In my ever continuing quest to bring my '81 Univega into the 21st century I keep running into snags-

I have been arbitrarily planning to use a 7 speed freewheel with this build and didn't know if it would work until I got a properly spaced hub for the rear wheel. So I got my 126mm Shimano 600 hub finally and threw an old no-name 7 speed freewheel from an old mountain bike onto it and tried to fit it into the frame- no go. It rubs against the dropouts (or more correctly against the backing plate of the adaptor claw).
Out come the calipers- this freewheel is about 36mm wide. So I check the only other freewheel I have for comparison- a 6 speed Shimano and it is roughly 30mm wide. Even though I haven't threaded the 6 speed onto the hub I'm sure it would work much better.

So my question is: Are most 7 speed freewheels about the same width as the one in question or could I find one smaller which may work?

I'm certainly not dedicated to using a 7 speed so going with fewer cogs isn't a big deal- but I am going to be using an 8 speed Shimano 600/Ultegra RD which in theory will work fine with 6 speeds (I will be running with friction shifting)- I just don't know how well I will be able to adjust the limit stops for a 6 speed. Also chainline may be an issue with the seatstay/dropout, though I won't know for sure until I actually mount the cranks and put on a chain.

i think if you can find it, a suntour ultra spaced 7 speed freewheel should work. i'm pretty sure the freewheel body is the width of a standard six speed but has thinner spacers to squeeze in 1 more cog. thus, making it a 7 speed.

for example: https://cgi.ebay.com/SUNTOUR-WINNER-7...QQcmdZViewItem

Will a flat washer or 2 on the drive side help?

Quite possibly. Though if I had to do that I would rather go with a 6 speed and deal with fiddling with the limit stops on the RD.

A seven speed road freewheel should be 31.85mm from the outside to outside of the cogs (5mm spacing cog/cog c-c). This is about 3mm wider than a standard spaced 6 speed (5.3mm spacing) I'm not familiar with MTB equipment but could they be wider? 36mm is too wide for a road 7 speed. Chainline should not be an issue, and your derailleur will handle it easily.

My own experience is consistent with vpiuva's observations. For most hub-and-freewheel combinations, you need 120mm overlock for a standard 5-speed, 122mm for an ultra 6-speed, 126mm for a standard 6-speed and 128mm for a 7-speed (all of which are "ultra" spaced). You can often cheat by a couple of mm either way, through judicious use of spacers and washers.

After much moaning, grinding and cursing, I spaced out my hub to 128 to make my 7-speed work on my Woodrup frame. John E. has it right.

Thanks for all of the valuable information everyone. My decision is pretty clear. 6 speed it is. It's not like I use very many gears while I commute anyway. With every new part or problem I get on this bike I gain loads of knowledge- thanks to all of you.

When I put a 7-speed on my Nishiki Ultimate, the chain also rubbed the dropout and I had to add about 1 mm of washers behind the locknut on the drive side for clearance. Here is the original 6-speed Suntour freewheel, the current 7-speed Shimano twist tooth freewheel and a Shimano 6-speed twist tooth freewheel I also tried showing their dimensions.

Very good illustration OLDYELLR. For people like me who are very visually oriented this is especially helpful.

If you don't mind I would like to see a picture of the way you did this. My problem is that on all of the freewheels I've seen there is a large void between the axle and the rest of the freewheel and I can't quite figure out where the washers should be placed (and what size washers for that matter). So even though I have decided to go with a smaller freewheel- I'm sure that I would appreciate seeing this theory in practice (as would people doing searches in the future).

Thanks.

I don't have a picture and would have to remove the freewheel to show where the washers are. However, I'll explain it using the picture below. I found 2 washers, each about .020" thick, the same diameter as the aluminum spacer. I drilled them out to fit over the axle between the spacer and the locknut on the right. This made the between-locknut dimension of the hub about 1 mm wider, but it still slipped in between the dropouts without much coaxing.



Here is the link to where I found this picture.

Some people are picky about wheel dish etc. Just to keep the rear wheel placement near ideal, I would add a washer on both sides in order to kep the wheel centered between the drop outs while creating the extra clearance on the drive side.

Frankly, I couldn't see that the wheel was off-centre by 0.5mm with the naked eye, but if it helps you sleep better, add 1 mm of washers on the left as well. That would make it slightly harder to get the wheel between the dropouts.