Rod bearing wear (S54)
The S54's rod bearings wear with miles and hard use, and a spun bearing destroys the engine. Failures are reported on Z4 Ms past about 75,000 miles, and many owners replace the bearings as a precaution; how often they actually fail is debated.
If a bearing spins: a used S54 is $5.5k to $7.5k before fitting, or a specialist rebuild of the failed engine $16,956, removal and fitting extra. Source: Lang Racing, which sells the rebuild; used-engine asking prices on eBay, October 18, 2026.
How to check
Ask for receipts showing when the rod bearings were replaced and at what mileage, or at least recent oil analysis. Without them, budget for the job, and listen for a knock on a cold start.
