F2 caliper pistons?
 

My Frankenstein race bike has front brakes that were alleged to be from a CBR600 F2. F2 pads fit it, but when I bought seals and pistons from a Honda dealer, they were too small. The old pistons are 27 mm in diameter, and the new ones are 25mm. The old ones have little discs glued into the tops of them (not shown), and the new ones are open.

Any ideas on what's going on here?

I thought you might have F3 brakes - but looking at the OEM fiche, the part numbers are the same for F2 adn F3, so apparently it is the same.

what was the Honda part number for the pistons you were sold? F2 and f3 all years use part # 45107-MN5-006.

I'm guessing you might have F1 pistons maybe which are a very similar part number - but different - 45107-MN4-016

I think that's entirely possible. Any way to find out what the diameter of the F1 piston is, short of buying it?

Lucky I have the PDF's for the f1 Honda Svc. Manual on my PC here @ work.

According to it, the piston OD is 25.335 - 25.368 standard, with a service limit of 25.33.

Checking the F3 manual I have here - it lists the exact same specs though.

I'll have to check my Clymer manual when I get home.

Strangely - I happen to have a 900rr manual for the 919rr series - 96-98 years.
that manual lists the 96&97 models as having 2 diff sizes of piston - one of them is a 26.xxx size, but you would notice the other piston being larger, right?

I think you might have Nissin brakes off some other bike maybe?

Anything's possible with this monstrosity. Looking at the fiches on bike bandit, it looks this way: Pistons and seals same for F2 and F3. Pads the same for hurricane, F and F2. Body appears to be the same for Hurr., F and F2 also. What threw me off was that the brackets are definitely F2, but if the castings were the same for F and F2, that would mean the bodies are probably interchangeable.

I'm gonna order F seals and see what happens.

weird, the 1000F has 3 different piston sizes mentioned, pre 92 1000f's were standard calipers, 92+ had a triple piston setup.

Apparently the earlier models had some different piston sizes? Confusing, but one of them was 27mm.

So the caliper bodies are the same for F and F2, except that the F pistons are 27mm and the F2's are 25mm. I bought F2 calipers on eBay and they bolted right up and worked like a charm.