Jörgs Motorcyle Pages:
BMW R-series gearbox overhaul

What you need

First of all, a warning: Overhauling the gearbox is a job that cannot be done with just the tools of the average do-it-yourself mechanic. You should have a sound amount of experience with maintaining your own motorcycle. Gearbox mechanics is a pretty interesting mixture of "brute force" and precision mechanics, so you will need a rather well-equipped workplace and several special tools, plus sufficient force to deal with torques in the 220-Nm range.

Your workbench must be stable and provide enough space and light. To fix the gearbox either in the horizontal or the vertical orientation, BMW shops have a special jig or "cage", but a simple, solid steel angle like the one shown here does the job. Drill two holes with 9 mm diameter and 215 mm apart, plus a third hole with ca. 22 mm diameter that is located 47 mm from the right-hand hole (and ca. 4 mm off-axis); the latter is to access the lower bolt that holds the internal shifting mechanism in place. Make sure that the device fits perfectly to the gearbox and that the complete assembly is firmly held to the workbench.

Actually you can perform the overhaul without such a support, but it helps a lot ;-)

As for the spare parts, the "usual" gearbox overhaul involves replacement of the five ball bearings (the sixth bearing is a large cylindrical roller bearing and usually does not need replacement) plus the replacement of all seals. You can download a list of the commonly required spare parts here as Open Document spreadsheet. Make sure you have these parts at hand before you open the gearbox.

Note that the gaskets are also available as a complete set; for the BMW paralever gearboxes, the part number is 23 00 1 338 597. This kit can also be used for the older gearboxes, but not the other way round.

Getting to the gearbox

The removal of the gearbox from a Paralever-BMW such as the R80GS, R100 GS or R100R is described on my R80GS page. For the "other" 2V-Boxer models from /7 onwards, you may find the notes about my R100TIC useful.

Opening the gearbox

1. Drain the gear oil (if not already done).
2. Carefully remove the speedometer drive parts near the exit flange.
3. Unscrew and withdraw the gear shift lever.
4. To open the gearbox, fix it horizontally into the device described above and attach the flange puller with its four bolts. A sketch of such a device can be found on the website of Daniel Dicke; Dean Verhoeven sent me a similar drawing (but with units in inches ;-) as PDF and AutoCAD file. Note that the inner bore needs to be sufficiently wide to accomodate a solid 24-mm hex insert. To remove the collar nut that secures the drive flange, several variants are possible: Either use two solid, long wrenches, or immobilise one of the levers with a vise. Please be aware that this nut is attached with 220 Nm, so you will need "some" force. Take care not to put too much stress on the housing itself, i.e. do not "twist" the assembly while you apply force! - Alternatively, you may use a pneumatic impact wrench to remove the collar nut.
5. At this point, the drive flange needs to be removed. Swap the hex insert above against a threaded bolt (here is a detailed view of the same device), then break the (conical) taper fit loose. Again, this will need considerable force; when you feel that the bolt is under tension, give it a well-dosed "smack" with a hammer along its axis to break it loose.
6. Now remove the handful of Allen head bolts that hold the cover in place. Note that one of these bolts has two washers (upper center bolt).
7. You do not need to unscrew the kickstarter assembly (if you've got one).
8. Now, use a hot air gun to heat the gearbox cover in the region where the three bearings are seated. When the cover is sufficiently hot, use a plastic hammer or yet another flange puller as described by Daniel Dicke to gently remove the cover.
9. Note that there will be a number of shims or "spacers" ("Distanzringe") between the bearings and the housing. Mark their location, then set all parts aside.
10. Remove the old gasket from the cover.

At this point, the gearbox is open and ready for disassembly. If you want to determine the axial play of the shafts (because you want to know if the gearbox was assembled correctly in the factory ;-), follow the instructions in section "adjusting shaft play" below.

