ACL Race Series main bearings for Honda B20A, B21 DOHC
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ACL Race Series main bearings for Honda B20A, B21 DOHC, part number 5M1957H. These support the crankshaft in the block and, in a tuned engine, are essential for ensuring oil properly reaches the entire crankshaft.
These are tri-metal bearings: a SAE 1010 steel backing, a copper-lead-tin layer approximately 0.3 mm thick, a nickel barrier, and a surface layer of lead, tin, and copper that conforms to the crankshaft surface and resists seizing.
The upper shells feature a full 180° oil groove, while the lower shells have a partial groove to avoid interrupting oil flow to the connecting rods, with a tapered groove end to prevent pressure spikes. ACL manufactures them with more "crush" (housing interference) than standard bearings to ensure they are held securely and dissipate heat more effectively, as well as increased eccentricity to help maintain the oil film when the housing deforms at high RPMs.
In the standard size, ACL specifies them for a crankshaft journal of 54.980 to 55.000 mm, and for this diameter, it recommends an oil clearance of approximately 0.057 to 0.068 mm. If the crankshaft has been reground, the measurement changes.
They do not have the silver-tin final 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.
Before ordering, confirm the measurement of your crankshaft (standard or reground); if you have any questions, write to us with your engine code.