Help Overboring an FZR/FZX. Options.

Got ideas swirling around in my head about boring out an FZR/FZX engine using the standard liners if possible (to save $$$ and get more grunt on the cheap). Chinese FZR pistons only go up to 48.5mm. Can get GSXR250 oversize pistons & rings that are 49.5mm. Without changing the stroke of the FZR/FZX (read: custom rods), and assuming the piston pin is in the exact same spot, this would bring a fizzer or zeal up to 265cc. Not bad.

Would need to know all the dimensions of the suzuki pistons & what size piston pin they use. Also need to ensure there are no piston to valve clearances issues, or problems with the piston skirts hitting the crank.

And that is assuming the standard liners will accomodate a 1.5mm overbore safely... I intend to find out.

 

The zeal has smaller ID throttlebodies. 26.5mm vs 28mm of the FZR's. I believe the carby boots are all the same (32mm, but in that weird oval shape where they join to the head) so running 3LN carbs wouldn't be a problem for additional performance/tuning. I was also thinking of taking a set of barrels down to an engine shop here and have them poke around to see what can be done, maybe press out one of the liners and see how thick it is.

I am just trying to find the max. overbore that can be done on the cylinder block with the standard liners and some appropriate pistons to suit. The idea being that all you are changing is the capacity of each cylinder. I was initially aiming for 300cc but that would most likely mean that custom liners would need to be made, and the cylinder block casting also would need to be bored to accomodate. Good long term project though.

The zeal has a lower redline and makes torque lower in the rev range, which I think would see the best power/torque increase with an overbore and some other tweaks.

 

Thanks for that. I brought up one of the rods I took off my stuffed 3LN6 crank and measured it. The OD reads 20mm and ID 13.8mm I'm no metalurgist but 0.25mm off the ID *might* scrape by? In any case it would involve taking the rods off and I was hoping to get away with not having to split the cases. Unless you're comfortable taking the rods off with the cylinder block off, and not losing the cap & other half of the bearing in the sump or gearbox... Otherwise, get custom mods made up.

Then there is still the weight of the pistons and piston pins to worry about. More rotating mass and more weight at the same velocity.

 

Having another poke around the eye of the conrod... at the thinnest point they are 2.7mm... Sadly I think 0.5mm off either side of the ID, not 0.25mm as I previously posted, would be a bit too much. Would need to have custom rods made I think, which defeats the point of trying to do it on the cheap.

 

You are a legend

Looks like I would need to trim the skirts if there are clearance issues (an easy thing to check) and maybe use 2 base gaskets to compensate for the taller piston.

I even have a fizzer engine with the head and barrels off to play with.

In any case I went ahead and ordered a set of those GSXR bits.