At least I think it's called an NSR F3. Looks just like a MC21 but has an F3 designation on the frame. Just wanted to know what the differences were.

The Formula 3 class was raced in Japan late 80's to early 90's. It was basically a chance for the factories to show off how good their production 250/400 were.

This meant that there were a bunch of very special parts developed that would turn the 45hp NSR into a 72hp (according to HRC) race weapon.

These parts had to be based on the standard parts, but thanks to the similarity of the NSR to the RS250 the parts were very closely matched to the GP bike.

A full F3 NSR for example would become almost a full on GP bike.

Chassis wise the F3 frame is beefed up in the required areas. The F3 swingarm has mounts to fit the RS brake system, and also RS wheels. The forks have different internals (no comp damping adjustment though!), and the rear shock is an aluminium bodied affair adjustable for everything including ride height.

On the engine it's pretty much take out the stocker and throw in a full unit. The cases are different in the respenct that they're machines to accept the RS250 ignition system, and HRC thoughtfully block off the oil injector for you. The crank is totally different to the NSR and much more resembles an RS. Single ring RS pistons are used, with 14mm pins instead of the heavy weight 15mm pins on the NSR. The cylinders are ported differently to stock and are shorter. Heads ran same as stock volume, but smaller squish band.

Transmission was pretty much all RS250, with a couple of specific gear shafts to use the RS gears with NSR clutch and sprockets.

Carbs were special too, and on the MC21 F3 HRC threw out the plastic intake rectifiers in favour of the 'old' MC18 carb insulator with the rubber tongues.

The brain was basically a modified RS250 PGMII.

Anything else worth mentioning.........errr......oh yeah, no GPS, no TPS and no air solenoids. And a huge great three core radiator.

Compatible parts?

Most of the F3 stuff will fit the MC18~MC28. They trickiest bit would be the ignition system as you'll have to machine the cases to fit the stator, and the taper on the F3 crank is different for the F3 flywheel.

Cylinders will go straight on, and have a different RC valve shape, but you'll need to watch your compression as this will change so good fuel must be used (100+ octane)

The F3 PGMII will plug straight into an MC18 in place of the original PGMII. Remember we did that at your place Andy? Sounded better for it, but we never rode it this way.

There's also an older type analogue CDI for the '88 MC18 F3, directly off the same year RS250. I've actually got a couple of these new, but never even tried them.

Suspension will go straight in.

Old type RS (NF4) pistons can be used on the stock NSR crank, but you'll need special small end bearings. These bearings can be sourced. There's a height difference in the pistons between stock and RS, so some cylinder mods would be needed.

Oh, there's so many different combinations that I could go on and on, but I think that'll do for now.

Thanks for the detailed explanation Matt. So, anyone interested in purchasing one? I believe there may be one for sale with HRC Magtek rims in Hong Kong. Not crashed just stored for the past few years.