| Also known as | E3789 |
| Diagram | Fig. 12 — GEARBOX SHAFTS AND GEARS · ref 2 |
| Subsystem | primary |
| Listed in parts books | 1964–1971, 1974, 1976–1980, 1983, 1985 |
TRAP DOOR PLATE 5 SPEED
Gearbox trap door / sprocket inspection cover plate for five-speed Triumph gearboxes, PN 70-3789 (E3789).
The trap door plate, part number 70-3789 (also seen in older parts books as E3789), is the small cover plate on the gearbox that gives access to the clutch sprocket area on five-speed gearbox setups. It sits in the chaincase area, bolting over the opening in the gearbox outer cover so you can get at the sprocket without pulling the whole cover off.
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This one shows up across gearbox shafts and gears figures as well as chaincase and chain listings, which tells you it's tied specifically to the five-speed gearbox conversion and later gearbox layouts rather than the earlier four-speed setup. If you're converting an earlier unit twin over to a five-speed box, this is one of the pieces you'll need since the door itself differs from what the four-speed cover uses.
When you order this, also check whether you need a fresh gasket, oil seal, and fixing screws to go with it — those are separate items and worth replacing at the same time since you'll have the cover off anyway.
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