Trying to plan the evening and no manuals at the office...

I installed my re-built front calipers. Back calipers still off so no fluid in yet. Then had the sinking realization that I put the pistons in 180 degrees wrong.

Does the cut-out section of the piston (maybe 0.5mm deep and a 40 degree arc) face the upper or lower side of the caliper. (i.e. away from disk rotation or towards disk rotation, when going forwards)




Toward disk rotation is correct. Or the rotor turns into the relief not away from it. Or points upstream... Not sure what is the best way to put it.

For people that don't know what Glen is talking about most caliper pistons have a relief towards the front of the leading piston. This reduces pressure on the leading edge of the pad so that the pads will wear more evenly. Because pads are located on an arc of the rotor the position of this relief needs to have some precession. This means that it needs to be at an angle, in our case and on most German rigs, 20 degrees to the leading edge of the pad side of the piston to the major axis of the caliper casting.

It's a bit of a mind bender because when you are looking at the face of the piston you are looking at the back of the pad so reverse logic is appropriate :) Holding the caliper near the rotor helps to visualize this.

You can't really rotate the piston once you press it in so you need to premeditate on this one.

Jay Kempf

PS this is the same reason why the leading piston on the multi-piston calipers are smaller...