My disc caliper never sat centrally over the disc, but the pistons originally adjusted to take it up. However, as the pads have worn, I’ve just about run out of adjustment on one piston (the inside one), which is going to cause all kinds of rubbing and nastiness.
I’ve used all the adjustment between the adaptor and the caliper. Mount is hard to the frame, but the caliper needs to move out more.

Ideas? Any way to face the mounts given their shape? Take a file to the mount?

I’ve used the file before and it’s worked.
In your case I would probably file the adapter, as if you overdo it this is less serious than either than frame or caliper.
Other option is to stick a washer twixt hub and frame.

Oops – I got a bit mixed up between ‘mount’ and ‘adaptor’ in my OP. What I meant was;

Mount Adaptor is hard to the frame, but the caliper needs to move out more.

Ideas? Any way to face the mounts given their shape? Take a file to the mount adaptor?

Thanks andrewh, hadn’t thought of the washer idea.

Start thin and build up to fatter one if required.
Looks like a steel frame? If so, making it a bit wider shouldn’t be a problem (I even got a 135mm hub into a 130mm frame!) May upset your indexing/chainline very slightly but easily re-adjusted.

Sorry to gently resurrect this after a few weeks!

I finally got the chance to take the adaptor off and rubbed it down using a coarse grinding stone.

It made the rubbing worse! Despite using a micrometer to check I was taking both sides down evenly, the caliper was twisted in towards the wheel and the upper edge of the rotor was fouling on the caliper. (We were assuming that the inside edges of the bolt holes were parallel to the outside edges for the micrometer to be accurate) I ended up taking about 30 thousandths off, which should have been enough to move the caliper away from the rotor, but in the end I had to pop two washers totalling 40 thousandths in between the axle and the frame to get the wheel to turn again.

So, as I don’t want to risk losing those washers during a trailside wheel change – I need another adaptor, with a smaller offset between the vertical, ‘post mount’ bolt holes, and the horizontal, ‘International Standard’ bolt hole faces.

Without resorting to trial and error, any ideas on a brand which is nice and ‘slim’? This one is a Hope.

can’t really visualise what you’re saying but I’d expect all mounts to be drilled to the standard spacing, regardless of how chunky they are around the holes

Is it that the calliper site too far inboard and is fouling the disc ?

Is that a bolt-up axle (eg shimano) ? could you get a washer inbetween some of the hardware that’s threaded on the brake side (though I suppose your locknuts might slip then). I imagine the trouble with spacing the calliper by “spreading” the chainstays is that the mount may well not be perfectly parrallel afterwards (though not by a lot)

Are the holes in the calliper ovalised to allow a bit of lateral movement ? Assuming there’s enough spare material, why not extend the oval to allow the calliper to move (out ?) a bit more ?

Get you frame mounts faced at a shop. it’ll flatten them off and take a bit of material off too.

Are you dead sure you’re moving the calliper the right way? I’ve spent hours aligning IS mounts with shims, to suddenly realise I’m adding, rather than taking away thickness.

And have you tried the same setup on another frame? Could be manufacturing error…

The caliper is sitting too far in and just fouling the disc, or more accurately, the inboard piston has run out of adjustment ‘reach’ and now the outboard piston has moved in, and rubs the disc. When the brake is pulled, the disc flexes inboard to meet the inboard pad.

I was removing material from the outside edge of the silver adaptor to move the caliper further out. The holes in the caliper are ovalised, but I’ve used all that adjustment! Spreading the chainstays is not too much of an issue, in terms of degrees it’s pretty small beer moving the end of the stay by 40 thousandths!

I could extend the caliper ovals, but cock that up and it’s a new caliper. Cock up filing an adaptor and it’s what £20?

I’m not sure the frame disc mounts could be faced. All the facing tools I’ve seen are set up for round mounting holes, rather than these ovalised holes. I don’t think it’s an error, just a Hub + Brake + Adaptor + Frame combination.