What may be the problem is that there are two versions of 6s freewheels. There's the standard 6s which is spaced like 5s except with one extra sprocket, and 6u (ultra 6) which uses 7s spacing with one less sprocket to fit onto 5s hubs.
A 7s freewheel should fit on a hub spaced for standard 6s, with the outer edge of the freewheel being about 2-3mm short of the locknut face. If you try mounting a 7s freewheel to replace a 6u on a 5s hub, the axl won't come out to the freewheel's outer edge.
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