i think you put it in before pushing the axle through as the axle has a fixed collar/washer that is too big an ID for the cap to push on over
It wasn't this. It is definitely the cup and cap diameter that has me puzzled.
You are right though about the cap having to be on before the axle.
@neilbah started
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i think you put it in before pushing the axle through as the axle has a fixed collar/washer that is too big an ID for the cap to push on over