GS500 motor into a GS425 bike?

[MidWestCoast442]

I'm considering options for my GS425. Love the bike. But more power would sure be nice. The motor that is in it right now is tired with one of the valve seals leaking oil (not a lot...but some) into the #2 cylinder. The spare motor is in much better shape...but there is a small crack originating at the threads for the oil drain plug.

So how about putting a GS500 motor into a GS425? Can it be done? Has it been done? If so, then what is involved to make it work?

Thanks!

-Ryan

 

[gsjames]

I'm considering options for my GS425. The spare motor is in much better shape...but there is a small crack originating at the threads for the oil drain plug.

So how about putting a GS500 motor into a 425

-Ryan

Seems to me you would need to change electronics, carbs, mounting tabs... could get spendy.
My 450 has a bottom cover with the drain plug in it. The cover is easy to get on ebay and easy to change. Welding and brazing are also an option. Take a close look at the 425. Perhaps I am wrong.

 

[Big Rich]

Can you put a GS450 motor in a 425 frame? I would think so, but never read about it..... if it's possible, than a 500 shouldn't be a problem.

Except for the exhaust ports sticking straight forward on the 500 head.

 

[MidWestCoast442]

Sounds like it would be best to work with the two 425 motors that I currently have

 

[Chuck78]

GS850 pistons bore right into the GS400 and GS425 cylinders! As do the Wiseco 844cc GS750 pistons, same diameter as GS850 but higher compression. Ask Mekanix about this, as your balancer shaft in the front of the engine in front of the crank may need a little weight added to it to balance out the extra weight of the 2mm larger pistons.

I may get custom 71mm pistons made for a 475cc GS425 come summer or fall 2015. Wiseco could possibly make one just based on the info they have for the GS750 844 kit and 750 combustion chamber to calculate the proper dome size for 10.25:1 compression (max you'd want on street 10.25-10.5).

Alternatively you could score some used GS850 pistons and then have the head and block milled to increase the compression ratio and give you better head gasket sealing surfaces, but then you would have to slot your cam sprockets and degree the cams back to near stock, as they will be slightly retarded now. The Wiseco pistons are a more expensive easy way out.

I badly wanted to buy a '77 GS400X chassis with a GS450 engine and GS500 head/cylinders bolted together. The engines all are swappable. The 500 head would require custom exhaust to fit the older frames, however. It has been done by a member on here though.

I have been even contemplating boring in shortened GS1000 (70mm stock) or GS1100G (72mm stock) sleeves into the GS425 jugs so that I could get JE or Wiseco to custom make me some 10.25:1 pistons in a 73mm (502cc) or 74mm (516cc) version, and sending out the head to Ray or someone to have larger valves and port work done to it. Once I get through some major house renovations, I will be building something along those lines. I figure I can get a kickstart GS400/425 down to 335-340lbs from 382lbs stock, and boost the performance substantially with less weight and more power with some GS400C/GS500 cams or possibly some $$$$ MegaCycle Cams (aftermarket), big pistons, head work, K&N/re-jet/performance exhaust. I suspect the bike will be at least as fast 0-60+ as a modified GS550 or close to a GS650. Top end will probably be lacking the torque to push against the taller gearing in 6th gear and the wind resistance, so I suspect it'd slow down then a bit, but I think this will be a pretty great combo. I think the longer GS1100E aluminum swingarm will be necessary to keep the front tire closer to the ground on launches instead of potentially launching you off the back of the seat on drag racing clutch dumps.