GS450/GS500?GR650engine swap...

[CarmeloLabadie]

Hello,
I hear you guys are the experts, so I was hoping you could help me out!
I am interested in swapping a GS500 motor in my GS450, or just using the GS500 top end. I read some of the threads and everyone says the problem is the GS500 exhaust comes straight out, hitting the GS450 frame. I have also read the GR650 head has similar exhaust outlets to the GS450. Could I use the cylinder off a GS500 and the cylinder head of a GR650? Does anyone know if this combo might work?
Thanks,

[Chuck78]

Look up the YouTube video "GS527 beast."
The GR650 head sort of fits but the cam chain tunnel area barely seals due to almost no overlap on gasket surfaces, it's not ideal and will leak oil.
The 550 cylinder is slightly too short
The GR650 cylinder in no way fits whatsoever, way too tall, crankcase end probably different also.
The 500 pistons' valve reliefs dont really line up with the 450 valves either but people have ran this combo with stock cams and not had interference. Re-machining the pockets is a better bet. The GS500 also is only like 472cc or somewhere near, not a huge upgrade.

The GS527 Beast was GR650 pistons bored into a GS500 cylinder (+ 2 base gaskets sandwiching a thick aluminum base gasket spacer?), And the compression ratio was lower than stock because the pistons & head are designed for a very long stroke.
And then there's the issue of the eventual nuisance oil leak since the gasket surface is barely overlap in one area in particular.

I forgot the name, but there was a member on here several years ago that had custom pistons made that would be the correct compression ratio to run the 650 head with a 450/500 crank.
Actually come to think of it I think he may have been using a GS400 bottom end and did some crazy work to move the stud spacing to match the 500/GR650, so the stroke was different on that even.

[Chuck78]

You would be much better off sending the head out for bigger valves and mild porting, and getting some custom big bore pistons made. Contact Rapid Ray Rains on here, or Dynoman Performance or APE Race Parts (Big Jay of APE is occasionally on this forum, he posted some GS400 big bore head mod pics with oversized valves a few years back.
Run GR650 cams with GS450 cam sprockets for good street torque, or browse the MegaCycle Cams catalog for GS400-425-450-500 cams

[Chuck78]

https://youtu.be/tQEVHd1cSyw

Stephen M (Manic Mechanic on here? Mechanix actually I think) said that's a 500 head and cylinder with GR650 pistons actually. I could have sworn it was the GR head. Take a close look at the exhaust, he says it is modified to clear the frame, but the only one I've seen before like that had a very obvious 90° turn, unless this head was modified and bored at an angle for proper exhaust exit.
The one I remember seeing was very clearly a sharp 90° elbow in the exhaust as it points directly at the frame.
Maybe machine out the head and then machining an insert to press in and machine at an angle would be the best bet, but that's some custom work.

If you search, they're formally was I believe a Wiseco GPZ piston, GPZ 1100? From a 4 cylinder. That was very popular to run in a gs500, but the manufacturer changed the dome design slightly, and it was no longer compatible with the GS500 head on the newer versions. That put a GS500 above 550cc if I recall.
Aside from the shift drum, a gs450 and a GS500 bottom end are nearly identical, slightly different engine mounts on the cases though.
The GS500 shift drum is reversed for linkage type rear set shifter.

[Chuck78]

You can go a lot bigger on the 450 stud spacing, but I am going to finally be building up this GS400 frame (I JUST GOT IT TITLED!) with the parts that I had for my stock GS425. The roller bearing bottom end should tolerate a lot more power, and I will be punching it out to 489cc 72 mm pistons as I have three left from a blown up GS750 920cc engine of mine.