Replacing a 6 speed freewheel with a 7 speed freewheel - is it simple?

[Gattopardo]

So been poking a bobbin brable, the bike is 6 speed on a freewheel and I'm not sure of the freewheel has faired well.  So was thinking would it be a simple thing of replacing the freewheel and tweaking the rear deraileur?  I know that the chain on the 6 and 7 is the same but is the frame spaced differently?

[Peter]

I have just done the opposite, temporarily, as I'm waiting for a wheel re-build for my 7-speed.  So I've put in the 6-speed rear wheel from my faster bike, just to see if it could be done.  When the bike was 7-speed, I had indexing on the rear derailleur so, to allow for the different spacing of the cogs, I turned the lever to the friction setting (same chain, by the way) and it seems to have worked fine.  Good luck.

[LittleWheelsandBig]

6sp = 126mm OLD, 7sp road = 127mm, effectively the same. 8sp = 130mm road, 135mm MTB.
5sp & 6sp chain was the same width (except Suntour Ultra-6). 7sp chain was narrower and 8sp chain narrower again. Nowadays, 8sp chain is pretty much used for everything <=8sp.

[Peter]

Sure, D.  Both bikes are steel and 40 years old and I'm trying to keep them going without cold-setting them to take modern stuff.  So I'm accommodating by realising I can only ride one bike at a time, so I swap the back wheel between the two.  I hope this will be a temporary fix while I track down builders who can still get hubs for 126 drop-outs.  It's a bit of a faff but ok, so far.  The arthritis in my hands is going to be the ultimate problem, I think.  I have relatively new 6 and 7 speed freewheels and they are easy to swap, which enables me to keep one back for "lightweight" and the other for general purpose - shopping, canals, hills and so on.

PS  Sorry, I may just be getting in the way of the OP.

[FifeingEejit]

I have done similar to wot Peter has, though...

old Raleigh atb
126 old cold set to 130ish
original 6 speed sis shifters retained
8spd wheel with freehub replacing original freewheel 6spd wheel
shifters in friction mode

7spd cassette which is I think an 8spd one with a spacer filling the gap

Sent from my IV2201 using Tapatalk

[LittleWheelsandBig]

Sure, D.  Both bikes are steel and 40 years old and I'm trying to keep them going without cold-setting them to take modern stuff.  So I'm accommodating by realising I can only ride one bike at a time, so I swap the back wheel between the two.  I hope this will be a temporary fix while I track down builders who can still get hubs for 126 drop-outs.  It's a bit of a faff but ok, so far.  The arthritis in my hands is going to be the ultimate problem, I think.  I have relatively new 6 and 7 speed freewheels and they are easy to swap, which enables me to keep one back for "lightweight" and the other for general purpose - shopping, canals, hills and so on.

PS  Sorry, I may just be getting in the way of the OP.

There are a few 126mm OLD hubs still made plus occasional NOS hubs and lightly used hubs/ wheels found in jumble sales and on the 'Bay.
https://velo-orange.com/products/grand-cru-high-flange-freewheel-hub-126mm