According to
THIS THREAD, the MP40
QD uses NMB 608Z bearings on both sides. The older regular MP40's used two different bearings, 607 (7mm x 19mm x 6mm) and a 698 (8mm x 19mm x 6mm).
I'm including
THIS THREAD on my replacement of a regular MP40 (that is, no QD) that discusses the most difficult part of this process.....picking the right bearings.
Brand does not matter (in this case NMB).
608 is the bearing size and type:
6 -> bearing series (deep groove in this case)
8 -> 8mm bore diameter
The 600 series are deep groove bearings with a 22mm outer diameter and 7mm thickness.
The letter indicates the type of dust cover over the race (the balls and their holder):
If one side has a metal dust cover, it's a
608Z. If both sides, it's a
608ZZ. One side rubber, it's a
608RS. Both sides rubber, it's a
608-2RS.
There are other designators, but not important.
Not all 608 bearings are created equal. Cheaper bearings have looser tolerances and less durable materials. Better bearings have tighter tolerances and more durable materials. The dust covers and tight tolerances can cause more friction for the motor to overcome but may be quieter and last longer. Rubber seals are more protective but create more friction than metal, and metal more than open (exposed races). The tradeoff is dirt ingress, which causes friction and damage.
Some bearings have removable rubber dust covers for cleaning and re-lubrication. Metal shields (Z/ZZ) are usually not removable without damaging them.
So there is a world of difference between a 608z or zz that cost $0.50 cents, and one that cost $10.50. As crazy as it sounds, the cheaper (less accurate) bearing may fare better in the powerhead, due to less built in friction.
In any case, never (ever ever ever) "clean" or "oil" these types of bearings with WD40 or any other solvent, especially those with metal races. They may spin better in your hand, but you just washed out all of the lubricant...
