Filing a carbon crank

I've just changed the outer ring on my Campag Centaur carbon crank (it was cheap - I'm starting to see why).

The original ring was stamped, and I've replaced it with a machined ring from another manufacturer (due to weird number of teeth).

Now I've stumbled across an issue that I've since read about on the interwebz. The cheapo stamped rings are a different thickness to machined ones, and this causes a wobble in the ring due to the crankset fitting method: 4 spider arms are standard, but the 5th chainring hole attaches directly to the back of the crankarm (bolts straight in).

So, the ring 'warps' 0.8mm towards the bike where it fits behind the crankarm.

I've ridden it, and it seems okay. Needed a bit of judicious front mech adjustment.

The simplest solution for a better fit would be to file away 0.8mm (or even 0.5mm would help) of material on the step+bolt hole behind the crankarm.

Worth it? Bad idea? Do I need someone with a dremel + workshop know-how, or would elbow grease and my flat metal file be enough?

All input appreciated.

if the nut is inside the crank arm, i don't think it would weaken it too much to file 0.5mm off
it should be thick enough there.

but then again, i would just get my favourite tool out, and bash it in, what do i know?

MDCC did a similar thing on his hack, but that was alloy...
https://www.lfgss.com/thread22479.html

I was half expecting Jorj to pipe up on this thread…

Has this been solved yet? I'd relieve the ring rather than the crank for carbon.

I'm no expert, but given the number of guys over on Bike Radar who go into a cold sweat with so much as a deep scratch I'd think long and hard before taking a file to carbon. No harm in getting a second opinion from a good tech familiar with carbon bikes first?

my mirage one (alloy though) came with a handy washer that I used to restore parity on my UT conversion.
(thanks to Jorj- who I got the idea off of)

Yes Campy had the 5th bolt offset, but there are manufacturers that do Campy-specific rings; TA and Stronglight for sure.

If yours isn't, why don't you file the hole on the ring and leave the cranks as they're meant to be???

It's not the offset bolt, which only applies to compacts, it's that the ring is effectively "sandwiched" between the spider and the crank, so it has to be exactly the right thickness (or a bit less, and use the 0.2mm shims which Campag supplies to take up the slack)

The thickness of the ring can be sorted, too.
And again, I'd rather not touch the cranks....

The thickness of the ring can be sorted, too.

Which is what I suggested, not out of any deference to a common mid-range Campag crank or any unease about the integrity of the structure, but simply because it will be easier to find a machine shop familiar with milling aluminium, and because it is easier to bolt the ring down to the machine bed than it is to hold the crank down in the required "face down" orientation.

I see this has been resolved, but in case anyone else has the same issue, file the ring(s).

This is exactly what I did to get some rings of my favoured size to fit a centaur compact chainset.

1 - elongate the hole on the ring that sits behind the crank arm to accomodate the offset.

2 - file the flat surface on the ring behind behind the crankarm if your ring is thicker than the campag standard.

  In my case I took material off from the inside surface on the other 4 arms to move the ring inwards slightly as well as it was so much thicker.

Took a while and isn't too pretty, but worked a treat.

Just catching up with this thread. Thanks for all the replies. I just left it. It works fine.

But if it ever stops working fine, I'll know what to do now*.

*buy a non-stupid chainset ;)