My recent purchase of a very tidy looking B31 appears to have been a mistake. Not only does it need to bored out to +60 but I think the crank bearings may be worn.
I am assuming that there should be absolutely no up and down movement on the conrod. Unfortunately I think I can feel a tiny bit of up and down but it's difficult to Identify because of the side to side movement.
Are there any tolerances on the crankshaft to allow for movement ?
I am not looking forward to stripping the engine completely, my confidence is starting to go a little.
The Haynes manual says side clearance should be 0.010 -0.012" (0.25 - 0.30mm), but as you assume there should be no vertical play at all. Could you slide a feeler gauge or two down each side of the big end to eliminate the side clearance, then try again? Good luck.
I have decided to do the job properly and replace all the bottom end bearings. I think there is some radial wear. I have also been advised that rebuilding the top end is pointless if the bottom end is not running a 100% it could cause bottom end wear eventually
If it is worth doing......etc
The good thing is that if done properly and you change oil regularly a well set up B31 engine will probably survive forever!