| Diagram | Fig. 4 — CYLINDER BLOCK AND HEAD, 6T AND TR6 · ref 19 |
| Subsystem | engine top |
| Listed in parts books | 1960–1969 |
Split cotter
Split cotter (valve keeper) that locks the valve spring collar in place on the cylinder head, now superseded by 71-3748.
WE259A is the split cotter (valve keeper) that locks the collar and spring retainer onto the valve stem up in the cylinder head. It's a small, two-piece wedge that seats into the machined groove near the top of the valve, and works together with the spring cup or collar to keep the whole valve train stack under tension. Without it, the valve spring has nothing to bear against and the valve will drop into the bore.
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This one shows up across a long run of cylinder block and head figures in our records — 6T, TR6, T120, T90, T100 in its several carburetor variants, 3TA, 5TA, and their derivatives — which tells you this cotter was a common fitment across a wide swath of the unit-twin lineup rather than something unique to one model. Given how many figures it turns up in, it was clearly a running production part for a good number of years before Triumph revised the number.
WE259A has since been superseded by 71-3748, so if you're cross-shopping parts books from different eras, that's the number to also have handy. Confirm which cotter your particular head calls for before ordering, since valve gear parts like this are easy to mismatch if you're working from an old parts list.
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