Trimetal connecting rod bearings BMW M20 M50 M52 M54

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

Introduced in 1977, the M20 was BMW's first compact straight-six: cast-iron block, single overhead camshaft head and two valves per cylinder, offered in 2.0, 2.3, 2.5 and 2.7 litres. In 1990 the M50 replaced it, with an aluminium DOHC head and four valves per cylinder; the 1992 M50TU version introduced VANOS variable intake valve timing.

From this architecture came the M52 (double VANOS, cast-iron then aluminium-silicon blocks) and the M54 up to 3.0 litres, as well as the North American M3 engines S50B30US and S52B32US. They all share the same connecting-rod journal diameter, so a single bearing set covers the whole family. Renowned for the strength of their crankshafts, these engines are highly sought-after tuning bases. As soon as power, engine speed or boost pressure rise, the standard bearings reach their limits and give way to high-performance bearings.

Trimetal connecting rod bearings

The connecting rod bearing links the big end of the rod to the crankshaft journal. It works under a hydrodynamic regime: an oil film permanently separates the bearing from the journal. As power, torque or engine speed increase, the load on this film rises sharply and the standard bearing saturates.

A trimetal bearing stacks a steel back, an intermediate high-load copper-alloy layer and an anti-friction overlay. This construction withstands far higher pressures than the original bimetal bearings, making it suitable for turbo, supercharged and high-rpm engines. The complete set includes 12 half-shells (6 pairs) for a six-cylinder engine.

Technical specifications

Technical guide

The ACL and King rod 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 the 1960s, 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 connecting rod bearing set fits all BMW six-cylinder engines that share the same connecting-rod journal diameter:

OEM reference: 11 24 1 284 849 / 850 / 851 / 852 / 853 / 854, set graded by colour class (red / blue).