6. Here we see the machinist measuring the main bores before line honing. Just like cylinder honing, the main bore housings must be honed to size and made sure that they are straight. Any time a different fastener is introduced to the mains (main studs, different bolts, main girdles, and so on), it’s best to have the block at least checked, if not honed. A nice, straight main housing allows the rotating assembly to turn much easier and not irregularly wear the main bearings.