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Taper tolerances of main journal bores when measuring for bearing clearances

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I've had an align bore performed on my Honda B18B block since I'll be running an aftermarket girdle and arp main studs to ensure minimal distortion.

With the main caps installed and no bearings fitted, I've measured the main journal bore to size up the main bearings correctly. When measuring both sides of the ID of each main cap (ie each side of the oil supply gallery in the block) I've found taper between each side by as much as 0.8 thou.

Each engine will have its own set of limits in terms of taper and out of round but what is would be a rule of thumb tolerance in this instance? After an align bore it should be pretty close to perfect I would have thought. Should I take it back to the machinist?

If you've had the main bearing journal line honed then it should be perfect. A taper of 0.8 thou is massive and certainly not something I'd expect or accept from a freshly honed bore. This sort of variation will affect both the crush on the bearing shells (which is essential for proper shell retention), and correct and accurate control of oil clearances. I'd be talking to your machinist.

Great, thanks Andre!