GR Pistons

[Andy.tech]

Has anyone ever tried to install a GR650 piston in a GS? What would the difference be in compression?

[The Buddha]

A GR is only like 2 mm larger but 13mm longer stroke if I recall. Its also old and kinda not actually light or strong compared to a forged piston like from wiseco.
You're better off getting 2mm larger wiseco's made to your preferred deck height etc etc.
No idea, but GR piston wont give you more than 40 or so CC. Maybe 1 point compression but there's a lot better way to get it.

Also compression will give you low end torque and power.
For higher overall output you need rpm. Which means cams, lighter pistons low compression and lower balance weight and lighter valves with stiffer springs.

Cool.
Buddha.

[mr72]

Also compression will give you low end torque and power.
For higher overall output you need rpm. Which means cams, lighter pistons low compression and lower balance weight and lighter valves with stiffer springs.

100% true except you can keep your high compression.

Only real problem in a gs500 is going to be heat. Higher compression or higher revs will both produce more heat and I suppose there is a point at which you overwhelm an air cooled engine. At least with higher revs you only make that extra heat while at those revs. Higher compression will run hotter all the time.

[Andy.tech]

Awesome, but is the GR650 piston the same compression as the GS piston? Is is the piston dome not as big  as the GS piston

[Big Rich]

Andy, do you have a GR piston to do a comparison?

Might look around for info from Mekanix over on the GSResources - i know he has done quite a bit of research with all of the GS twins (425, 450, 500, and 650).

[Andy.tech]

No I don't have one on hand but I think it would be less expensive than purchasing a new set of pistons. I have only seen pictures and they almost look the same but this is why I am asking.

[The Buddha]

I've played with GR pistons a plenty. I am not impressed. They're made of sheieteiey material - even worse than the GS ones, heavier, and a GR is a 3mm larger bore and 13mm longer stroke to get to the 650 cc.
You would be better off getting a 3mm larger forged one from wiseco if you really wanted that 3mm extra dia.
A GS head is lesser volume than the GS head and would get you higher compression if you installed the GS head on the GR. Not sure how much more, I haven't CC'ed the heads.
However I believe the pin height of the GR is less than the GS because its a lower compression motor than the GS if I recall. I suspect you'd lose a lot more compression from the piston height that you would gain from the lower head volume.

A GS head on a GR motor in a GS frame could work and get you a wee bit more compression which could help cos the GR motor can only spin to 8k, and better yet the GS head will solve the problem of the GR exhaust ports facing the GS frame down tubes.

I have contemplated doing that to the GS/GR and locking the disengaging flywheel to get that GR motor to not flipping do a Dr Jekyll and Mr Hyde on me @ 3500 rpm.

Thanks.
Srinath.

[Andy.tech]

I was just thinking of using the piston and installing it on the GS rods because they both use the 18 pins according to both repair manuals. It be cool to see them side by side to see the difference.

[mr72]

All this talk reminds me that I really want a GR650. That'd be a really affordable "British Twin" project bike. Lighter than a Triumph and maybe even a better overall bike than an XS650 or W650, loaded with classic British styling. Wonder if the rear can be converted to disc brakes.

[The Buddha]

All this talk reminds me that I really want a GR650. That'd be a really affordable "British Twin" project bike. Lighter than a Triumph and maybe even a better overall bike than an XS650 or W650, loaded with classic British styling. Wonder if the rear can be converted to disc brakes.

That and a XS650 Yamaha. And that's one more reason why I cringe when I see more and more and more of these turned into retarded a$$$ bobbers.
Cool.
Buddha.

[mr72]

That and a XS650 Yamaha.

No doubt. But the prices on the XS650s are pretty high. Seems GR650s are not on the classic/collectible radar yet.

And that's one more reason why I cringe when I see more and more and more of these turned into retarded a$$$ bobbers.

No kidding. Or attempts at café racers.

[Big Rich]

Snip......

Wonder if the rear can be converted to disc brakes.

Yep. Alloy swingarm from an 83 GS750ES, katana 750 wheel, Bandit 1200 brake stay, GS500 caliper & rotor, blah blah blah. As far as using a "stock" looking wheel? Not worth the headache to me....

No kidding. Or attempts at café racers.
[/quote]

Guilty as charged. Haha.