When you torque main caps onto a block, the bores they form aren't perfect circles — and they aren't perfectly in line with each other. Casting stresses, machining tolerances, and cap clamping loads all distort the finished bore geometry. Line boring and align honing are the two operations that fix this, and they're the reason you can't just bolt aftermarket main caps onto a block and expect it to run.

Here's the problem: the main bearing saddle in the block and the main cap each contribute a half-circle to the finished bore. When bolted together and torqued to spec, those two halves must form a true circle — and all five (or seven) bores must share a common centerline within about 0.0005 inch (12 microns) across the entire block length. Miss that, and the crankshaft binds, bearings wear unevenly, and you'll spin a bearing under load.

Line boring is the heavy-cut operation. A long boring bar spans all the main bore positions with cutting tools set to a common centerline. It removes metal to correct major misalignment — typically after installing new caps, welding a repair, or converting a two-bolt block to four-bolt. Line boring can remove 0.010–0.030 inch of material and re-establishes the centerline from scratch.

Align honing is the finishing operation. A honing bar with abrasive stones runs through all bores simultaneously, removing 0.0005–0.002 inch to true up the geometry and produce the final bore diameter with proper surface finish. It corrects small deviations but can't fix gross misalignment — that's what line boring is for.

Real-world example: Small-block Chevy builders converting a factory two-bolt main block to aftermarket four-bolt splayed caps must line bore afterward. The new caps have different stiffness and clamping geometry than the originals, so the bore distorts differently under torque. Skip the line bore and the crank will feel tight when you rotate it by hand — and it'll seize the moment oil pressure builds and the bearings load up.

Rule of thumb: After any main cap change, weld repair, or block modification, you need line boring. After normal machining or as a final trueing pass, align honing is enough. Total stock removal during align honing should never exceed 0.002 inch on the diameter — beyond that, you're shifting the crank centerline enough to affect deck height, cam-to-crank distance, and timing chain tension.

This is why swapping main caps isn't a bolt-on job. The caps and block are a matched machined assembly.