ACL Race Series connecting rod bearings for Mazda MX-5 NA/NB, 323... (B6/B6T, BP/BP-T, ZL-VE, ZM-DE)
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ACL Race Series connecting rod bearings for Mazda MX-5 NA/NB, 323... (B6/B6T, BP/BP-T, ZL-VE, ZM-DE) (part number 4B8351H), designed for engines that will operate under significantly higher loads and RPMs than stock.
These are tri-metal bearings: a SAE 1010 steel backing, a copper, lead, and 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 seizure. In the connecting rod bearings, the surface layer is thinner to better withstand fatigue.
ACL manufactures them with more "crush" (housing interference) than a standard bearing to ensure they stay securely in place and dissipate heat better, as well as greater eccentricity, which helps maintain the oil film when the housing deforms at high RPMs.
In standard size, ACL specifies them for a connecting rod journal of 44.940 to 44.956 mm, with a bearing width of 17.0 mm. For this diameter, they recommend an oil clearance of approximately 0.049 to 0.058 mm, measured with a micrometer and bore gauge. If the crankshaft has been reground, the measurements change.
They do not have the final silver-tin plating of standard bearings, so their dark, brownish-black and bluish color is factory-standard 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 standard size? Write to us with your engine code and we will help you choose.