Originally Posted by dooley
So 6mm then? Thanks.
It's just that I want to use minimal dishing.
Not necessarily. The 6mm difference is in the Over Locknut Distance - the distance between the two points on the axle that fit into the bicycle's dropouts. The OLD of your rear wheel axle and the spacing of your frame's rear triangle must match or be very close (within 2mm or so) for the wheel to fit the frame.
When seven speed freewheels were introduced, they were intended to fit on an axle with 126mm OLD, which fit frames with 126mm spacing. A five-speed wheel will have an OLD of 120mm. There are two main implications to all this:
1. In order to upgrade to seven-speed, you must re-space frame, through cold-setting (translation: bending it). If you don't have a steel frame, this is inadvisable, and you're really stuck with five speeds.
2. You will indeed need to re-dish the rear wheel, although not necessarily by 6mm. That has to do with a combination of the new OLD and difference in size between the five and seven-speed freewheels. In fact, I would hope that you don't have to re-dish by as much as 6mm - you are adding two cogs, sure, but the space between the cogs is also significantly reduced. I really have no idea, but I would hazard a guess that you won't need to change the dish by any more than 4mm or so.
This is, of course, assuming that your current hub and axle will fit a frame with 126mm spacing. It isn't guaranteed. It isn't necessary to buy a new hub, but if the axle is too short... good luck finding a replacement. You might just want to buy or build yourself a new rear wheel. Good luck with this project!