ACL Race Series connecting rod bearings for BMW V8 S63B44 (4.4L twin-turbo, M5, M6, M8, X5M, X6M) and N63B44
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ACL Race Series connecting rod bearings for BMW V8 S63B44 (4.4L twin-turbo, M5, M6, M8, X5M, X6M) and N63B44 (part number 8B1578H), designed for engines that will operate under significantly higher loads and RPMs than stock.
The structure is trimetal, consisting of a steel backing, copper-lead-tin lining, a nickel barrier, and a soft surface overlay that adapts and prevents seizing even under very high loads. In the connecting rod bearings, the overlay is thinner to better withstand fatigue.
ACL manufactures them with more "crush" (housing interference) than standard bearings to ensure they are securely seated and dissipate heat better, and with more eccentricity, which helps maintain the oil film when the housing deforms at high RPMs.
In the standard size, ACL specifies them for a connecting rod journal of 53.971 to 53.990 mm, with a bearing width of 15.0 mm. For this diameter, they recommend an oil clearance of approximately 0.056 to 0.067 mm, measured with a micrometer and bore gauge. If the crankshaft has been reground, the measurement changes.
They do not have the silver-tin flash plating of standard bearings, so their dark color—ranging from brown and black to bluish—is the factory finish and does not indicate any defect. ACL advises against polishing the bearing surface to adjust clearance; if more is needed, they should be combined with the HX version, which provides 0.025 mm of additional clearance.
Not sure if your crankshaft is at standard size? Write to us with your engine code and we will help you choose.