Collapsed Main timing side bearing

Oh dear - it’s possible it’s had the “wrong” timing side bearing fitted - apparently it was a common problem a few years ago as people didn’t realise there’s an increased radius chamfer on the bearing to butt up against the crankshaft web on the lightweights, but not there on the heavy twins which uses a similar bearing!

There’s plenty of info on here if you search about it.  If the failure is due to that then suggest you carefully check your crankcase halves for cracking/distortion etc - good luck!

Andy Sochanik & others on here can give far more detailed explanations.

Bruce

There are various ways used to make the cage some better than others, the ones that fail in the AMC gearbox on the layshaft are the ones with the wavy steel or brass cage fixed to together with spot welds or crimped. Best are pinned, phenolic in that application but in the hotter temps in an engine then pinned or solid brass. If you have the cage debris still then look at how the 2 halves were fixed together.

Just to emphasise the point about the inner radius on the crank cheek side of the main bearings of the Lightweight Nortons.

The actual main bearing Dimensions are the same as for the 500 & 600 Dominators, with one small (but vital) difference - the inner radius on the Lightwieght crank is 0.125" - whereas the Domi 88 & 99 uses a radius of 0.090".

This Radius only needs to be one one side of the bearing - but it MUST be fitted with that radius against the crank.

This radius has a big enough difference to be seen by the naked eye.

See the PDF attached.