That's quite an achievement to grind a crank that much and still be able to get shells to fit, as -040 is smallest ID shell readily available to fit std rods (as far as I know).
BSA originally recommended to grind to -030 but suppliers like DraganFly/SRM can supply -040 for SJ cranks.
I would check you have standard rods? Do you have a pic of the rods and shells? I suppose it's possible (but unusual) to machine "custom" shells to fit std rods!
As per Muscrat's "EEK" post the SJ crank big end journal is 1.460 dia as std, so your crank has been ground to well under the accepted min size.
The rh main is 1.374 as std, so that's well under the std size on your crank as well, but's relatively common to grind off the minimum to make that round again, and then get a custom bush made to suit the resultant journal size.
The lh shaft just needs to be a slight interference fit inside the std roller bearing. It's not used as a bearing surface of course.
BE journals do not wear much before an obvious "knock" occurs so it's unlikely the BE journals have worn to that size, from a std size, so they almost certainly have been ground to those dimensions to suit non std big end shells.
To assess the "state" of a big end journal it's notrmal to measure how OVAL it is as well as it's diameter, as nearly all the wear occurs on the top/bottom of the journal and very little on the journal sides.
Others may disagree but 002 ovality would indicate to me the need to regrind to the next size down. Any ovality adds to the "normal" clearance needed between rod and journal and of course fitting new (round) shells without grinding the journals will not remove that "extra" 002 clearance, so new shells fitted to an oval journal will not last very long.
For my BE journal regrind I asked for 0012 clearance between rod/shell and journal. In my case if/when the ovality has reached 002 there will be 0032 clearance on the top/bottom and that would almost certainly cause a noticable "knock".
As regards buying a used crank, you need to know it's undersize, and you then probably need to assume it will need grinding to the next size down AT LEAST, as it will probably be worn and have oval journals. Effectively this means that a crank already ground to max undersize is not worth buying, one on the next size up is stil a risk, especially if it's rusty or was used well after the knocking started.
Another risk with used cranks is that they are bent or cracked. The seller may be unaware of these faults, either of which might have caused the crank to be "put on the shelf" in the first place. Splines can be worn as well of course. Another fault is that either too small or no radius was ground on the crank during a prior regrind, and this cannot be fixed with a further regrind, once the metal in that corner is gone...it's gone.
A problem I encoutered with fitting -40 shells is that being thicker wall compared to the std shell they take quite a few foot pounds of torque on the rod bolts to make them go round in shape, not so much a problem for me as I was using billet rods which take a higher torque on the bolts than std rods, but it makes me wonder whether (even) if -050 or less shells WERE available for standard rods I wonder if there would be problems fitting them?
Anyone got any experience of buying and fitting BE shells smaller than -040?
