GS550 to 650 cylinders questions, compression, heads, vm vs bs

[Chuck78]

I'm considering finding some 650 pistons and cylinder jugs to swap onto my wife's 77 GS550. I would really like to retain the VM slide carbs, Which would mean I have to keep the 550 cylinder head. The 550 cam is definitely more radical than the 650, & I assume the 650 may have larger valves.
The biggest concern I have is what compression rato I'll end up with using the 650 pistons & 550 head. Any ideas? If the domes on the pistons are the same volume, then I Im probably in trouble as far as Running pump gas, As the larger Combustion chamber itself without any changes in combustion chamber volume will increase the compression ratio drastically.

How much this will increase the compression ratio is what I am unsure of. If the piston domes are smaller then maybe I would be alright. If anyone could tell me the volume in cc's of the cylinder head's combustion chambers on both the 77-79 550 2v head, & the 81-83 650 heads, that could potentially answer my question if they were both identical volumes - then I would know that I'd have the stock 650's 9.4:1 compression.

If the chambers are not the same volumes, then it gets a little more complicated and is over my head as far as calculations.

[tkent02]

The 650 pistons and 550 heads are not shaped the same, it may take custom pistons to use the 550 head. The 650 has a better piston shape for more power.

[Chuck78]

Well in that case has anyone ever fitted VM carbs to the later heads? I know that the 650 uses a unique exhaust and air box, so this maybe out of the question and the CV BS32 carbs might be the only way

[Chuck78]

From what I have been reading, The head and piston design on the 650 is far superior to any of the older 2 valve model designs. The 650 also have 9.4:1 Compression like most of the 1980's Models, Where as the 550 head is in the high 8 range.

So it appears that running a custom base gasket is necessary due to the thickness needed, the 550 block needs board out or die grinder massaged to fit the 650 sleeves, 650 jug head gasket cylinder head and pistons are necessary, 550 or 650 cams will do - 550 has more duration 650 has more lift.
On a cafe racer website as well as the GS forums, I have read of 2 instances of people modifying 750 pistons to fit in the 650 sleeve but it was quite a lot of work to make them match up with the head design.

[boontonmike]

Best bet is 650 head, barrel, and pistons. Hone out the 650 head and give pistons new rings. Use the stock ( but new) base and head gasket for the 650. Use 650 cv carbs. Carbs from the 550 are not worth trying to make work. I used later model 34mm CVs and the bike runs great. CV's give better streetability. Jet the carbs along with pods and exhaust and it will boogie. Gear the bike to your style.

Looks like you have a handle on searching for info on it already. I'm sure more will chime in. I was looking into the 750 pistons, but it looks like oversize 650 pistons are the easier, yet harder to find route.

[Chuck78]

Oversized 850 pistons are no longer available ( was looking into them for my 750), & I'm sure 650's are probably the same deal unless you get really lucky and find and old set laying around at some shop.

The Pistons domes on the 750 apparently need some particular machine work to make it properly to match the 650 head, but with maybe +1.0mm oversized 750 pistons, you can achieve 740cc on a 550 crankcase.

I'm going 920cc w/850 cylinders & 750 crankcase on my bike :-)

[Chuck78]

I also read that after checking deck height, previous builders of this combination determined you needed to make a custom base gasket in order to get the proper deck height with a .125" thick base gasket.

[Nerobro]

Your'e overthinking this. :-) The GS650 pistons, head, and cylinders drop directly on. (after relieving the crankcase that is..) the carb choice is dictated by your choice of intake boots. The 650 uses the 79+ larger boots,for the CV carbs. if you want to retain the VM carbs, just use the smaller boots.

The 550 exhaust bolts up just fine to the 650 head.

You also need to keep the GS550 cams. The 650 cams aren't clocked the same, and the crank sprocket is larger on the 650.

[Toasty]

Wow is that all it takes? It seems pretty simple... maybe once I get my bike back together from this last project... hmmm... I know the 550 and 650 engine are pretty similar, I wonder what the difficulty level is between boring the 550 to 650 or just dropping a 650 engine into the frame.

[tkent02]

650 engine in the 550 and you would lose the six speed transmission of the 550.

Boring the 550 engine, well why?

The 650 top end on the 550 is actually 675cc, and a better engine design, why not just go with that?

[Toasty]

By boring I meant putting the 650 top end on, poor choice of words. I haven't ridden a GS with a 5 speed tranny, I wonder what the gearing is like on those. I like my 6 speed though but I hear the 5 is stronger?

[tkent02]

By boring I meant putting the 650 top end on, poor choice of words. I haven't ridden a GS with a 5 speed tranny, I wonder what the gearing is like on those. I like my 6 speed though but I hear the 5 is stronger?

Where did you hear that?

[Toasty]

http://www.thegsresources.com/_forum/showpost.php?p=1163639&postcount=11

Very nice job! Keep this in mind, the 6 speed has narrower gears than the 5 speed & they are roughly the same total width. This means the 6 speed won't take the torque & abuse the 5 speed will. I think you will still be ok because back then they were building stuff so over kill that it lived anyway. Ray.

I seriously think I'm going to do this next. I'm gonna hold off on engine paint and frame PC, it's time for more power .

[Toasty]

I've been reading all I can find on the swap lately. Can you use the coils and ignitor from the 550 after converting to 650?

Is it possible to save the original 650 head gasket? I know the top end slides down on the studs from the block and I'm hoping it's not necessary to disassemble it too much. The head gasket is over a $100!

edit: I dug through the parts fiche, the 550's and 650's use the same ignition stuff except for the 650G. The coils are different on the G model, maybe for more tank clearance.

[Chuck78]

Well I ran into a wrecked GS650G parts bike yesterday that may potentially be a good donor for the whole top half of the engine to Reda's old 550 engine, as well as having a complete dual disc setup on the front still - which could be a donor to my single disc GS750B since they both share 35mm fork tubes! Things are looking good

Can you use the coils and ignitor from the 550 after converting to 650?

Definitely can do. Read up on the loudgpz's custom GM HEI ignition module, as the stock ignitors do fail with age.

Is it possible to save the original 650 head gasket? The head gasket is over a $100!

Bad idea! I think the aftermarket copper head gaskets are reuseable (but that's it), you can have cometic.com custom make a reusable one but probably easier to go OEM.