| Diagram | Fig. 7 — CLUTCH · ref 22 |
| Subsystem | primary |
| Listed in parts books | 1969–1971 |
BSA Drain plug, primary case
Drain plug for the primary chaincase cover, used to drain the primary oil bath.
This is the drain plug for the primary chaincase, the cover that holds the primary chain, clutch, and engine oil bath on the front of the motor. When it's time to change the primary oil, this plug is what you pull to let it drain out.
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It shows up in the parts books under the clutch and primary cover figures, and the catalog record we have places it in the 1969 to 1970 range. It's a small, simple part but an easy one to lose track of or strip out if someone's been careless with a wrench on it over the years, so it's worth checking condition before you button the cover back up.
One catalog listing for this same plug calls out a 5/16 UNC thread, which lines up with what you'd expect on a plug of this size, but confirm what your case takes before ordering if you're not sure.
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