| Diagram | Fig. 11 — CHAINCASE AND CHAINS · ref 1 |
| Subsystem | primary |
| Listed in parts books | 1974 |
GEARBOX SPROCKET,18T 500 TRIUMPH UNIT - UK
An 18-tooth gearbox sprocket for the Triumph 500 unit-construction twin, fitted in the primary chaincase.
This is a gearbox sprocket, 18-tooth, for the Triumph 500 unit-construction twin. It sits at the end of the gearbox mainshaft where the primary chain wraps around it, so it works alongside the engine sprocket and clutch to carry drive back from the gearbox to the rear wheel through the primary chaincase.
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The catalog lists this one under the CHAINCASE AND CHAINS section, which is where you'd expect a gearbox sprocket to live in the parts books. It also shows up under an older alternate number, SPR5TA/18T, which is worth knowing if you're cross-checking against an older parts list for a 500 unit twin.
Tooth count on a gearbox sprocket affects final drive gearing, so before ordering, check what's currently fitted on the bike and confirm the 18-tooth count is what you actually want or need to match.
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