Airhead transmissions

All 4-speed and 5-speed transmissions from 1970 through 1980 are interchangeable. The 5-speed started in 1974. For repair service on these or other BMW transmissions, please contact me.

Pre-1981 5-speeds

There were two real changes to the early gearboxes:

- shift dogs on 5th gear changed from round to square. The old gear is NLA and then replacement requires a new 4th gear to match it.

- the shift dogs broke off the second gear wheel on the intermediate shaft. This only applied to very early gearboxes; later on the gear was strengthened in that area, requiring a thinner shift fork. Part numbers didn't change! The intermediate shaft for these bikes is NLA so repairing this requires an upgrade to the 17.5° components (see next section) which means changing the fourth gear wheel as described previously. Unless used components are available, this damage effectively totals the gearbox.

There were a few smaller changes as well, such as the change to the shifter seal.

1981-on 5-speeds

Clutch and input shaft

For the 1981 model year, the input shaft was redesigned to accomodate a seal at the rear, which coincided with an overhaul of the clutch and actuation mechanism. This involved many parts: transmission cover, shaft, pushrod, all clutch parts, even the crankshaft.

17.5 degrees

March 1982 the tooth profile of the helical gears (input shaft gear and the 5th gear wheels on the intermediate and output shaffts) was changed; these are referred to as the 15° (early) gears and the 17.5° (later) gears. The first few years of the 17.5° transmissions are particularly susceptible a problem where the shock coupler ears break off the input shaft gear. In March 1985 BMW upgraded the input shaft and gear, eliminating this problem. Here (on the right) is an early 17.5° shaft with broken coupler teeth on the gear. The original part is available as a replacement and is much less likely to fail, or the entire shaft can be upgraded.

Shift kit

In mid-1981 (effective for most 1982 models) BMW improved the shift mechanism. The parts involved were available for a long time as kit 23 31 9 056 150 but are now only available as separate parts. The effect of this kit was to provide larger detents for each gear, to reduce the likelihood of false neutrals, and to keep the pawl from disengaging if the spring broke.

A) 23 31 1 451 563   shifting cam 3-4
B) 23 31 1 231 578   segment shaft
C) 23 31 1 242 892   pawl
D) 23 31 1 231 611   shifting cam 1-2-5
E) 23 31 1 242 910   spring

With the upper shift cam removed and flipped over it's easier to see the changes. The old style is on the left and the new is on the right. Cam A now has a round hub to work better with the revised pawl C. Shaft B has an offset to provide a bit more clearance between the pawl C and cam D (on the original version they basically rub against each other) which can be seen by the line where there is a crease in the part. Pawl C is totally revised and its movement is limited as to how far away from the shift cam D it will go, limited by hitting the hub of cam A. Cam D has taller and pointier peaks. The spring cannot really be seen in the photo; it keeps pawl C pressed against the pins of the shift cam D.

Circlip issue

At some point around 1985, BMW dropped the circlip on the front end of the output shaft. It was reinstated around 1994, but the gearboxes produced in that time frame are at risk for excessive bearing wear. Here is a description of repairing the problem.

4-speed transmissions

These were from my very early 4-speeed transmission.

I found an oddity with the shift lever return spring. There was a sleeve under it, which is not pictured in any manuals but is referred to in the BMW repair manual text. With this sleeve in place, the spring will bind and break (possibly explaining why this one was broken when I got it). Pictured is the shifter assembly; the sleeve was under the spring (you can see there isn't enough room for it).

During the /5 production, this part of the mechanism (the large bushing under the spring, and the washer separating it from the ratcheting arms) was changed. ETK only shows the new version; the old one is shown in the /2 literature.

Shims

48mm plate - input shaft rear, both transmissions
39mm - 4-speed cluster shaft
47mm - 4-speed output shaft
52mm - 4-speed input and all 5-speed

* 0.18mm
red = discontinued, NLA

4-speed transmissions originally used various sizes of the 48mm plate, but most have been discontinued. Shimming is now done using 52mm shims with a single .50mm plate. A 1.0mm plate may be too tight, even without shims.

Cycleworks sells shims that are NLA from BMW. Oilhead shims are available in 47mm and 52mm sizes (.30 to .55, +.05).

Bearings & seals

1 & 2 are input shaft front and rear, 3 and 4 are cluster shaft, 5 & 6 are output shaft.
6P refers to the Paralever bearing and seal, which is different from the standard 5-speed.
K is kickstart (seal only)
SH is shifter (seal only)
All bearings C3 clearance unless noted.

(1) Except a different bearing was used and then discontinued around the time that the circlip was omitted from the output shaft.
(2) '74 model 5-speeds used the 4-speed shifter seal.

Links

Duane Auscherman's website and transmission page (mostly 4-speed)

Joerg Hau's transmission rebuild page (5-speed)

Jim Buchanan's page on replacing bearings (4-speed)

Robert Fleischer's website and transmission page (mostly historical info)

Cycleworks' website (transmission tools and bearings)

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