Bicycle Mechanics - Will an 11 cog wheel work instead of a 13 cog on a Shimano megarange?

I currently have a 13-34 7 speed Shimano Mega 7 Freewheel, and the
smallest cog is a bit worn. I also find the smallest (highest) cog a
bit low, leaving me spinning but not englaging on some downhills. If I
invest in a new 11-34 Shimano Megarange Frerewheel, will I have any
problems with the existing chain / front crankset?

Also, is it possible to change single cog wheels on such a freewheel,
rather than replacing the whole freewheel?

Is your current gearing a freewheel or a cassette? I assume a cassette since there were no 11T freewheels ever made I know about.

You should be able to change the cassette to an 11x34 with no problems IF your freehub body is a "compact", that is it allows the fitting of an 11T high gear cog. Older freehub bodies required a 12T minimum. You can tell by removing the existing cassette. If the splines go all the way to the very end of the body, you can't fit an 11T without some modifications. If the splines end slightly before the end of the body, you are good.

Yes it is possible to change a few cogs without buying a complete cassette but the cost is almost as much as buying a complete new one. I presume the 13T isn't the only worn cog. BTW, you will probably need a new chain if your present cassette is that badly worn.

Is your current gearing a freewheel or a cassette? I assume a cassette since there were no 11T freewheels ever made I know about.

Au contraire, mon freire ;). http://www.sheldonbrown.com/mega7/

C3po - You will have no problems in switching to the new freewheel, though the 11 cog will wear faster than the 13 on your old freewheel. Still, go for it. These freewheels are what, $25? Not too much hurt on the wallet.

I currently have a 13-34 7 speed Shimano Mega 7 Freewheel, and the
smallest cog is a bit worn. I also find the smallest (highest) cog a
bit low, leaving me spinning but not englaging on some downhills. If I
invest in a new 11-34 Shimano Megarange Frerewheel, will I have any
problems with the existing chain / front crankset?

Also, is it possible to change single cog wheels on such a freewheel,
rather than replacing the whole freewheel?
If the sprockets are worn, the chain is definitely toasted. You will
certainly need a new chain. Shouldn't be any issue witth the cranks.
> Also, is it possible to change single cog wheels on such a freewheel,
> rather than replacing the whole freewheel?

No, not possible, the replacement parts are not available. Also, the
11-34 is a totally different mechanism than that used in the 13-34.
Shimano had to completely re-invent the freewheel to get an 11 to work.
The result is the finest freewheel ever made, and it's dirt cheap!
See: http://sheldonbrown.com/mega7
Sheldon "Seven Come Eleven" Brown

Au contraire, mon freire ;). http://www.sheldonbrown.com/mega7/

Whoops, forgot about that one. Thanks for the reminder. It is a recent addition, however, so that's my excuse. :)