I'm having a local machine shop work over the bottom end of a motor for my IT car. This motor was overrev'd, but continued to run until it dropped a cylinder about 2 hours into an enduro. The head was pretty well beat up, but we've gotten that back into shape with a bucket of new parts and a bit of machining. The machine shop's evaluation of the bottom end is more surprising.

Appparently the crank is within spec, but the mains come out with only 0.0005" clearance -- apparently *smaller* than the spec range. The machinst says he'd much rather see 0.0015" to 0.0020" clearance for an enduro motor. He's recommending align honing the mains to open up the clearance. I suppose an equivalent move would be to turn down the crank, but I haven't asked him about that.

I'm puzzled by the tight (but consistent) clearances on the mains. I'm surprised the clearances would be that far under spec on a used motor that was running well and didn't seem to have any oiling issues (at least that were apparent in the teardown).

Any insight would be appreciated. I thought the head would be the costly part and the bottom end would be a routine cleanup, measure, and reassembly with new bearings. Figures I'd be 180 deg off on that!

tom