There are three things you need to do:
1. Regularly oil the bearings with a light bearing oil. As soon as the bearings run dry heat will build up and the bearings will seize. This is especially true for the layshaft bearings as these run at much higher revolutions than any of the other bearings and thus will wear much quicker.
2. Make sure there is some side to side play in the layshaft once assembled. Ball bearings are not designed to take lateral forces and if there is no side to side play that is exactly the type of force exerted on the layshaft bearings. You need to sand the layshaft spacer lightly on the side of the layshaft pulleys until you have between 0.3 and 0.5mm side to side play.
3. Most XRay's come with a aluminum rear bulkhead brace that can be installed right behind the layshaft between the two bulkheads (). It is best to install this barce as it makes the two bulkheads more rigid reducing load on the layshaft bearings.
BTW, it is a good idea to keep some spare layshaft bearings with you as these bearings really do take a lot of load due to the high revolutions the layshaft makes and the small size of the layshaft bearings. If well treated they will be good for a long time, but they can break occasionally.
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