@Greg Watts Dou you know if the 2.5mm Magura Mdrs Discs (which are designed for the Gustav Pro Caliper's) will work with Shimano 4 Piston Caliper's? At the Moment i use og Shimano Discs(RT-76).and the always tend to deform.
@LWstner Straight answer: I wouldn't bolt those on without checking first, and here's why. The MDR-S at 2.5mm is thick on purpose - Magura's Gustav Pro/MT7 callipers are built with generous pad-to-pad clearance to run rotors that chunky for heat capacity. Shimano's 4-piston callipers (M8120, XT, Deore etc.) are machined around the industry-standard ~1.8mm rotor. Feed them a 2.5mm disc and you risk the pads not sitting square, poor piston retraction, or the rotor simply not fitting between the pads without forcing it - none of which you want discovering on a descent.
I can't point you to a verified fitment confirming that combo works, so I'm not going to tell you it's fine just because it bolts up in theory - that's exactly the kind of thing that looks fine in the garage and bites on the trail.
On the actual problem - RT-76s deforming - that's a known complaint on 4-piston Shimano setups under sustained e-bike heat loads, especially on long descents where the extra motor weight and speed load the rotor harder than a normal MTB. Rather than chasing a thicker rotor that may not fit your calipers properly, the more common fix people use is:
• A rotor built for Shimano's own pad clearance but with more heat mass - Galfer Wave discs are the usual shout, same 1.8-2mm ballpark but better heat-treated steel than the stock Shimano ones.
• Going up a rotor size (203→220mm) if your frame/fork clears it - more surface area does more for heat than thickness does.
If you want to chase the MDR-S route specifically, I'd get exact pad clearance figures for your calliper model before ordering rather than assume it'll just work.
