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I am working on a retro/new bike project and I have bought the Shimano SM-RT81-A 160mm discs whit Shimano SLX-M7100 brake calipers.

I have seen that the brake pads on the front rotor are about 5mm too far out.

Now I am wondering if there is a difference between the SM-RT81 and the SM-RT81-A version?

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    Can you clarify what you mean by 'pads on the front rotor are about 5mm too far out'? Do you mean the calipers do not line up with the disc? How about the rear brake, same problem there? – Argenti Apparatus Oct 24 '19 at 23:48
  • Looking at the Shimano site, it appears that the SM-RT81 rotors come in four different sizes -- 203/180/160/140 mm. Presumably the "A" designation specifies one of those sizes. – Daniel R Hicks Oct 25 '19 at 1:31
  • Disc brakes come with a number of rotors sizes, everything has to match. Are you sure the adapters/mounts on your bike are for 160 rotors and not 180s or 203s? – Deleted User Oct 25 '19 at 2:07
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    It would be helpful to see a photo of what you mean by "5mm too far out" – Swifty Oct 25 '19 at 9:18
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You need to determine the model number of the SLX brake caliper as the BR-675 caliper requires a wide-type rotor and pad as well. New BR-7000, 7100 & 7120 SLX calipers use narrow type rotors and pads. The SM-RT81 rotor is a narrow type and only compatible with the latter SLX model numbers. Shimano's Compatibity Chart for disc brakes/rotors. Also note the section "Interference Between Disc Brake Caliper & Rotor.". Significantly it advises one to avoid using an M675 caliper with RT81 rotor.
Whether this is the reason your pads are sticking out is still in question.

Determining the model numbers of your caliper and pads and comparing to the compatibility chart linked above would be a good place to start. The "A" at the end of a model number represents "standard" version of the small part (which is what the "SM" represents). Based on my interpretation of Shimano's published info, the variation in the model number SM-RT81 is related to differing diameters available. These different sizes are denoted SS, S, M, L as a suffix at the end of the rotor model number.

Some other considerations mechanically are that the rotor pistons have been pushed back in after the old pads removed and prior to the new going in. Check the new pads return spring is oriented correctly and that the eyeholes of pads and spring line up.

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