Hello. I have been riding my c.1985 Centurion Ironman for a month or so now after dusting it off and installing some replacement parts. I have been wondering how to get a bit lower gearing (I am not as young as I used to be). It came with a 52/42 chain ring and Shimano 600 13 - 24 6-speed freewheel. So, I thought I would look into upgrading to a 7-speed freewheel. I did some reading on the subject and it sounded like it might just work. Some articles said that you might have to widen the rear frame and re-dish the rear wheel due to the chain hitting the frame. Well, I thought I would go for it.

I ordered a Shimano 7-speed Hyperglide freewheel (13 - 28, same exact sprockets except for the 28-tooth) and a SRAM chain from Harris Cyclery. When I set the two freewheels next to each other, the 7-speed was about a half-cog wider than the 6-speed. I popped the freewheel on and, lo-and-behold, the rear wheel still fit on the bike. I shifted the chain into the highest gear and the chain still had (a little) clearance. Another worry was whether the rear Shimano 600 derailer would be able to shift to the 28-tooth sprocket; that too worked. I took it for a spin and it all works great.

Now I have a 14-speed bike with a decent low gear for all those hills (in Western Washington we have lots of hills).

I did the same thing with my '86 IM. The 7s slipped right in.

You might get a bit of chain rub against the chainstay by going from 6 to 7. You'll have to decide (if that occurs) whether you can live with that or not.

My bike is set at 128 mm in the rear so I changed out to a longer axle (one designed for 130 mm hub), placed 3 mm of spacers on the non-drive wheel side (and 1 mm on the freewheel side) and redished the wheel (less dish that way so it should be a stronger wheel). This may be more complicated than you want to deal with and someone here may have a simpler solution.

Congrats! If that didn't work, Shimano still makes 14-28 6-speed freewheels, too.

Getting that 7th 28t sprocket in, at no cost to wheel or axle strength, sounds like a win-win to me.

I did the same thing on my 1988 Cannondale. It worked good. It sounds like the same setup I used. I already had 7spd RSX DT shifters, so it was a low cost upgrade.

I have done several 6-to-7 conversions, with mixed results, in the sense that sometimes I end up w/ chain-to-chainstay rub, even though I never use a spoke protector/pie plate. The only fix I know is to insert a washer or two on the drive side, then recenter the axle and rim.

One more option for lowering the gearing further would be to replace the 42T chainring. If it is 130BCD you can find 39T a chainring as a replacement.

Good for you that the 7 speed freewheel worked out! It is certainly a good feeling when what is planned out on paper or in our heads works in reality when all the parts arrive!

I'm glad it worked for you. I did the same thing on my Fuji, except it was Suntour 14-24 6-speed to Suntour 13-28 7-speed. I ended up swapping a few cogs on the new freewheel to increase the range to 13-32. I had to add a small spacer to the axle to keep the outermost sprocket from rubbing the chainstay. I had to switch to 7-speed chain, of course. And it took a little trial and error to get the chain length right so that the existing derailleurs could handle the range. But other than that, it was pretty easy. Now I need to replace the decal that says "18 SPEED" on the chainstay, since my bike is now a 21 speed.

I did the same thing a couple of years ago on my Colnago. Ended up with an IRD instead 7 spd 13-28t (13,15,17,19,21,24,28)
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+1 Works most of the time, occasionally requires a thin washer to avoid chain rub (maybe one out of ten).

... this would be a good place to mention chain compatibility, but I'll leave that to the guru[s].

Just chiming in to say I also was lucky on the '83 SR. Just stuck the 7-spd Sunrace chrome freewheel on there and adjusted the Cyclone II. The extra gear does make a difference. It's a friction setup, so...

I have done this on about a dozen bikes most of the time it works fine. There have only been a couple cases where I actually needed to change the spacing and dish.

I recently acquired an 89 ST 400 that someone converted into a flatbar bike at some point. It's got modern Shimano 7 Speed trigger shifters that are in perfect condition, so I'd like to be able to take advantage of a full 7 Speed conversion. What cassette did you use ? What tool do I need ? Help is greatly appreciated. Thanks.

Actually, when I did that conversion I used a Shimano Freewheel. Since your bike is an '89 it might have a cassette. Here is a good Sheldon Brown description of the two types:

https://www.sheldonbrown.com/k7.html

The article also describes the removal installation process. If you don't have the tool, most bike shops will remove cassettes/freewheels for a couple bucks. I've even had them removed for free.
I like the SRAM cassettes, but shimano are good too. That really is all there is too it. Since you're replacing the rear cassette, get a new chain too. The Park Tool website has good videos on how to adjust the rear derailleur for the increased rear capacity.

https://www.parktool.com/

... or even a 38, giving you almost a full -- and noticeable -- 10 percent drop in gear ratio.

You can do everything with a 47-38 combo. that you can with a 52-42 -- just 10% less top speed and 10% easier climbing.

I just found a 128mm 46T Nervar ring for my 1959 Capo, so now I have a very nice 1.5-step 46-38 / 13-15-17-19-21-23. If I need to go lower, an easy-to-find 13-26 7-speed would be my logical next step.

Thanks for your response. Finished last night, was straightforward. Used penetrating oil and FR-3 tool to remove the Suntour freewheel. Replaced it with Shimano MF-HG37 13-28. Cleaned up the front chainrings, replaced the chain with KMC X8.93 & replaced the badly worn Suntour rear derailleur with a Shimano STX that came off a 1995 Raleigh M-80 that recently became a parts bike. Thanks for the links. I read the first one, and I've been setting up rear & front mech's for a while so no issue there. This bike shifts and looks 100% better now, I may sell it, I may not. I'll post pics after I take them later.