolts.

    How do I remove the final drive from the bike?

    First, remove the rear wheel. See elsewhere on how to do this, if you don't already know. For me, at least, this involves removing parts of the exhaust system, which also makes later parts of the final drive removal process easier.

    Second, undo the wing nut that adjusts the rear brakes. On the rear brake arm sticking out of the final drive unit, push the hollow round shaft that the wing nut contacts out sideways. Put it and the wing nut back on the threaded part of the rear brake actuator that's no longer attached to the final drive unit, so that you don't lose them.

    Third, drain the oil from both the final drive unit and the drive shaft. Both drain plugs are on the final drive unit. Make sure you can open the corresponding fill plugs before you drain the oil, or you may never be able to get oil back in. After the oil's been drained, put the plugs back in so you don't lose them.

    Fourth, undo the nut holding the right rear shock to the final drive unit, and undo the four nuts holding the drive shaft housing onto the final drive unit. It helps to support the final drive unit with something like a two-by-four before loosening everything.

    Slide the final drive unit back from the drive shaft, and then to the left off of the right rear shock. Put the nuts and washers back on their respective threaded shafts to keep them from getting lost.

    How do I remove the ring gear from the final drive?

    Remove the rear brakes.

    Undo the cover nuts.

    Use the two M5x30 bolts in the threaded holes in the cover to pull it up off of the housing. Heat is supposed to help, but I didn't find it necessary.

    The cover will likely take the ring gear along with it. Put the ring gear, bearing, and cover assembly in the oven, standing on the final drive splines (in a baking pan to catch residual oil), and send the oven up to 300F or so. When the cover falls down, (about 5 minutes or less for me) turn the oven off.

    While the parts are in the oven, the roller bearing in the housing on the inside of the ring gear comes straight out, and if you do so (Oops! I forgot to!) the inside oil seal should be right there. Since I forgot to do this, I don't know how to change the oil seal.

    Remove hot final drive unit parts from oven (with gloves or mitts), pull cover off the rest of the way. Watch out for the spacer(s) between the bearing and the cover.

    Use the hammer and flat punch to remove the bearing from the ring gear. There are four little holes in the ring gear for this purpose. Hammer against the inner race of the bearing only, and just a little bit going all around the circumference, until the bearing falls off.

    Take the ring gear to get fixed!

    How do I replace the ring gear into the final drive?

    With the cover off, removing the old oil seal is a matter of brute force. All the surfaces which were in contact with a gasket should be cleaned of all remains of the gasket. This includes the cover, and the housing where the cover goes, as well as the housing where it mates up to the drive shaft. Put the new cardan gasket in place over the cover studs. It can only go on one way.

    The new oil seal can be driven on the cover with an empty glass jar that has a lip of the proper diameter. I think I used an Eagle salsa jar, but I'm not certain anymore. Cut a pop can (Pepsi, etc) in half, throw away/recycle the top half and sand down the edges of the bottom half so they aren't sharp, or at least so they're not jaggedy, as this pop can will be used to protect the oil seal from the sharp edges of the newly machined lands of the spline. Insert it, closed (bottom) end first, into the oil seal, and push it until it is about a half inch from the cut. Later, after the ring gear and cover are installed, it'll be slid out the rest of the way, with the cut edges sliding past the lip of the oil seal. This is why it must not have jaggedy edges: it won't do much good as a tool to save the seal from the sharp edges of the spline if the seal is destroyed by that very same tool!

    [Ed Korn of Cycleworks suggested to me that a much easier way to preserve the seal is to tape up the splines. -RdR 19 Jan 1999]

    Put the bearing in the oven to warm it up, then slide it onto the ring gear. Use a hammer and punch on the inner race to get it to bottom out, if necessary. I don't think it matters on this particular bearing, but the engraved markings on the bearing were legible when it was on the ring gear, so I put mine back on that way, rather than the other way around. Let the bearing and ring gear cool off.

    Put the cover in the oven to warm it up. Put the spacer(s) back into the cover, and, with the ring gear and bearing on top, start the cover onto the bearing. The spacers will never stay in place if the cover isn't on the bottom during this part of the procedure. After the bearing has been started back into the housing, the ring gear can be put down on a block of wood or something soft and the cover can be gently hammered down until it bottoms out on the bearing. Don't get violent here: the impact is being taken by the bearing in a way that it wasn't designed to take, and if too much, can damage it. Once it's gone far enough, the oil seal will be clear of the lands on the spline, and starting on part of the ring gear that it normally rides on. It's time to remove the pop can: if the cover is bottomed out before the pop can is removed, it might get pinched down there somehow.

    Of course you've remembered to put the roller bearings back in place after replacing the inner oil seal, which I forgot to do, so it's time to drop the ring gear and cover down onto the housing and pinion gear. Any unyielding resistance is due to some form of interfering parts: the ring gear didn't slip inside the roller bearings, or the two gears aren't meshed, or the cover holes are not lined up with the studs, or something like that. The cover nuts can then be tightened up to 13 ft-lbs.

    How do I replace the final drive onto the bike?

    Don't forget the new oil seal on the drive shaft to final drive unit mating surface. Other than that, do the reverse of the removal. The torque for the shock absorber nut is 26-29 ft-lbs but the torque for the four nuts holding the final drive unit onto the drive shaft is not given in the Clymer manual. I tightened them to 35 ft-lbs.

    [It is maybe 15k miles since this repair and the new splines are still showing minimal wear. The brakes are dry, and after several hosings with the brake cleaner they started to grab again two seasons ago. RdR 19 Jan 1999]

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    Originally released 10 May 1996
    Last updated 27 Feb 2001