My brakes make a noise on a time interval, as the tire rotates around. I have Shimano BR-MT520 brakes. I hear you can loosen the two bolts and move it side to side to adjust.
What is the name of the tool and what size do I need?
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You need a set of metric hex wrenches, also called allen keys, hex keys, and allen wrenches.
They are hexagonal in shape, and come in sizes from 1.5 mm (maybe smaller) to 10 mm (maybe larger). Your local hardware store will have an appropriate set.
For bike maintenance, I recommend getting a set that covers 3 mm to 10 mm.
The 4 mm, 5 mm, and 6 mm keys will be the ones you most often use. Your brake caliper can be adjusted with one of those (my guess is the 6 mm).
Shimano BR-MT520 brakes are hydraulic discs. The alignment can be adjusted by loosening the two mounting bolts. The bolts typically have Torx T25 size heads, so you need a basic T25 wrench at least. If your rear caliper is mounted between the seat and chain stays the bolts can be difficult to get to, so a L-shaped rather than a T-shaped wrench is necessary.
The basic short-cut technique for aligning the calipers to loosen the bolts so the caliper can move but is not loose, pull the brake lever hard, tighten the bolts. See this Park Tool page for more details on caliper alignment.
If your pads are rubbing periodically as the wheel spins, your rotors are probably warped. They can be straightened fairly easily. There are special tools for this but an adjustable wrench works reasonably well. See this Park Tool video.
I read through the BR-MT520 Dealer Manual and it seems the mounting bolts are in fact 5mm hex heads.
The BR-MT520 utilizes size M6 caliper fixing bolts to secure the brake body to the bike. M6 bolts have a hex key hole in the head for the hex key (aka: Allen wrench) to fit. This hole is 5mm wide on an M6 bolt and therefore you'll use a 5 mm hex key to loosen and tighten it. (As an aside, the 6 in "M6" refers to the width of the threaded portion of the bolt which is 6mm in outside diameter. A metric socket head cap bolt like we're dealing with here accepts an Allen key that is 1mm less than the "M"# of the bolt).
To center your caliper to the rotor, loosen both of these caliper fixing bolts and squeeze and hold the appropriate brake lever. Snug the bolts back up enough to "fix" the caliper in position, release the lever and rotate the wheel to check that there is pad clearance of the moving rotor (no rubbing). If that fixes the intermittent rub, finish by fully torquing the fixing bolts to the spec'd 5-7 Nm. If a rub is still present, you can retry the above with both bolts and another technique is to alternate the loosening of bolts (one at a time, squeeze lever, snug up the single loosened bolt and check for rub after each bolt).
One thing that needs mentioning is that the VERY FIRST thing you should check when confronted with intermittent wheel noise is that the wheel is fully seated and properly secured in the dropouts. Less of a concern with thru axels but check the symmetry and security of the hardware in this case too.