Disassembling the gearbox

1. On the clutch side of the gearbox there are two bolts that hold the gear selection mechanism ("Schaltautomat") in place. Remove those (they fit very tight), then pivot and remove the complete plate with the internal shifting mechanism.
2. The input shaft ("Eingangswelle") can usually be withdrawn by hand; if necessary, use a plastic hammer.
3. The two other shafts must be removed using heat: Use a hot air gun or a soft propane torch to heat the clutch side of the housing, in the area where the two remaining bearings are seated. When the housing is real hot (about 100° C), extract the two shafts simultaneously towards the front. This is best done with two persons, where one heats the housing and the second constantly "rattles" the shafts to extract them (support the housing in a horizontal position).
4. Remove the shifting forks ("Schaltgabeln") - two of them come out together with the gear shafts - and the shifting fork shaft. Put all parts aside.
5. On the bottom of the housing there are a number of oil baffles ("Anlaufscheiben"). Clearly mark their location, then set the parts aside (attention, hot housing!).
6. Remove the old gasket if you did not already do so.
7. Remove the Neutral switch, if you like to do so.
8. Clean the housing and inspect all parts for damage.

An interesting point we found was the "bearing" of the kick starter shaft: The shaft runs directly in the the aluminium housing (!) instead of using a simple bronze bush. When we disassembled the gearbox of Niko's R90S, we found that the bearing for the kickstarter shaft was completely worn out! The sad conclusion is that you should not use the kickstarter, except if really necessary ...

What can happen to a bearing if the signs of wear are not noticed in due time is visible in this picture. The result of the broken bearing was that the shaft moved around in the gearbox and already started to "machine" material off the inside of the cast housing ... in extreme cases, the rotating shaft may break through the housing and the rear cover.

The gear selection mechanism

Upon inspection of the gear selection mechanism, verify carefully that the the curved disk and its axis (indicated by arrows in this picture) are connected and do not exhibit any play. If there is some "wiggle", have the two parts soldered together.

Another point worth considering are the three springs located on the gear selection mechanism. There are several reports that one of those springs (the shift pawl return hairpin spring) easily brakes so that that you cannot change gear anymore; the situation is simulated in this picture. As this is a piece that costs only a few cents, it may well be worth the preventive change. However, opinions on that subject vary widely: Some people recommend changing them every time the gearbox is opened, others say "don't change them if they are not visibly damaged".

Bruce Marshall pointed out that the length/strength of the cush-drive spring should also be checked; one his bike it had compressed so much as to allow some sort of float that causes a rattle on the overrun in 1st and 2nd gear.

The part numbers of these springs are also included in the file mentioned above.

Yet another suggestion is to change the plastic roller (visible on the left edge of the picture) against a "true" roller bearing; reportedly this improves the smoothness of the gear shifting process even further. According to Johannes Grauf the SKF 628/8 is a suitable bearing; Shaun Baisley pointed me to the 688 yet both have the same dimensions (8×16×5 mm).

Replacing the bearings

The common problem with BMW gearboxes is that one or more of the bearings wear out. For the Paralever models, this is "traditionally" the large bearing that is sitting on the clutch side of the output shaft. This bearing (6403-C3) does not only take the radial forces from the other shafts and the gears, but also the thousands of little axial shocks from the shaft drive ... in other words, this bearing is supposed to absorb forces that it is not designed for to that extent.

Once the gearbox is opened, it is advisable to exchange all the five ball bearings, as they are not very expensive. The four "small" bearings of type 6304-C3 cost about 10...12 Euro each, the big bearing (6403-C3) roughly 25 Euro. Interestingly, in the more recent airhead gearboxes, two of the small bearings are open, one is closed on one side and the fourth one is completely closed. The technical reason is probably that the "closed" bearings were supposed to keep their oil over a longer time, however the old /7 gearboxes reportedly did not use this "mix" of bearings (thanks again to Daniel Dicke for this information).

Changing the bearings is rather straightforward. Generally, use a two- or three-arm puller or a special bearing puller to extract the old bearing (if necessary, heat with a hot air gun to about 80° C). Then, fit the new bearing with a suitable tool, e.g. a large nut from a ratchet set plus a suitable press (you may use a hammer - as we did in the workshop -, although this is not a recommended way to deal with bearings).

• When changing the bearing on the input shaft, make sure you do not pull off the metal shim that is located just underneath the bearing.
• Do not forget to change the shaft seal ("Simmerring") inside the driveshaft.
• To remove the "small" bearing from the output shaft, pull the bearing together with the 1st gear pinion.
• The bearings for the intermediate shaft (layshaft) are simply pulled off.

It may be worthwhile inspecting the individual gears for signs of wear ... and for correct installation: on my R100TIC we found that one of the spacers on the output shaft was installed "the wrong way round", which lead to increasing wear on this ring and increasing axial play during use. As mentioned above, this was meant to be a factory-new gearbox!

