Most of the “I have an oil leak” diagnoses customers bring in are wrong. Not because the customer is wrong about there being a leak — the puddle under the bike is real — but because they have identified the puddle as the leak. The puddle is just where the oil ended up. The actual source is somewhere uphill, and the oil tracked down the engine until gravity caught it.

Here is how we find the real source. The tools are a degreaser, a rag, a flashlight, and an hour. The hour is not optional.

Clean the engine first

A dirty engine is illegible. Oil mixes with road grime, builds up at every casting joint, runs in tracks that have nothing to do with the current leak. Degrease the engine cold with Simple Green or a citrus-based degreaser, scrub with a brush, rinse it. Dry it with a leaf blower or compressed air if you can. Skip the pressure washer — water past the gaskets is its own problem.

Run the bike for fifteen minutes. Let it cool. Look at where the new oil shows up. The first place oil appears on a clean engine is the source.

The usual suspects, from top down

Triumph twins leak in roughly the same places after sixty years. Walk top-down because gravity does, and most diagnoses get easier when you start at the top.

  • Rocker box covers. Most common. Gaskets get tired, studs get over-torqued, oil weeps from the cover-to-head joint. Copper-faced gaskets, 15 lb-ft on the bolts, no more. About twenty minutes of work.
  • Pushrod tubes. Rubber O-rings at the top and bottom of the tubes shrink and weep. Common on bikes that haven’t had top-end work in two decades. Replace with Viton.
  • Timing cover and the dynamo/magneto drive seal. Pre-unit bikes leak here often. The seal behind the timing cover dries out and the dynamo or magneto drive weeps oil down the front of the case.
  • Primary chaincase. Bottom seam gasket on later bikes, two-piece gasket on earlier. Usually obvious because the leak is on the left side rather than the right.
  • Engine sprocket seal. Oil from the bottom of the primary, but the source is the engine. The seal between the engine and the primary chaincase fails and engine oil migrates into the primary.
  • Crankcase breather. Oil mist coming out the breather pipe and tracking back along the underside of the bike is not a gasket problem. The breather is plugged or the engine is making too much blowby. Pull the breather hose, check for blockage, look at the timing-side bush for wear.

Things people get wrong

Three failure patterns we see often enough to mention.

Silicone everywhere. RTV silicone has its place — sealing gasket joints that need a little extra, particularly on the gearbox top cover — but slathering silicone on every gasket is asking for the silicone to fail at points the gasket cannot reach, and getting the silicone back off when you pull the cover next time will take you longer than the original leak diagnosis would have.

Wrong threads on a British bike. Pre-1969 Triumphs are mostly Whitworth and CEI on the engine fasteners. Later bikes mix UNF and Whitworth. If you have been using metric or SAE bolts on a British bike, the threads in the case are unhappy and the gasket cannot seal. Match what is there.

Overtorquing the rocker covers to chase a leak. Triumph rocker covers are aluminum on aluminum studs. Past 15 lb-ft, the studs strip and the cover warps. You end up with a worse leak than you started with and a head that needs heli-coils. The fix for a weeping cover is a new gasket and even torque, not more force.

A clean engine and a flashlight tells you more than three forum threads.

Which leaks to chase, which to live with

Not every leak needs an immediate fix. A slow weep at the rocker box on a bike that gets ridden hard is not getting worse, and you can add it to the punch list for the next time the head comes off for a valve job. A drop or two on the floor under the timing cover after a long ride is the same.

Leaks that need attention now: anything that puddles, anything from the primary that is flinging onto the rear tire (genuine safety problem), and anything from the timing side that smells like fuel. A fuel smell in your oil means a worn carb slide or a sunk float is dribbling fuel past the rings into the crankcase, and you are diluting the oil.

The reputation

Triumphs leak. Mostly the reputation is overstated. A sorted Triumph that gets ridden regularly stays mostly dry, with a slow weep at the usual places that any vintage British twin will show. The bikes you see with a constant puddle under them are bikes that have been ignored for a year, or have a specific problem that the owner has decided to live with. Neither is necessary.