BMW S50 / S54 / S55 / S58

BMW M's straight-six — the S50 and S54, then the turbocharged S55 and S58 — in the M3, M4, M2 and Z3 M and Z4 M.

⚑ The S54's rod bearings are the failure this family is known for, and the pattern is well enough documented that preventive replacement on a high-mileage car is normal practice rather than panic. The clearances were tight from new and the bearings wear silently. Oil analysis for bearing metal is the cheap way to ask.

The S54's VANOS unit is the second known item. It fails gradually — a soft bottom end, a rattle at idle, a cam-timing code — and it is a known rebuild rather than a mystery.

The turbocharged S55 and S58 are a different engine and a different fault list: closed-deck blocks, direct injection, and the ordinary consequences of both — intake-valve carbon at mileage, and turbochargers whose life tracks oil condition. The bearing story does not carry across to them.

Establish which is fitted before reading anything: these span thirty years and four generations of M3 under names three characters apart.