Cog threading question...
 

Yo dudes,

So I got an 18t Roselli cog today, to put on my Zeus rear hub. The threading on the cog is 1.37 x 24TPI and I was assured the size was correct by a suitably reputable source (Shifter Dan).

The problem is this: When I start to thread it on, I get about half a turn in and it catches. I put a little pressure on it with my chain whip, thinking there might have been a minor deviation in the threading or something. Problem was, it didn't get easier to turn.

When I removed the cog, it looks like it's eating the hub threading alive. Nasty burrs and bit of metal coming off. It's only on the start of the hub threading, as I didn't go any further than that, so I haven't trashed the hub.

I'm guessing it's not really a good thing, but this is the first fixed gear I've built so I'm no expert on the matter.

Am I doing it wrong?

Any help is much appreciated, cheers.

make sure youre putting the cog on in the right direction....(thats probably not the case, but you never know)

Are the threads clean? There might be a little speck of sumthing in there.....

Cog's going on in the right direction, and the threads are clean. It's not the kind of resistance that comes from a dirty thread, as it's actually tearing the threading.

Shifter Dan is obviously no longer a reliable source.

cross threaded that *****. lbs should be able to chase it and maybe salvage your hub.

I'm with thatcher. You just tried to thread it on at the wrong angle.
With some concentration you should be able to thread it on right. Trouble is, now it'll try to follow the "threads" you just made so you'll have to force it on straight to make it catch the real threads.
1.37*24 is the standard size. Do look up Zeus to see if it's standard size.

Don't know if it helps (surly website) :

NOTE: Two reasons why cogs don’t always fit perfectly on hubs the first time:

1. Our cogs use ISO standard threads: 1.375" x 24TPI (34.92mm x 1.058mm). This threading was chosen due to its compatibility with other threads used on bicycle hubs. ISO threads fit English and Italian threads almost perfectly. It is important to not switch back and forth between cogs and/or hubs with different threads. FYI, the only hub that an ISO thread is not compatible with is one with French threading (34.7mm x 1mm).

2. We've found that the diameter of hub threads vary wildly from brand to brand. As such, we have tweaked our cogs to fit all hubs safely at the expense of being a little bit tight on brands that tend to be on the large side of an acceptable tolerance. American ClassicTM comes to mind; our cogs require a little extra force to install on these hubs than usual. In certain cases, only a professional mechanic will be able to tell the difference between a cog that is simply tight-fitting and one that is crossthreading on. If in doubt, call a pro!! We've also found that thread lengths vary from manufacturer to manufacturer. Unfortunately, there is no standard at this time. Our cogs are 7.4mm thick. They work perfectly on most hubs, but you may occasionally find a hub that has threads longer than 7.4mm. Or, you may find a hub that has cog threads substantially shorter than 7.4mm, leaving only 1 or 2 lockring threads exposed. In these cases, contact your local bike shop for recommendations and assistance.

Thanks for the replies guys. I'm having trouble finding the TPI for Zeus hubs, but I guess they must be fairly standard. Most Zeus stuff is Campy-knockoff, so I'd assume they're the same threading.

I'm fairly positive I didn't **** up the angle when I started threading it, but who knows.

Cheers.

Italian threads (as may be on your hub) are slightly different than English, but the two are interchangeable.

When installing cogs I always put the cog in place and rotate anticlockwise until I feel a click, which is where the threads line up.. I think it clicks at the top and bottom because two threads are "exposed" at the end (because of tpi and angle). At this point I just reverse direction to thread normally and so far it's never cross-threaded.

Yeah that's what I normally do when threading most things, and I did it this time as well.

I think it may have slipped though, so I'll take it into the LBS and see if they can clean up the very start of the thread for me.

Thanks for all the help.

Ok so I took the wheel and cog into the LBS today. John hit it up with a thread file, and it was still catching about two turns in. It was a bit sketchy, but it was threading on dead straight so he just greased the **** out of it and used a chain-whip to get it on. It wasn't cross-threading, but we think the threads on the hub were just a little high for it, so the cog was shearing off the tops.

Kinda scary, but it's on now and hopefully it'll stay on (and hopefully I'll still be able to replace it).

Fingers crossed.

wow, i didn't see that part about that being from surly's website, and with all the "we" jazz in there, you seemed like:

a) a borg

or

b) gollum