| Also known as | E3912 |
| Diagram | Fig. 1 — CRANKSHAFT, PISTON AND TIMING GEARS · ref 33 |
| Subsystem | engine bottom |
| Listed in parts books | 1963–1964 |
ENGINE SPROCKET DUPLEX
Engine sprocket for the duplex primary chain, mounted on the crankshaft ahead of the primary chaincase.
70-3912 is the engine sprocket for a duplex primary chain setup, mounted on the crankshaft where it drives the primary chain into the clutch. Triumph's parts book lists it under the CRANKSHAFT, PISTON AND TIMING GEARS figure, and it also carries the older E3912 number if you're working from an earlier book.
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This is a wear item that lives inside the primary chaincase, so when you're in there for a chain or clutch job it's worth pulling the cover and having a look at the teeth for hooking or wear before you button it back up. Our catalog record ties this part number to appearances in 1963-1964, so if your machine falls outside that span it's worth confirming fitment before you order.
We don't have a lot more written down on this one beyond the figure it appears in and the numbers it's known under. Give the shop a call if you want to double check it against what's on your crank before you buy.
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