Does any one know if a front break cylinder off a CBR600RR would fit the nc23??

NO! not at all or in any way.

The axial master off an '03-04 600RR has an internal bore of 11/16" and is meant for massive 4 pot calipers. The internal bore on yours is 1/2" and is meant for some relatively tiddly 2 pot calipers. Swapping to a bigger master will make the brakes LESS powerful, and also turn them more into an on-off switch. Same theory applies with a later radial master if that's what you're looking at.

Brakes are built as a matching set, you wouldn't go fitting a 50mm carb to a 125 , now would you? so don't go fitting massive master cylinders on little calipers neither.

If you are looking to upgrade then fit Firestorm fork lowers and mudguard (or figure a way to bracket up and use your mudguard), and fit the full brake system off the '03-04 600RR or a CBR929 or 954, it'll go together with just a little bit of judicious dremel action on the upper brake lugs of the leg castings. Then you're laughing with awesome brakes and stoppies at will. Your wheel and discs will fit, I forget if you need the axle and spacers off the firestorm but think they'll be the same as you have as both fork sets use the longer axle for 4 pinch bolt forks, some use 2 only, and the axle is correspondingly shorter. The fork internals go together so it's OK to get a pair of bent firestorm lowers. Although I did that and they were very bent, and the lowers were also damaged so be careful.

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