Hi all, new to this forum, looking for any useful advice re building an F3-based racer.

I have two f3's. One is really clean and worth keeping that way, the other is complete but a dog. I want to build it into a racer for post classics here in NZ. Rules here allow for up to 500cc twins to run in the F3 class, and while I won't stand much of a chance against the FZR's it will be a good start and a simple way to get going using as much of the current bike as possible. I can always upgrade later once I get a feel for what might work longer term.

Given the bike currently has a 70.5mm piston and 51mm stroke I figure the easiest route would be a 79mm piston. Keen to know if anyone else has tried this on an f3, or generally any other ideas people may have re building an f3-based racer but staying under 500cc.

Cheers
Roger

Check out this link on Brad Black's website http://www.bikeboy.org/duccapacities.html Brad is also a contributor on this forum.

I am curious to know if anyone on the forum has dabbled with different bore and strokes?

I recently learnt that this bike has 88 bore (standard for a 750 F1) and 74 mm stroke which apparently makes it a 900cc
https://www.facebook.com/100007753784746/videos/1593986710869773/?fref=nf

those engines all use the same deck height, and the longest factory stroke used is 66mm in the 800 motor. which given the 15mm difference means the piston has 7.5 mm of useless comp height. get some carrillos made that are 8mm or more longer and that way your custom pistons can be nice and short.

i think bruce meyers used to run rods in the 800 that were 3mm longer, so there's 10.5mm.

if you have to have cases that say 400, get a late m400 engine and use that as a basis. 2001 onward bottom ends are better, altho the swingarm pivot is different. otherwise get any 2001 onwards small block bottom end, like the yellow bike iii engine build.

using the 79mm bore almost gets you to the 80mm of the 600 pantah, so you could use those heads to get some valve into it.

love the f1 rules:

Formula One

431cc-1300cc multi cylinder 2
Stroke
601cc-1300cc single cylinder 2
stroke
601cc-1300cc multi cylinder 4 stroke

when the hell was the last time anyone saw a 601cc-1300cc single cylinder 2 stroke?

Then, if you want to go quick and cheap you could always insert a stock 58mm crank (500 to 600cc engines) and 74mm pistons (499cc engine). Maybe you could find a new/good set of pistons. But if you could build the rods and pistons using that 51mm crank it certainly should rev high. MikeV

PS: It is always hard to tell what the hopes/plans are for someone starting something like this; to win a club championship, or just go out for the fun of racing. One is expensive, the other less so.

you need the bore size to get the valve size into it. stroke is a byproduct when there's a cc limit. but it will help with the overall.