| Diagram | Fig. 9 — GEARBOX OUTER COVER · ref 35 |
| Subsystem | engine bottom |
| Listed in parts books | 1960–1970 |
Nut
A 5/16 inch, 26 TPI bar-cut full nut used across many spots on these motorcycles, from crankcase studs to fork and exhaust fittings.
S1-51 is a 5/16 inch, 26 TPI full nut, the kind of small bar-cut fastener that shows up all over these bikes rather than in just one spot. Our records show it appearing on crankcase studs, gearbox outer covers, front fork parts, exhaust and manifold fittings, footrest and chainguard mounts, mudguard stays, petrol tank fittings, and electrical brackets like coils and rectifiers - it's a general-purpose nut that the factory used wherever a 5/16-26 CEI thread was called for. Because it's a bar-cut nut, it wants a washer under it rather than sitting straight against a painted or machined surface.
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The alias 5/16X26F is just the older bare-spec way this nut was written up before it got a proper part number, and it later got superseded by 57-0224 as Triumph's numbering moved on - so if you're chasing this nut down in a newer parts book, that's the number to look for.
Given how many places this nut turns up across the catalog, it's worth having a handful in a drawer. Since the exact spot on your bike isn't pinned down by a single figure, check the fastener you're replacing against this thread size before you order.
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