GR650 top end on GS450 bottom end?

[Guest]

So, I have a nice dependable 81 GS450S and I want to make it into a 650. Short of finding a GR650 engine, what are my best choices? I do have a GS500 engine, complete. I know the exhaust ports are different causing frame interference, but I've heard the cylinder will fit the 450 bottom. Will the 650 head fit the 500 cylinder? Can I install a complete GR650 top end onto my GS450 bottom end? I would guess I'd need cylinders, pistons, head, carbs? Should I stop thinking like this and just ride the 450?

[Octain]

Yeah just ride it as is. If you want a bike with more power buy one.

[pete]

There was a thread around the place here that unfortunately got purged, but yes it can be done, or something along those lines anyway. It was definitely a 450 with a GR650 head on it, but it was a lot of work.

Somewhere out there in the internet there is a video of it, but I can't find it at the moment.

The guy who built it did post in the thread that was purged, but I have no idea now who it was I'm afraid...

[Charlie G]

There was a thread posted last summer, which I have heard was deleted, which discribed the process. I believe they used 500 cylinders on a 450 engine, with a 650gr head. I think it acheived 527cc, but I think I'm missing something. Scottychop posted the other day that he isnt going to complete his "mega build". He had the parts do do it. You might want to check with him on the particulars and if his parts are available. There were a couple of guys that posted that created this 650gr/450/500 combo bike for racing.

If it were me, I wouldn't want to alter such a nice 450 as yours, but would seek out a 450 bike that really needed some help. I have had the desire to create a project bike, altered and assembled from several bikes. In the end I've opted for a larger bike and my gs650g fits the bill fine, at least for now.
I like twins and had at first sought out a Yamaha xs650. They are expensive and hard to find. Good luck. Did you get the paypal?

cg

[olddaddy]

Charlie G, did get your paypal and the gauges are in the mail. The 650/450 engine will probably be built up on my spare motor. I've got a very low miles 450 and a 500 just sitting on the shelf. I like my little 450, but I am a tinkerer at heart and 650 just feels more right somehow. I've gathered up some other spares and will probably do something over the summer misery season here in sunny and wet Florida. I envy you living in Yreka, I have fond memories of Northern California.

[Charlie G]

Charlie G, did get your paypal and the gauges are in the mail. The 650/450 engine will probably be built up on my spare motor. I've got a very low miles 450 and a 500 just sitting on the shelf. I like my little 450, but I am a tinkerer at heart and 650 just feels more right somehow. I've gathered up some other spares and will probably do something over the summer misery season here in sunny and wet Florida. I envy you living in Yreka, I have fond memories of Northern California.

Small towns have their good and bad pionts but I believe this is how people are suppose to live. Sometimes I have to pinch myself as the community and the views can take your breath away. When it's bad I just remember commuting to work in S.F traffic.

cg

[Ranger]

Keep the engine that works as a back up incase the plan fails. Why not just buy a more powerful bike that doesnt run and fix it up? It'll probably cost less in the end.

[Flaming Chainsaws]

Send scottychop a PM. He's selling a GR650 head and GS500 pistons.

[scottychop]

Actually, just the top end from the GR650, and cylinders from the GS500. You would need to pick up pistons from a 500, or bore out the cylinder to fit the impossible to find GR pistons. I had heard a rumor that FJ1200 pistons would work fine too. GStwins forum has a few threads on this. There is also a GR forum too. New cam chain tensioner and GS500 carbs and you would be set.....

Pm me if you are interested in some parts.

[GregT]

A couple of things here....the GR650 head on 500 barrels will IMO give too low a compression ratio - might just as well use the 450 head as valve sizes are pretty much the same as the 500.
While the FJ1200 is the required 77mm bore size the valve cutaways are completely different - what caught my eye was the 76.2mm standard bore size of the 883 Harley. Don't know about pin sizes but I'm sure something can be done. There's got to be stacks of 883 pistons lying around as the common thing was to put 1100 top ends on them...maybe old stock forged pistons too ?

[olddaddy]

So, the best option would be a GR650 engine complete. Next would be to install the GS500 barrels and pistons on the GS450 crankcase and use the 450 head. My goal was a 650 engine in the 450 frame, but a 500 would be ok too.

[scottychop]

Hmmm...I hadn't thought about just re-using the 450 head. How do valve sizes differ between the two? I can run out and measure later if anyone is interested... I have both sitting on the bench.

[Charlie G]

I thought one of the points of using the GR650 head was it's larger valves? Better flow. I wish those threads wern't missing.

cg

[scottychop]

Pics:

GS450 head vs. GR650:


Valve comparison:


GS450 measurement:


GR650 measurement:

[GregT]

Agreed, the valves are bigger - as you'd expect on a 6mm bigger bore.

But the problem is the combustion chamber volume...the 650 head is too big for anything less than 77mm bore. All very well having it breathe well but if you can't get more than 7 : 1 compression it's not worth putting it on.

Looking at the head shape I see it's a bathtub - very modern. it may be possible to use the HD pistons I suggested as I believe they're flat - tops and it looks possible to deck the head to achieve a reasonable squish band.