| Also known as | S678 |
| Diagram | Fig. 2 — CRANKCASE · ref 36 |
| Subsystem | engine bottom |
| Listed in parts books | 1969–1974 |
Allen screw (5/16-18 x 4 in. U.H.)
Allen screw, 5/16-18 x 4 in., used in the crankcase assembly on unit Triumph twins.
This is an Allen screw, 5/16-18 thread, 4 inches long, listed in the crankcase figures for the unit-construction Triumph twins. It shows up across nine catalog rows spanning 1969 to 1974, which tells you it's a long-run fastener used on the crankcase assembly rather than something specific to one model year.
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The U.H. designation in the description refers to the head style. Being an Allen (socket cap) screw, it takes a hex key rather than a slotted or hex-head driver, which matters when you're reassembling cases in tight quarters.
If your parts book lists this under 90128A599 or S678, that's the same fastener under a different catalog number. Worth having a spare or two on hand since crankcase screws are easy to bugger up if the socket gets worn.
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