ok so I once again have the time to try and knock out the majority of my f2 build. I have a 93 f2 that has been completely torn down and ready for reassembly. Now I don't have the money to be throwing in aftermarket pistons and boring out the motor to fit them. but since I have the motor pulled apart I wanted to do something. Now here is what im thinking and need some answers on.....

to the best of my research the f2 and f3 upper and lower cases are the same. where the difference lies is internals. the f3 had lighter internals with higher compression then the f2 but the same bore (65mm). If this is indeed the case installing some f3 pistons (and maybe rods/crank) should be a simple bolt on type deal with an increase in power and performance along with a slight drop in weight. Unless im missing something which is entirely possible.
Can anyone here shed some light on this idea of mine? eventually I would like to install aftermarket cams. I already have full exhaust, intake, and jet kit for this bike and with the cams that will be more then likely were I stop.

 

While I'm not sure there actually IS a difference, there are other aspects to it than just the bore and stroke being the same.

Honda could have easily changed some things, as in: wrist pin height, wrist pin size, rod length, journal diameters for your mains and rods, etc that would make you unable to interchange parts


Again, I don't know if they did any of those things, but they could have. And if they did in fact "make the internals lighter", one of the ways you do that while keeping the same strength is to change some of those things so you're using less materials in certain areas.


Find someone who has actually measured the rod length, the journal diameters, wrist pin height, journal offset, etc

 

I understand that there is more then just bore and stroke. the wrist pin height changing would not necessarily mean it won't work. as long as the clearance is still good it would only change compression (another way to change compression they use instead of changing the dome at the top of the piston). and since there was a bump up in compression from the years I thought this might be how it was done if there was a change in pin height. As for the wrist pin size they are the same. any aftermarket pistons you get fit both the f2 and f3 rods. which to me would mean if an aftermarket piston goes into either one them then the pin size has to be the same right? ON that note the aftermarket pistons state particular compression ratios for both bikes which leads me to believe that the wrist pic height would be the same just different dome shape. (Ie example would be a 13;1 ratio for either f2 or f3 with aftermarket pistons) Any thoughts on that or am I still missing a part? as for the journal diameters wouldn't they be the same if both the upper and lower cases were the exact same? because the journals are a part of the case correct?

 

Well, according to BikeBandit:

So, something is different there. Not sure what. You need to figure out what the difference is between the rods. I look at pictures of the pistons, they have different numbers stamped into them (on the exhaust side of the piston). Also, the Rods (when looked up on ebay) have the same corresponding numbers stamped on them that the pistons do. You need to find definite measurements of everything and come up with the definite plan about what you're going to use. Is it just pistons? Or is it everything?

 

I get they are different.....hence why I said they are different.........(lighter weight and higher compression). so ya they would have different part numbers. the problem is they don't give much info on measurements so there is not a way to compare size and fitment. And I know I need to find a "definite measurement on everything"......why do you think im asking on the cbr forum? If I had a set of f3 internals I wouldn't need to be asking the question cause I would already know (I would have it in front of me to measure myself). But for the sake of buying a whole set to find out hey this isn't going to work I figured I would ask somewhere where there is a high likelihood of someone who has either done/tried this or knows the measurements cause they indeed have measured them. And as for what im going to do.....well if all the parts (pistons, rods, and crank) will fit and work then I will use everything but if lets say only the pistons work I will go that route.

 

I am not trying to be an A%$. so im sorry if I come off that way. I am just frustrated with the fact that as simple as I would think this is there is no info out there what so ever that I have been able to find. I think its crazy that I haven't heard of anyone trying, wanting to try and asking, or doing this type of swap. of all the crazy things I hear people swapping out like 900cc pistons swaps and full motor swaps and race bikes, yet nothing on f2-3 internals?

 

It's because you're asking about two different engines that haven't been relevant in 20 years (and pre-internet). Few people build 600cc engines in the first place. The difference between those internals will be negligible compared to the ease of slotting a 900 engine into the steel frame that the F2/F3 came with.

But, hopefully someone helps you that has more definite answers.