by Michelle (LaRose Racing)
If you hook up a stoboscopic timing light, and watch the light while the engine is running, you can determine if the noise is tapping every other revolution of the engine (noise coincides with light), or every revolution (noise taps twice for every time light comes on).
Noises that coincide with timing light are "top end" noises (valvetrain, valves, cam, lifters, etc), and noises that sound twice for every blink of the light, are "bottom end" noises (crank, rods, pistons, etc)
In order to determine #2 main bearing wear to decide if line boring is necessary, you can lay a machinist's straight edge (a ruler may not be completely flat/straight) across bearing saddles #1, 2, and 3 (not 4) in the case half you are working on, with the straight edge carefully held in a straight-up position (don't tilt it). Try running an .001 or .0015 feeler gauge under all 3 bearing saddles at the straight edge. If the feeler gauge goes under the straight edge at any of the 3 bearing saddles, then align boring is a good idea.
Be sure to get a reputable shop to do it--don't trust just anybody. BTW, it's *definitely* not something you can do yourself, to be sure. Just be certain that the case that is candidate for align boring is worthy of the investment---some other things can be wrong with the case, like the lifter bores (oversize), cracks behind the flywheel, etc.