How to fix: Worn gearbox output shaft seal: how do you go about it?

The diagnosis is confirmed: a leaking oil seal (a lip seal that rides around a shaft or tube). What's at stake depends on where it is. Fork oil seal: it keeps fork oil sealed around the fork tube; once worn, it lets oil run down the fork tube onto the caliper, disc and pads -- this is a major braking hazard (an oiled disc won't stop you), so it needs handling right away, and cleaning the brake is not optional. Output shaft seal (transmission side): it keeps oil sealed around the countershaft sprocket shaft; once worn, it lets oil seep onto the sprocket and chain. The fix, in principle: replace the seal that's failing. For the fork, you pull the tube, replace the seal (and the dust wiper), and refill with fork oil at the correct volume and grade -- an intermediate-level job that needs a few special tools (and involves the front end, so be thorough). For the output shaft seal, you need to pull the sprocket -- more involved depending on the model. Risk level: moderate, high on the safety side for the fork (oil on the brakes). On budget: a fork seal runs a few euros up to around twenty euros (the pair), fork oil around ten euros; an output shaft seal is cheap, but labor depends on access. Important: if you're not equipped or comfortable, especially with the fork (it's a safety item), hand this one to a professional. This tool helps you understand what's going on -- it doesn't replace a certified mechanic.

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Transmission / gearbox oil leak

You see traces of oil around the gearbox, the output sprocket, or, on a shaft-drive bike, the rear final drive and drive shaft. This is rarer than an engine oil leak, but shouldn't be ignored: a gearbox or final drive losing its oil risks a costly seizure. Important note: on most motorcycles, the gearbox shares the engine's oil (common sump); a "gearbox" leak is then often actually an engine oil leak located on the transmission side. On others (and on shaft-drive bikes), the transmission has its own oil. Main causes: a worn output shaft seal on the gearbox (oil seeping onto the sprocket and chain — common on 4-strokes), a gearbox case/cover seal (on the shift side or clutch side) that's failing, a poorly tightened drain/level plug (a slow leak, easy to fix), and, on a shaft-drive bike, drive shaft seals leaking (final drive oil around the rear wheel — a specialized job). You tell them apart by locating the traces: around the sprocket/chain? at the case/cover junction? at the plug? toward the rear wheel (shaft drive)? And by watching whether the oil level drops. Urgency threshold: a slow leak without a noticeable level drop isn't urgent, but keep an eye on the oil level (gearbox or engine oil, depending on the case) and don't ride if it drops — a transmission running low on oil can seize. Clean the area to better pinpoint the active leak: telling a genuine leak (seeping from behind the sprocket cover or from a seal) apart from simple chain-grease spray is key to a correct diagnosis.

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Possible causes: transmission / gearbox oil leak

These are the most common causes, from most to least likely. The guided diagnosis above helps you find yours, test by test.

Worn gearbox output shaft seal
Intermediate
The gearbox output shaft seal is a lip seal that seals the oil around the shaft on which the output sprocket (the one driving the chain) is mounted. Over time, heat, and rotation, this seal dries out, hardens, or wears, and oil starts seeping at the sprocket, soiling the sprocket, the chain guard, and the chain itself. This is a common cause of transmission-side leaks on 4-strokes (where the gearbox often shares the engine oil), and the seepage can, over time, lower the oil level. The tell: oil traces concentrated around the output sprocket and on the chain, behind the sprocket cover. The fix principle: remove the sprocket cover and the output sprocket (undo the nut lock tab), replace the seal without scoring the shaft, then reinstall the sprocket to spec and redo the nut lock tab, checking the oil level. This is an intermediate job, with difficulty depending on access to the sprocket. To distinguish from simple chain-grease spray: here the oil comes from behind the sprocket cover, not from the chain. This cause overlaps with the general oil-leak symptom (shaft seal). A chain and sprocket constantly coated in engine oil get dirty faster and wear out sooner: worth fixing.
Gearbox case / cover seal
Intermediate
The side covers that close off the gearbox and its controls (shift-side cover, clutch cover) are sealed with a flat gasket (paper, elastomer, or sealant). With heat, vibration, and aging, this gasket weakens, gets crushed, or cracks, and oil seeps steadily at the junction between the case and the cover. Since the gearbox often shares engine oil, this is an engine oil leak located on the transmission side. The seepage soils the side of the engine and, if significant, can lower the level. The tell: oil traces starting from the junction line between a case and its cover (shift side, clutch), on the side of the engine. The fix principle: remove the cover in question, thoroughly clean both gasket surfaces, fit a new gasket (or the appropriate sealant for that assembly) and reinstall to torque spec, following the tightening sequence (an under- or over-tightened cover warps and leaks again). This is an intermediate job; access to the cover determines the difficulty (the clutch cover may require draining and removing other components). This cause overlaps with the case/cover seal leak symptom. Pay attention to torque specs, which matter on these covers.
Poorly tightened drain / level plug
Basic
Like the engine, the gearbox (or the final drive, on a shaft-drive bike) has a drain plug and sometimes a level plug/screw, each sealed with a gasket. A poorly tightened plug, a crushed/reused gasket, or a dirty thread causes a slow but steady leak. This is the simplest and most benign cause of a transmission-side leak, and the first to check — often right after an oil change (of the gearbox or engine, depending on the shared sump). The tell: oil traces concentrated at a drain or level plug, rather than elsewhere. The fix principle: retighten the plug to spec with a new gasket, or remove it to clean the sealing surface and reinstall cleanly. This is an easy DIY job. Important: respect the torque spec (don't force it in an aluminum case, risking stripping the thread), use a new gasket of the right size/material, and check/adjust the oil level afterward. If the plug is properly tightened with a new gasket and it still leaks, the thread may be damaged (a bigger repair). This is often the good news of a transmission leak: a simple matter of a plug and a gasket, fixed in a few minutes.
Drive shaft seal (shaft drive)
Expert
Warning: on a shaft-drive motorcycle (BMW, Moto Guzzi, some cruisers and tourers), power passes through a shaft and a bevel gear at the rear final drive, all lubricated by a specific final-drive oil — never engine oil. The shaft seals (gearbox output, final-drive input) and the final-drive housing seals can leak with wear, letting transmission oil appear around the rear wheel and final drive. A leak lowers the final-drive oil level and, over time, threatens the bevel gear (a very costly seizure). The tell: oil traces toward the rear wheel/final drive, on a shaft-drive bike (no visible chain). Diagnosis and repair (specialized removal, seal replacement, specific torque specs, refilling with final-drive oil to the manufacturer's grade, checking shaft-drive play) require particular expertise: this job is classified expert-only, reserved for an equipped professional. Budget-wise, the seals are inexpensive but labor and the specific oil drive up the cost. Don't confuse this with a typical engine oil leak: here it's transmission (shaft-drive) oil, located toward the rear final drive. Don't neglect a shaft-drive leak: riding with insufficient final-drive oil destroys the bevel gear.
Repair to be left to a professional.

Worn gearbox output shaft seal

Gearbox case / cover seal

Poorly tightened drain / level plug

Drive shaft seal (shaft drive)