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.
I have been thinking about doing the same since picking up the spare engine. My machinist is brilliant with this sort of stuff, so i might have to take my cylinder's down to him and see what he say's. Fitting a 49.5mm piston should be no trouble at all, that's only .75mm off the bore wall. The 1.5mm O/S piston will take the FZR250 out to 266cc, it'll give it a bit more torque as it'll now have a 1.43 bore/stroke ratio like a late model engine does The CRF450 has a 1.53 ratio.
I reckon it'd just be the cam's and cdi that would be restricting the FZX250, rest would be the same, maybe some difference's in the head too (different part number) and the cylinder has a different number too You can get OEM 1mm o/s piston's for the FZR250 3LN, and 1mm O/S piston's for the FZX250 (different part number's)
Bigger piston's will help pull more fuel into the cylinder due to less shrouding of the valve's, so will make a bit more power through the whole rev range, especially in the higher rev's Some minor porting/clean up in the intake and exhaust port's will also help, as would slightly bigger valve's, carby's may need rejetting but they won't be too far off. And making a bit more power from the engine you might not need to rev it out as much so that would help longevity I'm guessing the bigger piston's will give 2-3 more horsepower, same with the head mod's depending on what's done
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.
FZR250 liners are 53mm OD. 49.5mm bore is as big as I'd want to go on std liners. Much thinner and you risk them cracking at the top flange corner. From memory GSX250 gudgeon pins are 15mm VS FZR250 at 14mm. Doubt if there's enough meat in the rods to open them up even 1mm... I'm assuming you guys have read my thread re the 150 twin - and thought "that looks easy" Ha - it isn't. But no one's stopping you having a go.
All good linkin. If you go to the trouble of bigger liners then the crankcase will have to be enlarged too.
Actually that's the easy bit...The top of the cases is simply open with the barrel sitting on the rim of a large opening. The only cutting required with bigger liners is to the gasket itself.
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.
if you're at the planning stage and giving serious consideration, shoot a message to some piston manufacturers, Wiseco etc, to see what's available with the correct cylinder bore, gudgeon pin diameter and deck heights from other bikes, you may be pleasantly surprised... If you get them from them you may be able to request specific head volumes for compression and valve reliefs machined for the Fizzer. Imagine Nikasil plated aluminium barrels, it can all get silly if you wanna pitch the money
It's what I was thinking too. But you're right - give it a go, throw some money at it - if it doesn't work at least you've either bought a few HP or wisdom...
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.
gregt did the tricky work on resleeving to 53mm successfully klx110 and drz110 bores are 53mm with gudgeon pin diameters of 13mm and there are 4 valve heads/pistons available for both which might have the correct valve relief locations already cut sleeving little ends on conrods with brass bushings is a done thing not sure on the deck heights though, and given that piston designs have evolved with less material in the skirts weight may actually be favourable resulting capacity would be 304cc
These Suzuki GSX250R piston's have a 14mm gudgeon, GJ72A GJ73A GJ74A https://www.aliexpress.com/item/Mot..._7&btsid=e38dbc1f-6d7d-4d59-b0c6-145f94807a68 They are 43mm tall overall, with 12.3mm compression height (questionable) Some of the piston kit's for the GSX250R are listed at 41.2mm tall and some taller again But looking at the pic's there's no reason you couldn't trim off the 6mm-7mm or whatever and taper the edge's There are also variance's listed in Compression height, and the Suzuki piston is listed as 12.5/1 compression too so will need to be double checked for clearance's Standard GSX250R piston, 49mm Dimension's/pic taken from here FZR250 standard piston, first one has the compression height listed at 13.3mm, second one is listed at 11mm https://www.aliexpress.com/item/mot...id=6c3f43c3-d33c-4e87-aa9a-c09c2ccdd4e3&tpp=1 https://www.aliexpress.com/item/Hig...341692121.html?spm=2114.40010208.4.133.d3bpcy
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.
I'm currently doing a 67mm piston/bore conversion to a KLX110 based engine (YX160, 60mm bore) for a mates' pit bike, 201cc
just to muddy the waters - 53mm sleeves and there are others available in larger diameters as big bore kits for the small capacity mx bikes https://www.lasleeve.com/cylinder-sleeves/l.a.sleeve-sleeve-h5197 those gsxr piston kits are too cheap to resist at least having a look though