| Also known as | E4003 |
| Diagram | Fig. 1 — CYLINDER AND CRANKSHAFT · ref 14 |
| Subsystem | engine bottom |
| Listed in parts books | 1965–1969 |
Small end bush
Small end bush for the connecting rod on Triumph 500 unit twins, where the wrist pin rides in the rod.
70-4003 is the small end bush that lives inside the connecting rod, up at the wrist pin end, on Triumph 500 unit twins. It's the bearing surface the gudgeon pin rides in where the piston pin joins the rod, and it takes the wear so the rod itself and the pin don't have to. Catalogs list it under the crankshaft and connecting rods figure, and the older parts books show it as E4003 before the numbering changed over to 70-4003.
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This is a small, simple part but an important one — a worn small end bush shows up as slap or knock from the top of the engine, and it's cheaper to replace the bush than to let it chew up the pin or the rod bore. Our catalog entries for this one span the mid-to-late 1960s, so if you're working on a 500 unit twin from that stretch, this is the bush the parts books call for.
If your book lists E4003 instead of 70-4003, it's the same physical part, just the older reference. Worth checking bore and pin fit before you press one in, since wear on the rod itself can affect how the new bush seats.
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