The brief
Mr Tan was referred to us by a regular customer with a troubling engine oil leak, a film building up under the engine and the level dropping between checks. He brought it in. An oil leak only spreads, the loss adds up, and an engine running low risks low oil pressure and damage. The trail led to the oil sump drain bolt, the plug at the bottom of the sump you take out to drain the oil. It seals with a soft washer that's meant to be renewed every oil change, and the bolt itself can be damaged if it's over-tightened or cross-threaded somewhere down the line. Once the bolt or its washer is past it, oil weeps out around the plug and drips. Some bolts aren't worth saving, so the drain bolt and its washer needed renewing, with the sump thread checked while it was open.
The diagnosis
With the engine cleaned off and run, the leak traced to the sump drain bolt, oil weeping from around the plug, not from the sump gasket or anywhere higher up. The drain bolt and its sealing washer were damaged, and the sump thread was checked and found sound. The rest of the engine's seals checked out dry. That's a straightforward fix: a new genuine BMW-spec drain bolt and a fresh sealing washer, torqued to spec, with the thread checked clean.
The work
The oil was drained, the damaged drain bolt removed, the sump thread cleaned and checked, and a new genuine BMW-spec drain bolt fitted with a fresh sealing washer, torqued to the manufacturer's spec, not over-tightened. Fresh oil and a new filter to finish, and the engine run and checked warm for any weep. A road test confirmed the sump was dry and the level held.
The outcome
No more oil under the engine, the drain bolt sealed and dry, and the oil level holding between checks. The BMW went home with the leak sorted at the cheap end of the job. A damaged drain bolt only weeps worse, but renewing it and the washer and torquing it properly stopped the leak, a quick, tidy fix.
