B58 buyer's guide: the modern turbo six, what fails by mileage, the inspection list

The B58 is the engine the community points to when asked which turbo six to buy: closed-deck block, a mature single twin-scroll turbo, the heat-management architecture the N54 never had, and a chain drive that was redesigned once and redesigned well. For a buyer the questions are narrower than on the older sixes — which revision, what coolant history, how much oil does it eat — and this page walks them.

The engine in one paragraph

Built from 2015 across the G-series platform — 340i/M140i/M240i (F- and G-chassis), 440i, 540i/640i/740i (G11/G12 era), X3 M40i/X5 40i/Z4 M40i, the G20 M340i, and the Supra 3.0 — with the 2018+ technical update (TU) carrying the one-piece chain drive redesign the factory's own functional description documents. Water-to-air charge cooling from the factory (the FMEM conversation is an N54/N55 topic, not this engine's), direct injection, and a revision-split data model our b58-factory-spec-sheet carries row by row.

The revision question — the first thing to establish

The chain-drive redesign is the TU's signature change and the community's shorthand for "the later engine is the safer buy." Both revisions are good engines; the TU removed the two-chain drive's complexity from the maintenance conversation.

What fails by mileage — the community curve, labeled

Community evidence, labeled as community. Our guides own the fixes.

The inspection checklist — one hour, in order

1. 01Cold start, untouched. First-second chain quiet (the TU especially), no blue puff, idle stability as the valvetronic-free warmup completes.
2. 02Full code read before the drive — manufacturer layer; the B58's DME history maps misfires, mixture adaptation, and the cooling-circuit codes in one pull.
3. 03Coolant: level against the tank's marks cold AND warm, stains at every hose junction, the tank cap's seal. The heat-management circuit means a small leak is a managed event and a neglected one is a limp event — the bleed law explains why.
4. 04Oil: level, color, cap — and the consumption conversation. Ask the number directly: how much between services? Half a litre per 3,000 miles is one conversation; a litre is another; "never checked" is the answer that ends negotiations on price.
5. 05xDrive cars: the 7-litre sump awareness. A shop that has been filling a 7.0/7.2-litre sump with 6.5 litres has been under-filling it for years — the factory overview data carries both rows and the spec sheet splits them.
6. 06The drive: smooth boost, smooth heat management (temperature rock-steady regardless of load), no misfire counters climbing at idle.
7. 07Tuned cars: flash history and boost honesty — the what-is-a-flash-tune page gives you the questions; the B58's tuning headroom is wide and the Stage conversation is well-documented.
8. 08Records: coolant-top-up frequency is the B58's tell. A car with a "topped it off monthly" story is a car with a leak that was never found.

The crankhub question — answered structurally

The B58 has no crankhub: no friction plates at the crank nose, no central bolt, no slip conversation. The damper bolts directly on with four single-use screws. The crankhub guides are for the older sixes; this engine's front-of-engine conversation is the belt and the water pump.

Years and options to seek — community consensus, labeled

Seek: 2019+ TU cars (M340i, Z4 M40i, X3 M40i, Supra 3.0) for the redesigned chain drive and the mature everything; the 2016–2018 revision-0 cars as the value pick with records. Options that matter more than folklore: the cooling and oil coolers of the M-performance variants for tuned ambitions. Treat with more questions: any car with an unnamed "coolant smell sometimes"; tuned cars without flash-slot transparency; high-mile cars with "doesn't burn a drop" claims that the dipstick contradicts.

The ownership math — honest

The B58 is the cheapest of the three to keep healthy: the wear-item cycle is plugs/coils/oil with long intervals, the decade items are plastics and the electric pump, and the catastrophe list is short and rare. The honest asterisks: oil consumption on high-mile cars is real and priced into nothing but your future, and the cooling circuit's complexity means small leaks find you early — which is a feature if you listen. The b58-factory-spec-sheet carries every number this engine's data holds; the torque master index carries its fasteners.