Modify a front derailleur cage to allow for non-round chainrings like Biopace or Rotor? I am repairing my road bike after a mishap where it swallowed the rear derailleur around the cassette and into the front derailleur.  As part of the rebuild, I'm swapping out the badly worn old chainrings and fitting some NOS Biopace chainrings.

Downside is that my Front Derailleur doesn't have clearance when the chainring's highest position is upward.

I am aware that my FD mech suffered and is probably somewhat bent, but it does clear the original worn chainrings.

Here's two shots showing the chainring teeth at both positions.

 
The lowest area of the chainring.

 
Highest point on chainrings, showing the overlap highlighted with red.

The front mech is positioned as high as it can go on the braze-on mount point.  If I tried shimming the mount, the tail of the cage would lift but this would not significantly change the inner plate's height.



The proper plan would be to buy a new front mech with a smaller inner plate.  

However I'd like to not spend too much on this repair.  Is it feasible to trim off some of the inner plate?   



I'd try to leave enough meat at the top to have something for folding a replacement reinforcement lip, but the lower half is lacking in metal for that, and will likely become quite flexible without additional reinforcement.

Is this a workable solution?   I figure if not, I'm up for buying a new mechanism anyway.

Part is a Shimano model FD-2403 so its a triple road, bottom pull, low normal, and the bike was 9 speed.



Update - the cage cut cleanly with a Dremel cutoff wheel.  Took a couple of minutes, and made plenty of pretty sparks.  I used the dremel to round off the edges.

On installation, I find the mount on the frame is still not high enough to work as required.   I've ordered a couple of Band-on to Braze-on adapters to see if the mech can be mounted higher up the seat tube.

I still don't have enough "reach" to the right to get the chain onto the big ring.  Hopefully this will also be addressed by a bandon mount with more range.
For completeness, I ended up replacing the bolted-on brazeon mount with a band-on braze-on mount.

This allowed me to raise the derailleur by those critical last few millimetres.

If the bike had a brazed-on braze-on mount then I might have had to cut it off and file the seat tube to a round finish, but fortunately this Cannondale frame had a small bracket held on by two 3mm bolts into rivnuts.  The Rivnuts are still in the frame under the band.  

(I'm wondering if I should have put two grub screws in the holes to seal them up and reduce rain ingress from the rear tyre.)

Here's the completed fitment: