jonwvara 

I have customized lots of freewheels, but have limited experience with cassettes, so I'm reaching out for a little help.

Here's the deal: I would like to build up an 8-speed cassette for my touring bike that would let me use a 13-tooth small cog instead of the (to me) uselessly-high 11-tooth cog that seems to be the default choice on every 8-speed cassettes I have ever seen or heard of.

Per Sheldon Brown, it appears that I can disassemble new 11-28 HG50, ditch the original 11 top cog and 13 second cog, add a new 34-tooth cog and spacer on the other side of the 28, and install a new 13-tooth first-position cog.

I can tear down and reassemble a cassette easily enough. But what's not clear to me is whether it's possible to use a 7-speed cog as the 13--since, as far as I have been able to tell, no one ever made a designated 8-speed 13 for the top position.

I'm hoping I can get away with it even if the spacing is a tad different, since I'm still using friction shifters. (This is C&V, right?) But I'm not sure about threading, etc.

Enlighten me! Please!

rccardr 

Yes, it will work just fine as long as you use a genuine 13 tooth first position cog and lockring.
Friction makes it all so easy, there just needs to be sufficient space between the first and second cog for the 8 speed chain, no problemo with a 7 speed first cog.

Dfrost 

I’ve got several 13T and even a 14T first position cog in my stash of loose cogs. They probably came from 7-speed cassettes that I’ve torn down. BTW, spacing between 1st and 2nd cogs doesn’t need to be all that precise, even for 8-speed indexing.

jonwvara 

Well, heck.

I don't want to mix old and new cogs, so my plan had been to start with a new 8-speed Shimano 11-28 cassette. I can source one of those okay, but I'm striking out on finding a 13-tooth first position cog in 7 speed. I can find a new 9-speed cog in that size, but I wonder if the ever-so-slightly narrower spacing (relative to 8 speed) will be an issue.

No luck at all in finding a 34-tooth 8-speed cog. Anyone know of a source for one?

BradH

13-26 8 speed should be easy enough to find. 13-23 used to be common but sadly, for me, has been discontinued.

On paper 9 speed cogs are thinner. Actual measurements indicate they are the same. The HG50 models are nice for cannibalizing. All the cogs are loose once the rivets are ground off.

I agree on the 11t being useless.

Dave Mayer

Yes, you can apply a 13-tooth cog on the front and dispense with the useless 11 and 12-tooth cogs.

13-28 used to be the standard 7-speed cassette spec. 7 and 8-speed spacing is close enough that you can mix and match the cogs, but use your 7-speed spacers.

There is something rattling around in my brain that indicates that the cog spacing for the smallest cog in a Shimano 7-speed stack is larger than the rest of the cogs. 7-speed spacing is nominally 5.0mm cog center to center, but I think the first spacer (built in) is thicker. But you should research. Perhaps some grinding is in order. IIRC, 8-speed spacing is all uniform at 4.8mm cog center to center. In the real world, 0.2mm variation is immaterial.

The ultimate test is to inspect your bastardized new cassette from the side to check whether the spacing looks even. You may have to apply one or more shims to correct the spacing.

Make sure you use a lockring appropriate for the 13 small cog. Shimano lockrings come in two flavors: ones for a 11-tooth, and a larger diameter one for 12 and larger first cogs. Using the 11-tooth type lockring will not provide enough engagement on the small cog.

bamboobike4

Make sure you clear the seat stay with that 13T. Some frames keep it pretty tight back there. I'd imagine a more classic steel would have room.

rccardr 

It’s friction shifted, spacing is irrelevant as long as the lock ring cinches down tightly and the chain fits the space between cogs.
To answer your question, Jon, a 9 speed 13 tooth would work just fine. If you want to get really precise, there are thin spacers out there as thin as half a mm.
But…not necessary.

TPL

Friction ....the thinking man's first choice

John E

Been there ... done that. Using a screwdriver and a drill, respectively, I have removed the pesky screws or rivets holding together those detestable "spiders," so that I can gear-phreak properly. I built a 12-13-15-17-19-21-24-28 cassette for my mountain bike, mixing and matching cogs from two different Shimano cassettes. Works like a champ with my 7-speed lever in friction (sensible) mode.