I came across a problem with an oil pump the other day, I assembled the pump and it turned nicely by hand, I then assembled it in the case and tightened it up whilst trying to turn it by hand, I'd barely nipped it and it locked up so I backed it off a bit and it freed off again, but I wasn't happy with the tension on the bolts, they were barely beyond finger tight, so after a lot of head scratching I eventually put it in again with the shaft on the pump turned 180 degrees and hey presto it tightened up to an acceptable torque and still turned by hand. The conclusion was that the slot in the end of the drive shaft was machined a couple of thousandths of an inch over to one side from the centre line which caused a side load on the drive on the pump itself, this must have happened when it was assembled from new. None of the parts were pattern parts and were with the engine since at least the 1960's. The pump appeared in very good condition with no wear as the pump was undamaged I guess they must have spotted this on the production line and turned the shaft 180 degrees to suit, there must have been a slight difference on the machining of the drive too for it to have been assembled in the first place, either that, or it had worn very slightly to compensate for it (it must have been only very slight, but enough to make a difference.)
This just goes to show how important it is to assemble the pump before putting the crank in the cases and checking it will turn by hand.
Just one to watch out for as it's the first time I've ever come across this in many BSA rebuilds