Just for the record, this picture shows the famous "X" mark of a gear wheel. In April 1982, BMW changed the pressure face angle of the teeth on the input shaft helical gear, the cluster helical gear and the output shaft 5th gear and it changed from 15 degrees to 17.5 degrees. The little "X" stamping denotes that those revised helical gears can't be retrofitted to an earlier gear set (= prior to April 1982) because they won't mesh correctly and will self destruct.

Reinstalling the shafts

1. Position the gearbox housing horizontally.
2. Insert the washers ("Anlaufscheiben") into the housing. Note that the two "hat-shaped" baffles belong to the layshaft.
3. Heat the (clean!) gearbox housing with a hot air gun or a soft propane torch in the area where the two bearings for the output and the layshaft are located.
4. When the housing is real hot (about 100° C), insert the two shafts and the (single) shifting fork for the 3rd/4th gear simultaneously - below is a short video. Push the bearing fully into their seat (plastic hammer).
5. Insert the remaining two shifting forks (short ends facing each other). The gearbox now looks like shown in this picture.
6. Make sure the shifting mechanism is in Neutral position, then insert it into the housing and attach it with its the two bolts (24 Nm).
7. Change the shaft seal for the gear shift lever, then insert and attach the shift lever.
8. Insert the input shaft.

At this point, make sure you can shift through all the gears and back: It seems that it is possible to assemble the gearbox "wrongly", so that you can shift gears up - but then you are stuck .. :-(

Time to adjust shaft play.

Adjusting axial shaft play

Adjusting shaft play is a crucial step to ensure that your gearbox "behaves as designed". If the axial play of one or more of the three shafts is too high, they may move during use, which leads to "hanging" gears or other problems during shifting, in particular under high load. If the axial play is too low, the friction in the bearings is too high and you will experience loss of performance and rapid wear.

In theory, once your gearbox is well-adjusted, you do not need to re-adjust shaft play when you change the bearings. Yet ... how do you know your gearbox is well-adjusted? In the case of my R100TIC, I was told that the original gearbox had been replaced against a new one at about 50 Mm. When we opened this gearbox during the workshop it had run for about 35 Mm ... and we found not only that it was a refurbished one (clearly identified by traces/scratches on some parts) but that it had not at all been installed with the tolerances specified by BMW. The "allowed" axial shaft play is 0.05 mm, but we found the following axial shaft tolerances in this "original BMW" gearbox:

• input shaft ("Antriebswelle") 0.35 mm (0.30 mm too much)
• layshaft ("Zwischenwelle") 0.50 mm (0.45 mm too much!!)
• output shaft ("Abtriebswelle") 0.05 mm (exact)

A little more play than 0.05 mm would be normal (think wear and tear), but 0.30 and 0.45 mm are really a lot. The large play explains the problems that I observed during gear shifts: the shafts move along their axes. Indeed, the troubleshooting list of the BMW workshop manual lists incorrect axial play as diagnosis for trouble during gear shifts ;-)

Adjusting the shaft play is not very difficult. Essentially, you determine the depth of the recesses in the gearbox cover and the thickness of the ball bearings "above" the gearbox housing, then you add shims (German: "Distanzscheiben") until the difference between those two is 0.05 mm or a bit less, but not zero.

This requires, however, some precision instruments. You need a depth gauge with an accuracy of 0.05 mm ("Tiefenlehre"). The second part needed is the measuring plate ("Distanzierplatte"), which you may want to borrow from your friendly BMW dealer ... or, if you have access to a CNC-equipped workshop, you may build it according to the sketch shown here (also available in drawing exchange file format, .DXF). Note that the exact thickness of this plate is not of much importance (the original has 7.50 mm), but the surfaces must be exactly parallel.

If you think of commercial or "mass" production, make sure you have read and understood the legal stuff, copyright and usage notice!

To adjust shaft play, insert the three shafts into the housing as described above. Use a new gasket and attach the measuring plate with three bolts and the prescribed torque (10 Nm).

