GS550 Bored 740cc Officially Done!

[wera90ex]

Just finishing up a GS 550 project

[steve murdoch]

Just finishing up a GS 550 project

New business card? lol.
You should start a new thread with some details on your new hot rod.

[Guest]

imagine my shock to see one of my 6 year old threads bumped to the top. Makes me feel like a OG around here.
lol

bike looks good wera90x. What class are you gonna run that in? lol

[Nerobro]

imagine my shock to see one of my 6 year old threads bumped to the top. Makes me feel like a OG around here.
lol

bike looks good wera90x. What class are you gonna run that in? lol

... Does this mean my 5/650 is even older? Dear god i'm old.

[Gammaboy]

Stupid question - why would you do this (ie GS550/650 big bored to 740cc) vs using a GS750 motor?

[boontonmike]

Easy, 750 is larger. 673cc has same power as 750, and 550 base is 6 speed trans.

[Guest]

Hey brveagle, How about an update on the 740 operating experience? Ray

[tkent02]

Stupid question - why would you do this (ie GS550/650 big bored to 740cc) vs using a GS750 motor?

And the 550 is much much lighter, shorter, quicker and more nimble than the 750.

Ray, you can ride mine all you want.

[Guest]

Ok Tom, Once the weather returns to a more suitable (for me) temperature of at least 75
f, I'll take you up on that. Did you bore yours out or is it still 673cc? I'm thinking the 740cc version might be a bit of a terror? Ray

[tkent02]

No, I just bought another 550, very nice original one. I think it will get the 673 treatment, plus nice suspension and brakes, and then my original cafe clone will get the kicksass 740 or whatever it is engine.

The problem for me is, I am always short of round tuits

[Guest]

673cc + 6 speed tranny + nice and light =

[Gammaboy]

Right, got it. makes perfect sense

Are the GS550/650 a pressed together crank?

[Stahlgrau333]

brveagle, what adaptations were necessary to make the GS750 pistons fit?

[Guest]

Sorry for the delayed response guys, don't log into here too much.

I ended up "detuning" my gs550/740cc bike. It was just running too hot. I was even running an oil cooler and full synthetic.
It's back to a 673 for more reliable performance.

To answer some questions, I did not really know how to build a motor when i did this. It was a HUGE learning experience to me. The gs750 2 valve pistons share the same wrist pin as the 550's (80-82). However, they were hitting the head. I then had a block spacer milled in the amount of 4 mm. It was "just a guess" after much "measuring". at this time i did not know much about squish and how to measure for it. I ordered it through cometic. Great company.
This sunk the pistons in the cylinder, which is not ideal, but the bike ran great. (other than being really hot, ALL THE TIME).

It if had more free time, i would really love to experiment with smoothing out a 650 head to remove the high spots of the combustion chamber in the head. I would imagine by doing this, you would have a "high compression" motor that would not require a block spacer.

Summary: Still love and ride my gs. Just have it as the 650 top end.

[Stahlgrau333]

Thanks for the update, brveagle. I have been wondering about the heat issue, so that input is definitely appreciated. I was also dying to know how you got the 750 pistons to 'fit', thanks for clearing that up.