Mounting an F3 rear rim on the F2 is a pretty common upgrade, which affords a 1/2 inch wider rim, but with practically no modification necessary.
The F2 rear is 4.5", and the F3 rear is 5", which allows one to safely employ a 170/60 rear street tire, giving a little more contact patch, but without sacrificing any stability, due to any deformation in the tire's profile.
In racing, the 5" rear rim allows making better use of the common 165 rear slicks, sold by Dunlop and Bridgestone, and maybe others.

The sprocket carriers are interchangeable between the F2 and F3 rear rim, and employ the same cush drive, the same sprocket side wheel spacer is used, and the length and diameter of the axle is the same, - there is only one part necessary to make the swap, which is the brake-side wheel spacer from an F3- with that part, the F3 rim is a direct bolt-up.

I created this thread, to provide the dimensions of the F3 spacer - another member requested these measurements, as he wanted to make comparison with a spacer from a 900 RR wheel, to see if they had the same dimensions, and I figured, why not make this info available to all, in case someone else could use it... a note to that member: sorry it took me so stinking long to get this done!!

The picture below is not of an actual F3 spacer, but it is very close in its design - the measurements, however, are from one of my extra F3 spacers.



If any of those numbers are hard to make out, the measurements are:

Inside diameter - 20mm
Total height - 14mm
Height of inner race - 11mm
Outside diameter of inner race - 30mm

 

Well, according to one of the "Cheap Cycle Parts" diagram, the 95 CBR F3 & 95 CBR 900rr brake side rear wheel spacer is the same part #. So I went to the garage and measured my 900rr wheel spacer. First I converted JNS's measurements from millimeters to thousands. Everything checked out except the 14mm total overall thickness. Mine is about .018 short which comes out to about 13.5mm total overall length, which I dont believe would cause any problems. Also the 900rr/F3 spacer looks just like the F2 spacer, not like the one in the picture. Which JNS clearly stated. Thanks JNS

 

I definitely wouldn't sweat 1/2mm.

Regarding the pic I posted above, I mentioned it wasn't exactly the same, but similar in design, and what I meant was, the spacer actually has that flange, like the one above, but just a little bit differently sized.
I went ahead and took a couple of pics of what I have, which is what I should have done in the first place!





It's highly likely they've had different incarnations of the parts performing the same function, because that larger outer diameter flange is not really necessary, but this is what all three of my F3 spacers look like.

 

The spacers in the OEM diagram for the sprocket side look like the one you showed us but not the brake disc side. I have a spacer that does look like the one you showed us but it is the sprocket side. Either way I'm not concerned, F3 rear wheel is going on next time I buy tires. Do you have a picture of your F3 wheel mounted and we can see the spacer in action?

 

Well, I sanded, cleaned, and painted my F3 wheel. It looks like poop. I used engine enamel. Smooth in some places, rough in others. I followed the instructions, 2 light coats with 10min drying time between them. Then 1 med coat. I have complete coverage. Oh well, I guess I'll just have to start wet sanding 1st with 1000grit then 2000grit then wax the crap out of it with scratch remover wax.