Overview
Much is made of the weaknesses of the final drive, however despite these, the shaft drive has remained a successful signature of BMW motorcycle design for nearly 100 years. It’s true that the drive spines wear down relatively quickly (when compared to the longevity of other components in the airhead design) and then necessitate a complex repair, however the actual gears of the final drive are very durable if setup and maintained correctly.
For the following, the most important resource was William Plam’s video of Boxer2valve which is a must watch if you are planning on resealing your final drive. Also useful was Rob Fleischer’s (Snowbum) article.
Goals
For the purposes of my project I wanted to replace all the seals of the final drive (oil leaks are common) and clean up the aluminium housing. Given the relatively low mileage of my bike I did not intend to replace the bearings. I will briefly touch on the concepts of backlash and preload (side) however I will not go into detail about how to re-shim the final drive as this is generally not necessary if no bearings have been replaced.
Special tools
There are a number of special tools which make working on the final drive easier. Some are essential.
- Support bracket
- Pinion lock for removing the input shaft nut (and measuring backlash)
- Input shaft seal carrier removal tool
The support bracket is a simple piece of flat bar with some holes cut in it. I made mine out of aluminium. It definitely makes working on the final drive a lot easier as it allows you to hold the drive in a vice in various orientations. The diameter of the central hole is 67mm and the support holes are M10 and they are spaced 58mm centre to centre.
Disassembly
I pretty much followed the Boxer2Valve video here. First the brake shoes and the brake cam is removed preserving the washer. It is worth using a scribe to index the position of the camshaft and the arm so that you can put it back together the same way. After that, with the final drive facing up in the bracket, the nuts securing the cover (cardan plate) are removed. There are two threaded holes at the edge of the plate which are tapped for M5 and allow you to screw in two M5 bolts and gently “push” the cover off. Generally the crown ring will come off with the cover- I then used a rubber mallet to tap the crown ring (gears) out of the cover. Be careful to preserve the two shims- the small diameter, thicker brass shim is on the bottom and sets the backlash. The large, thin silver shim sits between the large bearing and the cover and sets the preload (or lateral play).
Next I removed the large nut securing the input gear on the pinion shaft. This requires a tool to hold the pinion still while a 24mm socket is used. I used a tool from Cycle Works as shown. After the pinion input gear is removed you can access the large seal behind it. This is in a seal carrier which is screwed in to the housing. To remove the carrier requires another special tool to engage the 4 notches in the carrier. I used the tool from Boxer2Valve. Behind the seal there is a large washer and a shaped spacer which engages a reciprocal groove in the seal as shown. Note- the boxer2valve tool is sadly not great- it didn’t fit perfectly and it was difficult to torque the threaded ring back up at the end. I would recommend buying the cycleworks tool which is a different design.
To remove the inner (outside or right hand side) seal, the crown ring needle bearing needs to be removed. To do this, the final drive is flipped over in the support and then heated (for a long time) to a temperature of at least 75c. Around this temperature the bearing simply falls out, revealing the seal. Some people put the final drive in a hot oven to achieve this. To remove the seal there is a groove in the casing to receive a seal puller so you aren’t levering on the bearing surface (shown).
The wide diameter seal in the cardan plate (cover) can be tapped out using a screwdriver or similar taking note of location (the seal sits in a specific groove) and orientation (the spring faces in towards the gears).
The thick blue/green seal in the seal holder on the input shaft can be driven out and replaced with a bearing driver- it helps to heat the seal holder first.
Drive shaft fill threads
It was apparent on my bike that a previous owner had been over zealous when tightening the driveshaft oil fill bolt and stripped the threads. This is a common problem. Unfortunately someone had attempted a very poorly executed repair involving;
- a helicoil insert which isn’t square, and isn’t M12 (i think possibly imperial)
- cutting the bolt to try and get it to fit??
- applying large amounts of some kind of hard expoxy to try and build up the surface to the “not square” plug bolt.
To repair this I made up a jig to put my final drive into my small milling machine. I then used an end-mill to clean up the surface and drilled out the hole. I then retapped the hole to M14 and used a final drive drain bolt (M14). I also re-tapped the driveshaft drain hole which was in a lot better shape. I had to cut the M14 final drive bolt because it was too long.
