ACL Race Series main bearings for Opel Z28
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ACL Race Series main bearings for Opel Z28, part number 4M2273H. These support the crankshaft in the block, and in a tuned engine, they are responsible for ensuring that oil reaches the entire crankshaft correctly.
These are tri-metal bearings: a SAE 1010 steel base, a copper, lead, and tin layer of approximately 0.3 mm, a nickel barrier, and a surface layer of lead, tin, and copper that conforms to the crankshaft surface and resists seizing.
The upper shells have a full 180° oil groove, and the lower ones have a partial groove to ensure oil flow to the connecting rods is not interrupted; the end of the groove is tapered to prevent pressure spikes. ACL manufactures them with more "crush" than standard bearings to ensure they are held securely and dissipate heat better, along with increased eccentricity, which helps maintain the oil film when the housing deforms at high RPM.
In standard size, ACL specifies this for a crankshaft journal diameter of 67.992 to 68.008 mm, and for that diameter, it recommends an oil clearance of approximately 0.067 to 0.081 mm. 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.
If the crankshaft has been reground or you are unsure of the required size, please write to us with your engine code before ordering.