1. The housing and shafts must be cold.
2. Determine the thickness of the plate.
3. For each of the three shafts, measure how much the outer races of each of the three ball bearings protrude above the plate.
4. Add the two values above.
5. Measure the depth of the three bearing recesses in the gearbox cover (again, all that counts is the flat surface where the outer race of the ball bearing comes in contact).
6. For each of the three bearings, subtract the latter value (5) from that obtained unter (4). This is the thickness of the "perfect" shim to achieve zero axial play. Subtract 0.05 mm from this value and you obtain the required shim thickness.
7. Get the necessary shims. Your BMW dealer carries them in thicknesses of 0.200, 0.280, 0.38 and 0.50 mm.
8. Well ... you can remove the measuring plate now ;-)

A simple spreadsheet for these calculations can be downloaded here in Open Document spreadsheet format (this is the same file as mentioned above).

Adjusting axial shaft play - an Alternative

What can you do if you do not have the plate? Martin Beurlen from South Africa reported that the following procedure "sounds complicated but it actually works well" ;-)

1. The gearbox is mostly assembled, only the cover is still removed.
2. Remove all the shims but leave the oil baffle in the layshaft.
3. As a gauge, use some soft metal - such as 2-mm hollow-core resin solder wire - and secure it into position on each of the three bearings so that the solder just does not pass the outer bearing race. The position can be seen in this picture which was taken after the final step; note the wrapping of the wires to secure them in place.
4. Put the cover gasket in place, reheat the cover, replace and tighten the securing bolts to the required torque.
5. Then remove the cover again and simply measure the thickness of the flattened wire.
6. Establish your shim thickness and allow your tolerance.

The only downside that I can see is that you have to close and then remove the gearbox cover once more.

Y

Reassembly and closing the gearbox

1. Insert a new shaft seal ("Simmerring") into the gearbox housing cover. Note that there are two different part numbers and two different constructions, according to the type of the motorcycle:
   • For the Paralever models, the flat end must be outside. In addition, there is a small triangular cut-out in the housing at the upper end of the seal seat, which formerly served as a vent ... this must be sealed, otherwise oil will leak from the gearbox into the rear drive.
   • For the "earlier" Boxers with oil in the swingarm, the seal must be installed with the spring visible and the small cut-out remains open.
2. Position the gearbox horizontally.
3. Insert the shims, as calculated in the previous section, into the gearbox cover (you may, of course, put them onto the bearings and attach them with some grease, but we found it easier to have them in the cover).
4. Put the paper gasket in place. Some people recommend using a small amount of liquid sealant here; it does not hurt anyway ...
5. Heat the gearbox cover in the area where the three bearings are located.
6. Fit the cover onto the gearbox, then use the small Allen-head bolts to attach the cover step by step (remember: two washers for the upper center bolt; 10 Nm). From time to time, use a plastic hammer to release the tension from the cover around the bearings.
7. Make sure the shafts do still turn absolutely free - if this is not the case, you made an error during the determination of shaft play ...
8. Insert the seal for the input shaft: place it onto its seat and gently push with a suitable tool, e,g. a nut from a ratchet set.
9. Carefully clean the taper of the output shaft and that of the drive flange (best: a strong degreasing agent, such as brake cleaner). Set the flange onto the taper (i.e., the cone).
10. Bolt the flange puller used above to remove the collar nut to the drive flange.
11. Oil the threads of the nut with some viscous oil. Then fit it to the output shaft and tighten it with 220 Nm (!).
12. Insert the speedometer drive parts.

That's it. The gearbox is ready for use!

Final notes

• Before reinstalling the gearbox, make sure you can shift through all the gears and back. It seems that it is possible to assemble the gearbox "wrongly", so that you can shift gears up - but then you are stuck .. :-(
• A second point to verify is the seal (Simmerring) at the exit flange of the gearbox: If this is a Paralever model gearbox, it must be installed with the flat end outside, and the small triangular cut-out in the housing just above that seal must be sealed, e.g. with some silicone. For all other models, the seal must be mounted with the spring visible and the cut-out must be open (it is a vent for the rear swingarm).
• Upon re-installing, connect the neutral switch first. It is pretty difficult to access it later, so if you forget this one you may run into trouble ...
• Don't forget to fill in the appropriate gear oil - some people swear by the efficiency of synthetic gear oil.
• When you refit the gearbox in the bike, grease the clutch spline (on the gearbox side, not on the clutch side) with MoS2 paste (copper paste or Staburags will reportedly do the job).

Links

• Airhead transmissions details by Anton Largiader. Many detailed pictures plus the historic evolution.
• Daniel Dicke's Website.
• Putting new bearings in a /5 transmission, by Jim Buchanan.
• Cycleworks.net, Tools and videos by Ed Korn.