From Lathes & Co write up.
This is an important step in re assembly don't ask how I know
The top bearing in the shaft assembly is an ordinary ball race, but the lower pair are a pair of angular-contact type. When the assembly is rebuilt some end loading is required on the bearings - and this can be done with a length of tubing, the setting being locked by a grub screw through the top collar (ensure that it bits into a new part of the shaft, not the original dimple). At first the bearings will be slightly tight and to correct this it's necessary to seat them properly by giving the end of the shaft a single smart tap with a 2 lb lump of brass. After this the shaft should spin freely and sweetly.
A good one fetches strong money