ACL Race Series main bearings for Mini 1275cc (1983+)
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ACL Race Series main bearings for Mini 1275cc (1983+), part number 3M2205H. These support the crankshaft in the block, and in a tuned engine, they are responsible for ensuring oil reaches the entire crankshaft effectively.
Internally, they feature three layers on a high-strength steel backing: a copper-lead-tin lining, a thin nickel barrier, and a lead-tin-copper sliding layer, which is the surface that works against the crankshaft.
The upper bearings feature a full 180° oil groove, while the lower bearings have a partial groove to ensure oil flow to the connecting rods is not restricted, with a tapered groove end to prevent pressure spikes. ACL manufactures them with more "crush" (housing interference) than standard bearings to ensure they are securely seated and dissipate heat better, along with increased eccentricity, which helps maintain the oil film when the housing deforms at high RPMs.
In the standard size, ACL specifies this for a crankshaft journal diameter of 50.831 to 50.843 mm, and recommends an oil clearance of approximately 0.054 to 0.064 mm for this diameter. If the crankshaft has been reground, the measurement changes.
They do not have the final silver-tin plating found on standard bearings, so their dark, brownish-black-bluish color 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 an additional 0.025 mm of clearance.
Not sure if your crankshaft is standard size? Send us your engine code and we will help you choose.