Will Honda CBR900RR/Fireblade calipers work on a 1991 MC21 NSR250SE? I'm looking into this as an inexpensive replacement and not from an improvement aspect. The brakes on my MC21 were awesome. It has sat in high humidity for a long time, unused and now the front brakes are locked.

How difficult is it to rebuild the MC21 front calipers? Where can you get the special tool to loosen the fasteners? Mine have a very strange bolt head.

I appreciate any and all responses.

Skunkbait

Very easy to strip and rebuild the calipers. You need an external/female torx socket to undo the bolts which are cheap and readily available, you can get away without using a brake piston removal tool if you pump the pistons out before you remove the calipers.

All the seals and pistons are available from Honda or as pattern parts, its best practice to replace all the seals before reassembly including the small o rings that seal the two halves (you need to split the halves to remove the pistons) use red rubber grease on the new seals during assembly.

Have a good look at the grooves where the seals sit in the caliper, if your calipers are sticking corrosion in these grooves could be the cause, pushing out the seal so it nips up the piston. If its not to serious you can get away with cleaning the grove with a dremel and rotary nylon brush.

Personally I would rebuild my calipers before buying second hand as used calipers are likely to be in the same condition as yours anyway, spend the money on a rebuild and you know your brakes are good.

Paul,

I appreciate the advice. Would you happen to have a part number for the seal kit? WIll a CBR900RR/Fireblade seal kit work? When I mention NSR 250 to my Honda parts people they have no idea what I'm talking about.

Thanks again.

Skunkbait

There is a parts manual you can access on here but I had a look for you, they list them as individual seals or a piston set i.e. 1 seal & 1 dust seal and you have 2 different piston sizes on an mc21.

P/n: 06451-166-405 & P/n: 06451-GE2-405 you would need 2 off each kit to do one caliper plus 2 off P/n: 45103-MR7-006 these are the small seals which seal the 2 caliper halves. They arent KV3 part numbers so are probably common to other models so your dealer should be able to get them no problem.

Be sure to inspect the pistons as well and replace them if they are corroded, the corrosion can tear the seals and cause them to leak which will only end in tears!

Thanks again. From my part number search it looks like the calipers are the same as a 1993 CBR600. I greatly appreciate the help.

I agree with the rebuild sentiment, then you know it's done properly and the brake will feel light but firm. It's an easy job - I use a releasing agent then clamp each piston back and use the airline to move the pistons individually before disassembly. I've never had a problem - if you get a stubborn one then I have a spare master cylinder rigged up with a short hose; I just pump the sticky piston out with the extra pressure.
The reason for my post is that I'm rebuilding an MC21 (with a Magtek front wheel) for a friend and he's supplied me with standard and CBR900 calipers. The obvious difference being that the 900 calipers are threaded in the mounting holes. This makes it fiddly to tighten them so I guess I'll get the threads drilled out - can you see any reason why this might be a problem?
Cheers, Paul

I'm confused.

Does anyone know if the CBR900RR front caliper seals work in the 1991 MC21 front caliper?

The part number from NSR-World for the front brake caliper piston seals matches with 1993 CBR600F2 part numbers on the websites I've found; Ron Ayers and Honda Parts Nation. But the CBR600F2 calipers have two pistons of the same diameter and the calipers on my 1991 MC21 clearly have pistons of different diameter. Maybe it's just the incorrect diagram but both parts websites show the same thing. Also, the brake pads for the CBR600F2 are clearly different.

I know my MC21 uses the same brake pad as the CBR900RR and the early (1993) CBR900RR calipers bear a very close resemblance to the MC21 calipers, at least in the diagrams. It seems logical that the CBR900RR seals would work.

Thanks.

Skunkbait.