Unsure of fitment on MC21 but I used a 160 on my MC28SP without issues. Bike handled great on the track with a 160. I tested it at an open track day (no sessions) at Buttonwillow Raceway. I had a mounted Pirelli Supercorsa slick on a spare wheel, did a stint with my 150 Pirelli Supercorsa DOT tire, came into the pits, quickly swapped rear wheel, and continued to lap at identical lap times. I was running a 110 Pirelli up front.

Now using a 115(?) sized 125 GP rear slick up front? Definitely does not work.
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MC28SP, '16 KTM 500EXC, '16 Beta Xtrainer, '07 BMW 1200GS, '96 Ducati 900SS