gt750 cylinder swap / mod

[2strokeforever]

has anybody tried or manage to fit a cylinder from an other two stroke engine on the gt750 engine?

[Suzsmokeyallan]

There are no other two stroke engines that have a cylinder even close to a GT750s in design, it would have to be CNC machined from a piece of aluminum billet or casted.

[tz375]

I had a picture of a set of H2 top ends grafted onto a GT bottom end with a big fat spacer plate to get the right height and presumably to rotate the transfer ports.

A bigger issue is the tight spacing on the left two cylinders. Modern top end have much wider transfer ports and to fit them requires moving the center cylinder towards the right. It can be done but there's no point. A GT motor can make over 200HP with a Brett deStoop cast top end on it and can make an easy (? 125Hp) with conventional porting - and that's at the rear wheel.

80RWHP is easy on a street motor with pipes a spacer, 34mm carbs and a port job. That's good for way more speed than the rolling chassis was designed for and almost twice stock HP (46-48 at the rear wheel).

[2strokeforever]

i had this crazy idea about 2-3 years ago when my brother in law bought a big bore cylinder kit for his yamaha banshee guad from trinity racing in the US. As a lot of you may know the Banshee is a 350 twin 56 bore x 54 stroke and use the same engine as the RZ/RD models.
The big bore kit called Cheetah is a monoblock desigh with nikasil coating, exhaust boost ports, pneumatic power valves and comes in various bore and stroke types, great stuff!! Modern two stroke tech cannot be beaten.

As tz375 correct said the bigger issue is the left and center cylinder spacing, being 102mm center to center.the yamaha engine has the same spacing between cylinders.Of course that involves a lot of welding on the crankcase in order to mach the transfer ports,relocate the cylinder bolts,rebuild the crankshaft with shorter rods... Oviously this is not the easier way if you want big hp but i have to say i like the challange.

What makes everything more difficult for me is that over here we don`t have engineers that can make custom jobs, they will do rebore and crankshaft rebuild(as long as the big end pin is not weld).My brother in law has a crankshaft with the pin weld and they cannot do it,so i made a call to Chris Applebee in the UK when i ask him if he can rebuild it he said "man made it ,man will doing it again" . As far i know nobody ever made a porting job to a two stroke engine here before.
The Cheetah 535cc cylinder kit 73x64 is the same stroke like the gt motor but for $2,500 for the top end kit (plus that i need a 2nd cylinder to make a triple) is a lot.I do know that they make a monoblock triple cheetah cylinder that will fit the Mattoon billet cases but that is 102mm spacing between all 3 bores and that will not work on the gt motor.He is the web site of the foundry that make the castings http://www.cpindinc.com/" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;
How much do you think will cost to have your own cylinder casted?

Last winter i start looking for other donor engines and like others i turn out to snowmobiles.Ski doo rotax cylinders seem to be possible for the job,i bought 2-3 different types just to study them.The good thing about snowmobiles is that you can find a lot of used parts on ebay cheap and although you will be modifing your engine you will be using stock parts from an other, so in terms of reliabilty i think is better.
Ski doo mxz 600 use (2) 76mm bore x 65.8mm stroke cylinders, is 1.8mm longer stroke from the gt. If i just seat the cylinder 0.9mm lower will this work? ports will work? also in order to fit these cylinders not only i have to move the middle cylinder more to the right but the left more out ,this is the only way to make more space.To do that i need to machine off 3-4mm from the crank wheel/throw(narrow the width of the wheel). To make you understand better on the left pair of wheels i need to narrow down only the left wheel and on the middle pair of wheels to narrow down the right wheel , this is the olny way to space out the rods.
What i do not know is after all this job the crankshft will balance?

forgive me for my english, but i`m tring my best..

[tz375]

You have done the same research as I did. I bought top ends from sleds and MX bikes and measured all sorts of things. I looked at Matoon cases and CPI top ends and the problem is lack of room between L and C cylinders.

The smart solution is a set of Matoon cases with three CPI top ends for around 180HP but that's beyond my budget. I also looked at welding up the stock cases and that's a good idea to get the studs moved as well as bigger crankcase ports and allows for a larger block.

The block can be heavily modified but to get power up to crazy numbers needs at least new liners and a lot of welding ad preferably a Bret deStoop big bore block and head. There's a sidecar race team in NZ who just machined a new head from billet and they could probably modify that for a larger motor. IIRC, their motor is 900cc or something in that region.

What we are talking about is a complete redesign to add a lot of port area to a motor that was designed as a tourer 40 years ago.

Did you see the German or Swiss motor ( I don't recall which) - 1000cc, powervalves, intake reeds, etc. A huge amount of welding and machining.

PS - Your English is excellent.

[ja-moo]

You have done the same research as I did. I bought top ends from sleds and MX bikes and measured all sorts of things. I looked at Matoon cases and CPI top ends and the problem is lack of room between L and C cylinders.

The smart solution is a set of Matoon cases with three CPI top ends for around 180HP but that's beyond my budget. I also looked at welding up the stock cases and that's a good idea to get the studs moved as well as bigger crankcase ports and allows for a larger block.

The block can be heavily modified but to get power up to crazy numbers needs at least new liners and a lot of welding ad preferably a Bret deStoop big bore block and head. There's a sidecar race team in NZ who just machined a new head from billet and they could probably modify that for a larger motor. IIRC, their motor is 900cc or something in that region.

What we are talking about is a complete redesign to add a lot of port area to a motor that was designed as a tourer 40 years ago.

Did you see the German or Swiss motor ( I don't recall which) - 1000cc, powervalves, intake reeds, etc. A huge amount of welding and machining.

PS - Your English is excellent.

A point of dimishing returns? It's kinda like saying you have a fast harley, when only the decals are Harley. Say you do spend huge bucks, it's not a buffalo anymore........... To me, the accomplishment is taking what you got and making it the best it can be, within it's own design barriers......

[tz375]

I agree JA. It's where the thinking takes you though. It's the old process of taking one apparently logical step after another.

I wonder what I can do to improve porting
Maybe different barrels
Oh that needs a different case
Ah that needs a different crank
Ah well I may as well change the trans and clutch while I'm there
Uh Oh. Nothing left of the original.

So then we go back to step 1 and try a different way to "improve" it.

[ja-moo]

I agree JA. It's where the thinking takes you though. It's the old process of taking one apparently logical step after another.

I wonder what I can do to improve porting
Maybe different barrels
Oh that needs a different case
Ah that needs a different crank
Ah well I may as well change the trans and clutch while I'm there
Uh Oh. Nothing left of the original.

So then we go back to step 1 and try a different way to "improve" it.

I always tried to keep things simple, here is the pile of parts, let's see what I can do..........