Tri-metal crankshaft main bearings BMW M20 M50 M52 M54 S50 S54

The BMW M20, M50, M52, M54, S50 and S54 straight-six engines

Launched in 1977, the M20 introduced BMW's compact inline six: cast iron block, single overhead camshaft, two valves per cylinder. The M50 followed in 1990 with an aluminium cylinder head, twin camshafts and four valves, joined by VANOS variable valve timing in 1992. From this architecture came the M52, then the M52TU with double VANOS and the M54 up to 3.0 litres, plus the engines of the BMW M3 and Z3 M: S50B30, S50B32, S52B32 and S54B32, up to 360 hp in the M3 CSL. All share the same crankshaft journal and housing dimensions: a single bearing set covers the whole family.

The crankshaft of these engines runs on seven main bearings. Each journal turns between two half shells, one seated in the block and the other in the main bearing cap: they carry the crankshaft. The ACL Race Series and KING RACING tri-metal sets shown here replace them on built engines.

Tri-metal main bearings

A tri-metal bearing stacks three layers: a steel back, a copper-lead intermediate layer with high load capacity and an anti-friction running surface that protects the journal. The complete set contains 14 half shells, seven pairs for seven main bearings. The upper and lower half shells are different: each goes in its own position.

An aluminium bi-metal takes around 8,000 psi, a performance tri-metal up to 12,000 psi, 50% more. Hence better resistance and better durability on turbocharged, supercharged and high-revving engines.

Technical specifications

Technical guide

The ACL and King main bearings shown here are trimetal bearings, designed for engine building and racing. Compared with an original bearing, they handle far higher loads: they cope with increased power, high rpm or the addition of a turbo or supercharger, where a standard bearing quickly reaches its limit.

The two brands

	ACL Bearing Company, an Australian manufacturer of high-performance engine bearings since 1949, offers with its Race Series precise tri-metal bearings, available in many sizes and proven on built engines worldwide.
	King Engine Bearings, an Israeli manufacturer of high-performance engine bearings since the 1960s, offers with its pMax Black tri-metal alloy bearings designed for heavy loads, with a pMaxKote coating available as an option (XPC version).

Construction and advantages

Criterion	ACL Race Series	King Racing
Cross-section
Type	High-performance trimetal bearing	High-performance trimetal bearing
Country of manufacture	Australia	Israel
Construction	Trimetal: steel back, copper layer, soft surface	Trimetal: steel back, copper layer, soft surface
Alloy	Race Series (copper-lead)	pMax Black (hardened copper-lead)
Protective coating	No	pMaxKote (optional, XPC version): polymer coating on the surface that limits wear when lubrication drops (start-ups, thin oil film).
Available sizes	Standard to +0.50 mm (+ increased clearance HX)	Standard to +0.50 mm (+ increased clearance STDX)

Vehicle compatibility

This main bearing set fits all BMW six-cylinder engines sharing the same crankshaft journal and housing dimensions. For the M20, this set applies to engines built from 1988.