Spline assessment
To determine whether you need to replace your splines try and measure the proportion of the wear by inspecting the splines carefully. The splines don’t engage to the wheel over their entire length, so the portion of the spline closest to the crown gear shows you the original thickness. You can then work out how much wear you have. Anything more than 50% probably mandates a repair however you can decide based on how much riding you do and when you are planning on taking the drive apart again! The repair is probably best left to a specialist as it involves grinding the crown gear off and welding it onto a new spindle with new splines. The reason for this is that the pinion and crown ring gears are manufactured as a matched pair and you cannot replace one without replacing both.
Cleaning and Polishing
For cleaning I used my standard technique- WD40 and red scotchbrite followed by fine steel wool and Autosol aluminium polish. This brought things back to an acceptable clean lustre. There are also a couple of oil weep holes which are often plugged up- I cleaned them out with wire and blew some isopropyl alcohol through them.
Resealing
To replace the seals it helps if the parts (casing and cardan plate) are fairly warm. And to replace the needle bearing the case has to be very hot- it should practically fall in to place. I used a standard bearing driver to tap the small seal in and gently help the bearing into fully seated position it goes in with the spring facing up (towards the oil).
The bigger seal in the cardan plate (cover) was hard to get in. Much harder than some videos I have seen. What worked for me was heating the plate up hot in the oven. Then using the old seal as a pusher and two bearing drivers as a kind of press. Unfortunately I didn’t have a bearing driver exactly the right size but I was able to make this work. Make sure it is not proud above the surface that will contact the large shim. I replaced the paper gasket- in theory this gasket thickness will contribute to the lateral play shimming equation so make sure the gasket surface is meticulously clean- I used a razor to completely remove all traces of the old gasket and dressed the surface with 1500g wet paper.
When putting the newly sealed cardan plate over the splines it is possible to damage the inner lip of the seal. To avoid this I used the aluminium can sleeve trick- just make sure there are no sharp edges on your can as they will probably be worse than the splines! The thick blue/green seal for the input shaft can be driven into the seal carrier using a standard bearing driver. When the cover is back on I torqued the 13mm nuts down to spec (about 20nm) in a criss cross pattern.
Backlash and Lateral Play
Just a final note here on some commonly heard terms – backlash and lateral play (often called preload) and gear surface contact. Understanding these requires some basic understanding of how a bevel gear works.
The diagram of a bevel gear below shows the pinion position adjustable along the yellow axis (surface contact) and the crown gear adjustable along the red axis for backlash. The diagram differs from the airhead design in that the position of the pinion and crown are reversed and also the gears are straight cut not helical.
In a spiral (helical) bevel gear like the airhead final drive, the contact between the two sets of teeth (crown and pinion) is largely determined by the position of the pinion. This is set by the shim between the pinion and the inner bearing (not seen in my pictures because I didn’t take this apart). The contact is checked by inking the surface of one gear and seeing the imprint on the other. The ideal contact area is in the middle of each tooth, slightly towards the thicker end. You can determine this using machinists dye like Prussian blue- you paint one set of teeth, put the gears together and spin and then inspect the imprint on the other. I did this with a better (more available) dye called Pillar box red which is great for making red velvet cake and was in my pantry. It worked, but not great. If you are replacing the bearings on the pinion I recommend getting some good dye.
The backlash refers to the amount of free play in the system- the amount of movement of one gear before movement of the other gear occurs. In the airhead final drive this is set by the small diameter inner thrust washer which sets the height of the crown gear on the pinion. According to the factory manual this should be 0.15-0.2mm. To measure the backlash the pinion needs to be immobilised (using a jig similar to the one used to remove the nut) and a dial gauge used to measure movement. The movement should be measured at approximately the level where the two gears mesh which is roughly level with the outside edge of the big bearing. To achieve this I made a small jig similar to the one shown in the workshop manual. As expected, my backlash was in spec (0.15mm)
Finally, the lateral play (side to side movement of the crown gear) is set by how tightly the cardan plate squeezes the crown onto the pinion- the preload. This should be tight enough to have no side to side movement but not putting pressure on the bearings. The exact amount of shim required can be established by measuring the depth of the bearing from the cardan case and then subtracting the depth from the edge of the casing. It should be measured with the gasket in situ as this effectively “loosens” the cardan plate. An alternative way of measuring this is the method sometimes used for shimming the gearbox case- using a crushable material such as solder wire and placing the lid on, crushing the wire and then measuring the thickness of the crushed section using a micrometer.
Replacing on the swingarm
Finally I replaced the drive on the swingarm. A new gasket and new nuts were used and the nuts torqued to the specified 30nm. You can see here that I have also put the brake shoes and brake hardware back on but anyone who has read this far is undoubtedly dying of boredom so I will save those details for a separate post on brakes.
