channelz28

I have an late 80's/early 90's mountain bike with a 6-speed freewheel. I acquired a Megarange 7-speed freewheel and figured it would be an easy swap. When I tighten the axel nuts on the wheel the whole thing locks up. The freewheel won't spin and the wheel gets really tight. I couldn't see what was wrong right away and before I try to figure it out I thought I would see if it is doable. Sheldon Brown says it is.

Any easy check as for why it would lock up? On another note, the gears are a little bit loose. Is that normal or is the freewheel broken?

operator

It locks up because the freewheel is probably contacting the frame, check visually

dabac

Keep in mind that if you ran indexed shifting on the 6-speed, the indexing is unlikely to work very well on a 7-speed freewheel.

Well...
Doing the axle nuts up shouldn't mess with the bearing adjustment (much) so my guess would be that the new freewheel is a tad wider than the old, and when you do the axle nuts up the freewheel gets pinched against the inside face of the dropout. You may have to undo the locknut and add a spacer/washer to create clearance. Maybe even shorten the stack on the left side and tweak the dish on the wheel a bit.

Depends.

If the sprockets wobble against each other, then something is wrong. If all sprockets wobble a little as a unit, then it's OK.

The sprockets should be firmly attached to the freewheel shell, but it's very common for the freewheel shell to have a bit of play in it.
Remember that the parts of the freewheel are only moving against each other when you're freewheeling, so those bearings pretty much only have to carry the load of the chain. Easy job.

mrrabbit 

Measure on the hub:

Freewheel Stop to End of Locknut

If = 31.5mm = No
If = 36mm - 37mm = Road 7-Speed Freewheel (MF-HG-37 7-Speed)
If = 39mm = MTB 7-Speed Freewheel (MF-HG-40 7-Speed MegaRange)

=8-)

channelz28

Thanks for all the replies. I made a list of all these things that it could possibly be, I then went outside to check it out. As i was taking the wheel off, I noticed that there is a funky spacer stuffed in one of the dropouts as a stop of some sort. Well it's about 2mm wider than the dropout and that's what was pressing against the freewheel. I took it out and now it's all good. I guess I need to be more observant.

FBinNY 

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.