| Listed in parts books | Not catalogued by year yet |
SPROCKET 18T TRI PU TO 63
18 tooth gearbox (countershaft) sprocket for Triumph pre-unit four-speed twins, PN 57-0471.
57-0471 is an 18 tooth gearbox sprocket for the Triumph pre-unit twins, the sort of shop sometimes calls the countershaft sprocket. It sits on the gearbox mainshaft where the primary chain picks up drive from the engine and hands it off toward the rear wheel, so it's a key link in the whole primary drive setup.
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The catalog code for this one reads PU TO 63, meaning it belongs to the run of pre-unit four-speed gearboxes built up through 1962. You'll also see this same part listed under the older SPRPU/18T reference, which is just the earlier way the same sprocket got written up in the parts books.
If you're rebuilding a pre-unit primary or chasing worn teeth and chain slap, this is one of the pieces worth checking closely, since sprocket wear shows up as chain noise and uneven drive long before anything else lets go.
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