How big can 750-4 go?

I’ve got a 1980 KZ750 engine and it’s kinda my pet project. It was given to me for free, in pieces, in a box. I’m a machinist by trade, so it’s been sitting in my shed for some time, but the wheels in my head have started to turn in kooky directions... the cylinder and head are off of the bottom end, doesn’t appear anything is actually broken, but the sleeves do have a bit of rust, one quite a bit more than the rest. I know there’s a few options out there for overbore (69mm Chinese 810 with cast pistons, or Wiseco high comp forged 810cc) but... me being me... I want to see just how insane this thing could be... theoretically, if the engine was factory assembled how I think it probably was, with the cylinder liner sleeves pressed into the block, one could bore them to an extremely thin thickness, then have them nikasil plated... of course finding one off pistons would require $$$$ but I kinda know a guy, I could go full Burt Munro style, but with a little modern tech, CAD my own factory pistons up, but then add to the diameter, and 3d print a set of 4 of them, then lost wax cast each one, to end up with my own custom size piston, and do the necessary finish machining work myself in my lathe and mill. I’m thinking 74mm bore would be possible, with the factory 54mm stroke, and with the bored nikasil plated and also getting the pistons coated (ceramic coating the domes, Teflon coating the skirts) it could make one heck of a mean little rev happy 930cc monster, and the displacement would be enough for gobs if bottom end torque, I could drastically port the head (I know all about the horrid casting lip inside the factory head, just wish I could increase the valve size, but the deck just won’t allow it) I wouldn’t go too ridiculous with the compression, keep it in a cast-piston and pump gas friendly 10 or 10.5:1 ratio. Is there anything else I could do to further add more cubes? Shy of a full billet cylinder block or starting with a whole other engine? I wouldn’t need it to last 100,000 miles, as it would just be a pet project, just curious what it could really do with a bit of mad scientist engineering.

Your brain is working overtime on this one...

I think machining some new larger liners would be a more viable option than messing with plated bores. And most of the big piston houses will design you a piston so no reason to go off the rails there either. Sounds like a viable project, but you won't get near the bang for the buck quotient of the cruzinimage pistons.

Is there a bigger stroke crank I can get without custom regrinding the journals off center? Granted, if I do that I’ll need to move the wrist pin higher on the piston or custom design a shorter rod, (maybe even make my own Ti rods if I want to go completely psycho) depending on how much I can wiggle the wrist pin on the piston. Rod/stroke ratio starts coming into play, but if I just go 1.5mm off center on the journal regrind, 1.5mm higher wrist pin to piston dome height, that’ll give me 3mm more total stroke, which would be a huge jump to 980cc!!! I had considered just going to bigger liners, but the nikasil also has another benefit of lowering the drag, less friction. Bigger liner will hold more heat, thin liner will dissipate heat better (more modern engines just use bare aluminum block with nikasil to get best cooling as well as the other benefits of nikasil) Lots of variables to toy around with... haha

My thoughts are, with the equipment I have on hand, I could go 74mm bore pretty dang easy. Even casting my own custom pistons. Could even do it for about the same price as the cruizinimage 810 kit, given that my after hours evening time is ‘free’ and I can get a set of genuine Japanese rings for around $60 if I go with an actual factory bore size (Yamaha FJ1100 uses 74mm bore to just name one)

If you make new bigger OD liners,,, some part of it will be directly exposed to air...no aluminum. Take a look at the cylinder.

Plus... I kinda want to just be able to say I actually created the pistons myself. Who else does that? Haha

undiablo wrote: If you make new bigger OD liners,,, some part of it will be directly exposed to air...no aluminum. Take a look at the cylinder.

You referring to the part that sticks down inside the crankcase?

Nessism wrote: Your brain is working overtime on this one...

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I know of a GPz750 turbo that has KZ900 cyl. liners and a 3 mm stroked crank. 1010 cc and make over 300 hp.

And yes you can see the liners between some of the fins

You would be money and time ahead by simply installing a hopped up KZ/GPZ 1100 engine in the 750 frame.
82 GPZ750 frame with a modded 82 GPZ1100 engine swapped in.

Well I just purchased a set of sleeves lol guess I’m committed now.
The 900 uses the same 66mm bore as the 750, so they’d be gaining zero cc there. I don’t have a GPZ11 engine laying here collecting dust. (I don’t even have a KZ/GPZ frame, gonna slap it in my raked out, stretched frame and swingarm CB frame haha)

Plus, huge bore and short stroke is a perfect recipe for a high rpm screamer anyways, look at all the modern bikes.