Atlas bottom end.

Is this an Atlas engine you have previously stripped or one you are assembling from parts? If the latter, are you sure the crankcases are from an Atlas?

New longer conrod bolts?. bigger nuts?.Aftermarket rods?

There is not much clearance in a 750 Commando engine. When I seized my big end, the extra stoke allowed the conrod nut to contact the inside of the crankcases. Do the conrods foul the crankcases when they are constrained to move in the correct manner with the pistons and barrels fitted? If you just have the bottom end assembled but no barrels fitted it will be easy to jam the condos when turning the crank.

Strange??

650 through 20M3 commando all use the same case casting #s.

The unmodified rods... R1 series (650 and early atlas ) through R9 MKIII commando should fit without problem if using original bolts/nuts.

I'll be curious to see what resolves your problem.

Hi Keith - Are you quite sure that you're holding the top of the rods vertical to the crankshaft ? i.e. in the plane of the piston direction. If they aren't quite vertical the nuts will touch at the bottom of the stroke. Also are you using genuine Norton nuts ? The extended Triumph type will touch. Cheers, Howard

Keith, just confirm that you are talking about the nuts and not the bolt heads. I would expect longer nuts to foul but the bolt heads should be seated down in the counterbores of the con rod and come nowhere near the cases

I would expect the bolt heads to be eccentric so probably not a case of just reworking them in a lathe. ( unless you can be bothered to faf about with a four jaw chuck. That process will mark the bolts and who knows where that will lead) As I'm sure many here will know, you need to pay particular attention to the bottom of the counterbored holes in the rod and ensure there is no sliver of loose alloy under the head of the bolt before re-fitting and tightening.

Oops. Tried to delete post. Duplicating someone else's answer.