I doubt this will be the issue with your crank locking up. The Mains to Bearing Clearances are very small - 0.0006 to 0.0026" / 0.015 to 0.066 mm - and it doesn't take much to up-set these. Everything needs to be super clean, I really can't stress that enough. Clean, then clean again, then clean again, then wipe over with paper towel & paint thinners and you are getting close to how clean it needs to be.
I'll list some checks here, but this is not an exhaustive list.
When you say you've 'tightened up' the bolts on the Main's Caps, was that just spanner tight or tightened to torque ? They need to be torqued up to take up the right shape.
Assemble the caps back on the block, without the shells, torque up the bolts (lightly oil the threads of the bolts). Here you need to check the diameter of the tunnels - it's a bit tricky as there's oilways there you need to avoid with your measuring tools and you'll need to measure each one in a few places to check for roundness. These should be 1.8960 to 1.8965" / 48.158 to 48.171 mm.
Check the backs of the bearing shells that they are super duper clean. If you have your old bearing shells, a loose ball bearing and a micrometer, measure the thickness of them. Start off measuring the diameter of the ball bearing, then place the ball bearing on the inside of the shell. Using the Micrometer measure over the bearing shell and the ball bearing, subtract the diameter of the ball from that figure and that's the shell thickness. Standard size 850 shells are 0.0719" or 1.826 mm thick.
Re-assemble the shells back in to the block and caps, check where you have slid then in that they haven't shaved in the edge over which they were slid in to place, clean that off if there's a small bit of shaving, then without the crank, torque them all up, then release only 1 bolt on each cap. This should open a small gap between the Cap Face and the Block. You should be able to get a feeler gauge in here. There should be a 0.003 to 0.008" / 0.075 to 0.20 mm gap. This is the bearing crush. This is what stops the bearing shell from spinning in the block.
Take that apart, then re-assemble it with the crank, but leave 1 set of shells out and leave that cap off and the crank thrusts. Check that the crank spins and then on the journal where you have left the cap off, measure with your DTI for crank wobble. This will show up if the crank is bent. If everything to this point checks out. Do these checks again, but try it with another set of missing bearing shells - this will show you which set is binding and which set you need to look at closer.
Check to that it's not binding up on the thrusts.
I'll add here too, that it usually take me around 4 - 6 hours to fit a crank in the block and that's if there's no issues to rectify !
20240414_130022.jpg 45.46K
2 downloads
She is an early Mk3 from 1971 and I would like to save it from hobby. 
