ACL Race Series connecting rod bearings for Honda K20A2, K20Z1, K24
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These ACL Race Series connecting rod bearings, part number 4B1972H, are for Honda K20A2, K20Z1, K24 engines. They are the standard choice when opening an engine for performance upgrades.
These are tri-metal bearings: an SAE 1010 steel backing, a copper-lead-tin layer approximately 0.3 mm thick, a nickel barrier, and a lead-tin-copper overlay that conforms to the crankshaft surface and resists seizure. In connecting rod bearings, the overlay is thinner to better withstand fatigue.
Race Series bearings feature increased crush and a slightly more eccentric shape than standard bearings; these two details are designed to ensure the bearing remains seated and well-lubricated as the block and connecting rod deform under load.
In standard size, ACL specifies them for a crankpin diameter of 47.980 to 48.000 mm, with a bearing width of 15.4 mm. For this diameter, they recommend an oil clearance of approximately 0.051 to 0.061 mm, measured with a micrometer and bore gauge. If the crankshaft has been ground, the measurements will change.
They do not have the silver tin final coating of standard bearings, so their dark color—ranging from brown to black and bluish—is their factory finish and does not indicate any defect. ACL advises against polishing the bearing surface to adjust clearance; if more clearance is needed, they should be combined with the HX version, which provides an additional 0.025 mm of clearance.
Before ordering, confirm your crankshaft measurement (standard or ground); if you have any questions, write to us with your engine code.