I’m in the process of building a hardtail using a new Carbon 456 frame and various bits I’m reusing from another bike.

My intention is to go single up front with a 11-34 on the rear, so I’ve built it without a front mech, and was going to reuse my Firex GXP cranks (triple) by binning the rings and getting a dedicated single ring for the middle. Theory goes there that the middle will be the correct chainline. Backed up perhaps with a bash guard (already had bash in place of the big ring anyway) and a Jump Stop on the other side just to ensure it’s not going to drop off.

Anyway, got the chainring (104bcd FSA DH single) but turns out the crank seats on the middle are a mm or two smaller and the ring wont fit. It will fit on the outside though where the seats are fractionally bigger. The bolts positions are fine.

Any advice for this? I can sit the chainring on the outside but the chainline will be wrong which surely will be bad when going to the big rings on the cassette, and aside from wear it’s surely more likely to drop the chain?

I could go with the old chainring on the middle, but wanted a 34t instead of the 32t I had and it’s ramped so more likely to slip off.

Otherwise it’s new cranks which I didn’t want to do at this stage. What options do I have there? Noting that I’ve fitted a GXP bottom bracket now. I guess I can bin it, but I’m happy working with GXP stuff so would like to keep it, or something similar.

One thing I’m finding is there don’t appear to be many options for just the crank arms/spider with no rings, particularly for single speed.

If it’s only a millimetre or so can you just file down the chainring a bit?

Seems to be a fairly common thing to have to do.

Don’t worry about a chain slipping off because its Ramped, Did it slip off before.

I’d go with above if you want 34t get the file out.

True, it didn’t slip off before. I read around and people say as soon as you bin the front mech it’ll drop off instantly. Hmm, not sure. Sat in the middle almost all the time before and in fact it was a git to try to drop to the granny.

Could file the chainring but getting thin there between edge and bolt holes, but might just need a small amount and it should stay strong. Or I file the spider. Guess that may be tougher stuff to through.

Don’t have a file however. Got a Dremel but not sure it’s wise to use it for that :D. Else, trip to B&Q.

Can in the meantime go with the current 32t. Standard steel shimano affair, bit heavier. Would like to just fit the 34t and size the chain appropriately for it.

I filed down the spider before when coming across the same problem, no problem, chainring is still pretty much centered thanks to the bolts even though I didn’t do a neat job on the cranks.

I’ve filed spiders and chain rings before, spider would probably be better in this instance. If you’re not too ham fisted a Dremel will probably be fine.

Well, not the neatest of jobs, but thankfully it’s on the inside so can’t see it. Dremel did the job, after going through a few of the filing things. Still wasn’t perfect fit, but once the bolts were tightened up it sorted it out.

Problem solved.

Just the gears to sort out now and my black stealth machine is ready to go.