I’ve bought a used bike which was shipped to me with the caliper unbolted from the frame. It’s my first time using hydraulic discs, so I’m not really sure how to adjust them. I’ve fitted the wheel, followed by the caliper. I left the caliper bolts a bit loose, and checked that the wheel rotated without any significant rubbing. I then applied the brake by holding the lever, and tightened the caliper bolts, but then the wheel was jammed.

What’s the proper method for adjusting it? By the way, the bike would have been running fine before it was partially disassembled and sent to me.

That is the proper way, don’t have the bolts too loose through.

If it’s rubbing one side make sure (with the caliper off) that both pistons are moving equally – but don’t squeeze too hard, if a piston comes out it’s a major faff. You may need to free one side off by holding the other in with a tyre lever or similar so the other has to move, or freeing it up with a brake cleaner.

If the rub is minor you can minorly bodge it by spacing the rubbing side slightly with something about the thickness of a business card. I’ve got some sparkplug feeler gauges I use for this.

Thanks very much. I’ll have another go later in the week.

The method you describe generally works. Things to look out for are sticky pistons, movement when you touque the bolt, and wonky posts.

Firstly, take the wheel and pads out, and very carefully push the pistons back into the caliper. Then squeeze the lever a little and check that they move together. Then push them back again. BE VERY CAREFUL – you don’t want to eject the pistons, or damage them by sticking a screwdriver in there to push them back!!

When you put the pads and wheel back and get ready to align the caliper, you need to tighten the bolts down just to the point before they bind the caliper to the post. Then when you pull the lever, tighten the bolds down a bit at a time to ensure that the torque doesn’t spin the caliper around the bolt.

Also, when you tighten the bolts, watch the caliper very closely to see if it twists in relation to the surface of the rotor. You may need to have the posts faced, or they could be bent.

Personally I wouldn’t rely on any caliper having equally moving pistons, especially a used one.
My preferred method would be to remove the pads, and the looking down the top and bottom slots in the caliper, align them so that the gap either side of the rotor is the same. That way you KNOW that the caliper is centred. If the rotor is a little out of true, you may need to rotate the wheel and try and tweak the caliper psotion slightly so its centred as best you can.
Sure its a little more of a fiddle, but it is definitely right, not probably right.

+ 1 for Matts, but after i have done it i look at the rotor when squeezing the lever, if one caliper moves the rotor first i then adjust it once more

One other thing to look out for is tension added by the hose being tied to the chain/seatstay very close to the caliper; I usually cut the last zip tie when doing an adjustment so the caliper moves freely.

I am with bigyinn on this – get up close and alight the çaliper slot to disc by eye.

If you are still running, try cleaning Pistons carefully with the pads removed and the tiniest smear of brake fluid – then actuate Pistons back and forth lots to free up any that are ‘lazy’.

They seem to retract further from rotor than my old Shimano’s – and the feel is great, plus superb power.