International Standard, or IS callipers are bolted to the bike using bolts which pass through the mounts on the frame or fork and then screw into the calliper, which is threaded. The horizontal alignment is adjusted with shims (thin washers) between the frame and the calliper.
The instructions that follow are to fit IS callipers to an IS mount fork, but if you’re fitting them to a frame then just substitute the word ’frame’ for ’fork’. It’s wise to make sure that your fork has been correctly faced before fitting an IS mount calliper. Facing involves shaving off the inner face of the mounts (A) to make sure that they are exactly parallel. It’s a job that requires a specific and very expensive cutting tool, so is one for the bike shop only, but it is worth doing as even a brand new fork may not be straight.
(A) Rear International Standard disc mounts
’Shim’ is a fancy name for an accurately sized washer. Shims have to be of a precise thickness because the spacing between the calliper and the frame must be the same for each bolt. If the washer stacks are of different thicknesses, the calliper sits crookedly over the rotor and the pads will drag as you ride, rubbing noisily as well as making the brake feel mushy and soft. You’ll need a variety of different shim widths: the thicker ones sort out the general alignment and get the rotor to pass through the centre of the slot in the calliper, whilst the wafer-thin ones fine tune the position. Front and rear callipers are fitted in the same way: the bolts pass through the mounting tabs and thread into the calliper, whichever end of the bike you’re fitting them to. If you’re reusing calliper bolts, make sure you use threadlock (new bolts will have threadlock already applied). Braking forces will twist and vibrate the bolts, so they need extra care to stop them rattling loose.
To fit the calliper
With the rotor fitted securely to the wheel, slide the calliper into place over the rotor. Fit a single washer to each bolt and pass it through the mount into the calliper. Watch the relative positions of the calliper and the rotor as you tighten both bolts in turn, a little at a time. Look along the rotor, in through the slot in the calliper – it helps to hold a sheet of paper behind the slot.
Step 1 : As you tighten the bolts, the calliper will draw closer to the frame. If the inside pad starts to touch the rotor before the bolts are snug, you’ll need to add shims. Make sure you have shims in a variety of thicknesses. You will have to remove the bolts to fit the shims. Add an equal thickness of washers to each bolt and refit the bolts, repeating the process and watching the gap between pads and rotor again.
Step 2 : Use the thicker washers at first, then use the thinner ones for fine-tuning (A). The gap between both pads and the rotor should be the same on callipers where both pads move. For a calliper where only one pad moves, the rotor should be as close to the stationary pad as possible, to avoid unnecessary bending of the rotor. Squinting at gaps through callipers can be confusing so holding a piece of white paper up behind the slot really will help you to see what you’re doing.
Step 3 : The process of fine-tuning an IS calliperlike this is made significantly simpler if you’ve been lucky enough to get hold of some of Shimano’s ’tuning fork’ shims, which look like open-ended tuning forks and can be inserted with the bolts still in situ (B).
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES