Transmission & Final Drive

The airhead gearbox has a reputation it half deserves. It is not fragile, but it is unforgiving of assembly error, and it hides its complaints behind symptoms that sound like clutch problems.

Shimming is the craft at the centre of it. Three shafts, each wanting five to ten hundredths of a millimetre of end play — measured cold, because the aluminium case grows faster than the steel inside it. The factory method is elegant once you see it: shim every shaft to zero, then let the crush of a new gasket donate exactly the running clearance you need. Fit a gasket of unknown thickness and you have silently changed the specification.

Further back, the final drive ratio stamped on the case tells you more about how your bike will feel at seventy than any other single number on the machine.

Every figure on this page resolves to one model and year in the book. Pick your bike and the rows that apply to it come forward, with the rest marked.

Open this chapter in the book

Film · in the book

Exhaust off, rear wheel out.
Header nuts, then the pipes, then the rear wheel comes away from the final drive. Same bike, same day. The final-drive figures are in the tables above. Silent, 0:27. Watch it in the book →

5-speed internal ratios

Gear	Ratio		Applies	Notes
1st / 2nd / 3rd	4.40 · 2.86 · 2.07	—	All airhead 5-speeds	Overall gearing = gear ratio × final-drive ratio (no separate input reduction on airheads, unlike oilheads).
4th / 5th	1.67 · 1.50	—	All airhead 5-speeds	5th is an overdrive-feel 1.50, not true overdrive.

Gearbox shimming — the heart of a rebuild

Spec	Value		Applies	Notes
Shaft end play — all 3 shafts	0.05–0.10 mm	.002–.004"	input · intermediate · output	MEASURE COLD — clearance grows as the alloy case out-expands the steel shafts. Too much input-shaft play = grabby clutch; too little = rapid bearing failure.
Factory shimming method	shim to zero, gasket gives the play	factory gasket ≈0.1 mm	BMW workshop practice	Measure through a shim plate over a NEW gasket. Aftermarket gaskets differ in thickness & crush — that changes your end play.
Shim plate diameters	52 mm — all 5-speed shafts	48 input-rear · 47 & 39 (4-spd)	shim sourcing	Many 4-speed sizes NLA from BMW; Oilhead 47/52 mm shims (0.30–0.55, 0.05 steps) substitute. Cycle Works sells kits + measuring plates.
Bearings	C3 clearance	—	all positions unless noted	Case heated to ~100 °C for shaft removal/install. Cover bolts: 72 INCH-lb (see Torque §21–23).
Assembly feel check	output shaft turns stiff cold	frees up warm	post-rebuild	Splines should bite your fingers slightly when twisting cold. Verify all 5 gears shift before the cover cools you into a corner.

Known 5-speed issues

Issue	Fix		Applies	Notes
Output-shaft circlip deleted	machine groove + fit clip at rebuild	—	many later 5-speeds — check yours	BMW dropped the retaining circlip during production; without it the bearing can drift on the shaft. Production range disputed — early-'80s boxes (yours included) are prime suspects. Standard rebuilder practice is to cut the groove and install the clip. Ask whoever opens your box.
6403 input bearing	use improved "TM"-marked bearing	same part number	5-speed input	Later production of the same bearing number is the better part.

Final drive — ratio ladder (twin-shock era)

Stamp	Ratio	Character	Typical fitment	Notes
32/9	3.56	shortest airhead ratio	R65	Ratio is stamped on the final-drive case — trust the stamp, not the model chart. Crown/pinion travel as a factory-matched, number-stamped set.
31/9	3.44	very short	R65	Small-airhead ratio. Absent from most model charts.
37/11	3.36	short — acceleration	R60/6, R60/7, R80G/S, R80ST, R80RT, R65 '86+	R60/5 only in some older books. Trust the case stamp.
32/10	3.20	short	R75/5	Pairs with 1.186-ratio speedo.
34/11	3.09	middle	various 750/800
33/11	3.00	tall	R100/7, RT typ.	Pairs with 1.112-ratio speedo.
32/11	2.91	tallest — relaxed cruise	R100S/RS typical	Confirm the ratio stamped on the housing. Tall gearing + café weight loss = a very relaxed 5th. Swapping ratios without matching the speedo (W number on its face) skews speed/odo ~7%.

Rear-drive rebuild rules

Rule			Applies	Notes
Re-shim triggers	gearset · any bearing · gasket change	even gasket→sealant	always	Shimming sets pinion depth + crown backlash, verified by dye contact-pattern — same craft as any hypoid diff. Careful-amateur territory WITH a dial indicator, shims, dye, and a mentor; otherwise send it out.
Ratio ID without opening	mark tire, count coupling turns	per wheel revolution	any airhead	Feel the driveshaft coupling bolts through the rubber boot and count.

Transmission & Final Drive

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