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I am servicing the front hubs on two identical bikes. Upon reassembly, one bike rides fine, but the other one the hub immediately became loose (quick release axle, loose bearings). I tightened the cone and locknut, but the same thing happened again.
I measured the width of the hubs with electric calipers. The one which always stays is 99.8mm across. The one which keeps coming loose is only about 99.4mm. The dropouts are 100mm wide.
Could this point me to the fault? Is it possible something is missing inside? 0.5mm is too thin for a washer, surely.
Any chance that the bearing race(s) is gubbed in the one causing trouble?
I wouldn't take too much notice in the measurements as this could just be differences in manufacturing.
If the cones are coming loose after they have been properly tightened against the locknut then I would look at the threads on the axle. If the axle has stretched at all then nuts may loosen. You can usually tell by unscrewing the nut along the length of axle. If the feel tight at any point then the axle has stretched.
If not it could be a bent axle - but you should feel that when adjusting the bearings. The other thing could be the cup is starting to fail in the hub - I've only seen this once and there was an obvious crack in the cup surface. But it would cause flex.
Which hubs are they out of interest