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

The running clearance is checked during assembly (bore gauge or plastic gauge); the instructions supplied with the set always take precedence.

Technical guide

The brands

	ACL Bearing Company, Australian manufacturer of engine bearings. Its Race Series range offers high-performance trimetal bearings: hardened steel back, high-load copper-lead bonding layer and conformable overlay, machined and metrology-inspected.
	King Engine Bearings, engine bearing manufacturer founded in 1960. Its performance range is based on the pMax Black trimetal alloy, with the optional factory-applied pMaxKote polymer coating to improve wear resistance.

ACL Race Series trimetal construction

Cross-section of an ACL Race Series trimetal bearing, from the sliding surface to the back:
Overlay (lead-tin-copper): anti-seize, low friction, conformable.
Nickel barrier: separation layer.
Bonding layer (copper-lead-tin): fatigue strength, high load capacity.
Steel back (SAE 1010, high strength): rigidity and retention in the housing.

Higher load-carrying capacity

The copper-lead bonding layer withstands significantly higher pressures than the original aluminium-tin bimetal bearings, improving reliability under heavy boost or at high engine speed.

King Racing: pMax Black and pMaxKote coating

Cross-section of a King Racing bearing. The XP range is based on the pMax Black trimetal alloy (steel back, copper-lead layer, hardening treatment). The XPC version adds the factory-applied pMaxKote polymer coating, with no added thickness, which improves wear resistance and protects the bearing under marginal lubrication (start-up, intensive use).

How to choose your size

Checking the clearance during assembly

Once the bearings are fitted, check the running clearance: the tiny gap between the bearing and the crankshaft journal, through which the oil film flows. Too small, the engine overheats; too large, oil pressure drops. The simplest method is the calibrated strip (Plastigauge): lay a strip on the journal, torque the rod cap to spec, then compare the crushed strip against the supplied gauge, which reads the clearance directly in millimetres. Always follow your engine builder's specification and the instructions supplied with the set.

Order the Plastigauge →

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).