Aimed at riders who are looking to lower RPMs at highway cruising speeds, the 21 tooth gearbox, or “countershaft” sprocket can be fitted in place of the original 19 tor 20 tooth sprocket. Modifications to the clutch door aperture are necessary to accommodate the larger sprocket.
Suitable for fitment to Triumph 650 and 750 5-speed transmissions, this high-quality sprocket is manufactured from high-carbon steel and sourced from Emgo. Designed for use with a standard 5/8″ x 3/8″ 530 chain.
The countershaft sprocket transfers the power from the primary drive to the final drive through the transmission. As the size of the front sprocket increases, the overall gearing is raised. This can affect the charging system capabilities at certain engine speeds in higher gears, as well as the potential for increased wear on the clutch plates when pulling away from a stop. Torsional strain on the mainshaft is also a possible consequence of operating at very high speeds in excess of 100 mph.
The entire clutch assembly and clutch door must be removed to access the sprocket. The rim around the clutch door aperture must be notched to allow the introduction of the larger diameter 21 tooth sprocket to the splined high-gear assembly on the mainshaft. This involves scribing a pattern to the rim and using a small grinder to remove the 21 notches required to pass the sprocket through the opening in the primary chaincase.
For the stock 19t 5 speed sprocket click here.
For more sprockets and other related spares click here.
We also stock a 5 speed gearbox for Triumph 750 twins (which can also be fitted to 650 twins including pre-units). Click here.