| Also known as | B313D, E3999 |
| Diagram | Fig. 1 — CRANKSHAFT AND CONNECTING RODS · ref 3 |
| Subsystem | engine bottom |
| Listed in parts books | 1964–1970, 1974 |
SLUDGE TRAP 350/500
The sludge trap tube fitted inside the crankshaft on Triumph 350/500 unit twins, catching debris before it reaches the oil feed.
The sludge trap is the tube that sits inside the crankshaft on Triumph 350 and 500 unit twins, catching the grit and sludge thrown off by centrifugal force as the crank spins so it doesn't get pumped through the oil feed to the bearings. It's a simple piece of tube, but it does real work keeping the oiling system clean, and it's one of those things nobody looks at until a rebuild forces the issue. The catalog places this part in the crankshaft and connecting rods figure, which is exactly where you'd expect it.
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This part number, 70-3999, also shows up under the earlier alias E3999 and B313D in older records - same physical piece, just different paperwork depending on which parts book you're holding. Given how it lives buried in the crank, most owners only think about this trap during an engine rebuild, when the crank gets split and inspected anyway.
If you're pulling the crank apart for a rebuild, it's worth checking the condition of the sludge trap while you're in there rather than assuming it's fine because the engine ran. A trap that's partially blocked or split can starve the bearings of clean oil, and that's a lot more expensive to fix later than replacing the tube now.
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