The original cassette, a Shimano Mega Range (13-34) says ZH37 on it. Can I replace it with an HG50 (7 speed of course) ... either 12x28 or 14x32? If I can switch, will the rear derailler need adjusting b/c of the different cog sizes?

Right now it is like having 6 speeds because the mega range jumps from 24 to 34 which never is smooth when riding.

Thanks.

Yes, they will both work.
You won't need to adjust the rear derailer, but you might want to check both adjusments and chain length after the change (especially if you go to the 12x28).
See:
http://www.sheldonbrown.com/derailer-adjustment.html

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David Ornee, Western Springs, IL USA

I have a shimano compatible lockring tool which fits like a dream into the new cassette ... but doesn't reach far enough into the hub to engage the lockring on the old cassette.

Is it the right tool? What should I do?

Which tool do you have?
http://www.parktool.com/tools/FR_5G.shtml
should work for all Shimano Cassettes.

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David Ornee, Western Springs, IL USA

Yup, I have the FR-5G ... I think there might be a cover. I see a disk with a couple indentations at 3 and 6 o'clock. And I just saw a pin spanner for a freewheel body cover. Is that what I am looking at?

If you have a freewheel, a cassette lockring tool will not work on it.

I think I'm sunk.

I went to the Park Tools site, Repair Help, Removing and Installing Rear Cogs. My wheel looks just like the one for which they recommend the FR-1 (not a cassette type freewheel at all). Further down Park says there's not much point in threaded freewheel removal as individual cogs are not typically available and the freewheel is not intended for service. Is that what "Alloy sealed" meant on the specs for my rear hub? Sealed up w/ no need of service just toss out like a bic pen when done? (I.e. buy a new one, cassette/cogs and all.)

Any advice?

repeat after me, "pay more for next bike" ...

Well, you may be sunk, but take a look at Sheldon's site on this freewheel at:
http://www.sheldonbrown.com/mega7/
You can get a 7 speed cassette rear wheel or a 8/9 speed cassette rear wheel and use a 7 speed cassette with a spacer.
If there is nothing wrong with what you have now other than only 6 usable rear cogs, just think back to the 3 speed and 5 speed days before digging into your hard earned savings.

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David Ornee, Western Springs, IL USA

I'll just ride alot ... six rear cogs isn't so bad ... just seemed like a waste of space that I was willing to utilize for the price of a cassette. A new wheel is more money ... so I'll go along on my 18 fully functional gears, every once in a while having my legs spin out of control as I jump the 24 to 34 forgetting where I was on the cog. That is a drop annoying. I suppose if I were in San Francisco (I.e., steep), I might appreciate the radical shift.

Thanks for all your help.

Sealed,means the bearings in the hubs have seals to keep crud out. they can be serviced. Freewheels themselves can be serviced, indivudal cogs are available from several sources like loosescrews.com and Sheldon Brown, and the whole freewheel can be replaced.....................The problem you are having with current freewhel may be nothing more than maladjustment. Parktool has the adjustment drill in the repair section.

sounds to me like your rear derailure is not adjusted rite if you are having problems with your chain jumping from 34-24 and you dont have a rapid rise derailure your cable is loose

[QUOTE]Originally posted by swimmeronwheels
[B]The original cassette, a Shimano Mega Range (13-34) says ZH37 on it.

If it the ZH37 is a cassette and not a screw on freewheel the replacement should fit straight on. The jump is because your gear cable is too tight and pulls the chain too close to the 34 tooth sprocket. try scewing in the adjuster on your derailer a little bit say 1/2 a turn to start with. You can't beat the 34 tooth granny gear for mine. If you replace the gear it will probably jump the same from 2nd to 1st gear like the ZH73.

[QUOTE]Originally posted by neile
[B] already established that it is a freewheel.

I'm not having trouble with the chain jumping ... I just don't like the jump. When pedaling even if I would go one gear easier than the 24 tooth cog, jumping all the way to the 34 tooth cog is rather extreme. the gears do the jump smoothly enough. I just don't do alot of sudden inclines here around chicago. I might love it in San Francisco.
The seven gears are set up 13-15-17-19-21-24-34. The first six are really nicely spaced.

I'm just going to work on wearing it out and then see where I am down the road for a new wheel, bike, whatever. Then I'll also know even better exactly what gear setup I want.

There are plenty of 7 speed Shimano comapatible freewheels available with smaller large cog choices (26,28,30.) A LBS with a QBP catalogue should be able to get you any of them. Simple solution.