I’m trying to fit a standard post mount Magura 4-pot via a 180mm IS to PM adapter, I have previously always run the caliper bolted straight to the fork without any need for side to side alignment via conical washers – but for some reason I can’t get the alignment this time without the pads rubbing.

So, ive fitted some thin as I can find conical washers and now I have perfect pad/rotor alignment on the vertical axis, but I dont have full pad contact on the rotor as the washers have effectively added circa 5mm of additional height to the caliper.

Whats the best idea to get the caliper back square onto the rotor?
I dont think its a brake mount alignment issue as i’ve tried 3x pair of forks and im getting the same result on all of them to some extent.
The issue does seem to have been made worse when switching from 2x standard Magura pads to the ‘upgraded’ 4x pad configuration, but none of the pistons are protruding more than the others so I can’t see what is causing the issue.

Wouldn’t adding conical spacers above and below the caliper mounting points always cause this issue of pad/rotor spacing too? (ie, pushing the caliper away from its optimum position)

IS mount forks (all the ones you have tried)? Could be they all need facing. Are all the pistons moving equally as with 4 pads it could be one is moving more.

I’v e only just fitted a Salsa rigid fork with IS mount, both other forks (Fox 32 and Travers prong) were post mount, and was finding the same issue, although I had no issues on the FOx 32 initially, it was mainly when i switched down to 180mm from 203mm, and changer to 4x pads at the same time.

I guess I need to pay closer attention to the movement from each piston, but as I mentioned above, now that Ive added the conical spacers im no longer getting any rubbing on the rotor – which is confusalating me.

The adapters don’t account for spherical washers unless you’re using Avids which are designed to use them.

Your best bet would be to carefully file the least expensive bit of metal to account for the angular misalignment and if you want to be thorough use a longer bolt and spherical washer above the caliper (to prevent the bolt head being pulled over to one side).

Then just use the slots in the caliper to align it.

Or get the mounts faced. I have a PM mount that’s slightly misaligned and could do with facing, but could only find shops with IS mount facing tools.

Shimanos approach uses conical washers under the bolt head, not between the mount and caliper.
Try it that way as this will keep your pad contact correct.
Back off 1/4 turn from tight, Spin wheel, squeeze brake lever, gradually tighten bolts up.

Good tip about leaving washers above the caliper, i’d always thought that was pointless but when you put it that way it makes sense – i have had bolt heads pulled to one side before.

Cheers, i’ll go give those ideas a try.

The whole point in the conical washers being above the caliper on +20mm post mounts is that the two mounting surfaces aren’t parallel, so without the conical washers above the caliper, the bolt heads bends as it’s not square to the mount.

There is no point in conical washers in IS (fork) to PM (caliper) situations afaik. They are for single-bolt post-to-post adapters when the point made by @BearBack and @Woodentop applies.

Get the mounts on the frame faced at good LBS.

I used a card business card on either side of the disc slid into the calliper, then pulled on the brake lever then turned up the bolts.

Normally I just brake, nip it up and then release the lever, spin wheel, then brake a again while using a torque wrench without moving the calliper.

Sometimes the torque on the bolts can move the callipers.

Having a pal hold the brake lever or rope, freeing a hand to hold the calliper too while you nip up the bolts.

Callipers with massive holes for the screws allows too much movement.

Sometimes it’s a few seconds to sort and sometimes a bloody hour.

OK, a combination of giving the pistons several cycles with application of mineral oil, and a light filing of the IS/Post mounting point faces and a very careful centering of caliper and gradual tightening seems to have virtually done the job. Theres not much room for error between pads and rotor, but the pistons do now seem to be applying pressure and retracting more equally.
Need to take on a good shakedown ride but so far so good.