Before reassembly, check that the conical lower burr and impeller fit well onto the driveshaft with both steel washers underneath. Make sure all parts in the cutaway diagram are accounted for.

The lip on the drive shaft should be within about 0.5mm of the top surface of the conical burr. When the nut is tightened it will squish to about flush.

If the burr is too high above the lip on the shaft (higher than in the pic), try putting the acorn nut + washers on (See 7, 8, 9 in the pic), tightening hard, and then taking it back off.

If the lower burr is still sitting too high on the shaft, take a quick detour to Step 20 and then come back.