Details before pulling the trigger on 550/673 swap

[Kara25]

Hi guys i wanted to ask some info about a couple of things that i cant manage to figure out after reading tenths of posts here about the 673 swap on a 550. 1. My 550 has a marving racing exhaust K&Ns RC-2382 dual filters and a Dynojet Stage 3 fitted. Right now after a lot of back and forth the bike RIPS in comparison to how it was when i got it. Question No1 is. Can the dynojeted Bs32ss that i have be used on the 650 head? I know that boots etc are the same the question is if the bike will fuction properly bcs if i can avoid buying Bs32ss for the 650 will be perfect European ebay has a set which is all gummed up for almost 350 euros!!! 2. I want to use my 550 camshafts on the 650 head. Does this eliminate the need to fit a 650 advance mechanism on the 550?? From what i have seen here you only need the 650 advance if you use 650 cams or sprockets. Is that correct? 3. How possible is to do the grinding without splitting the cases?? I done a top end rebuild 4 years ago and i am feeling confident up untill that point when i will remove the 550 pistons but to go further down and split the cases makes me nervous i am thinking of stuffing tons of paper towels on the holes of the rods to catch the shavings and maybe even using a small vacuum cleaner did anyone do it without splitting the cases?? I got a dremel on order for the job my old one unfortunatelly bit the dust. 4. Cam chain tensioner and tensioner blades will be used from the 550 they were in very good shape at least 3.5 years ago but what about all the other hardware for the head? Can i use the 550 bolts nuts etc and at the same torque specs as i did back then with my 550 rebuild or do i need to source 650 nuts etc and see the torque specs for the 650 model? 5. And last. If i use the 550 cams assuming i wont need to change the advance mechanism as stated in 1. The timing will be the same procedure of counting 20 pins if i remember correctly? Thank you all in advance sorry for the HUGE ESSAY

[dorkburger]

Chuck78 Is very knowledgeable on hot rodded GS's and mix and match engine builds. Hopefully he'll check in.

[Nessism]

I'm not an expert on this swap, but my understanding is that you can reuse pretty much everything, other than the cylinder and head. The carbs may need to be rejetted.

Regarding grinding the crankcase, use burr bits in the rotary tool, not sanding sleeves, which will give off grit as they wear. Personally, I'd split the cases, but I don't know for a fact that it's strictly necessary.

[Kara25]

I just ordered the head its complete no broken fins with the 4 round cosmetic plugs and with 650 cams(which i wont use) and valve cover. The only thing thats missing is the little breather cover on top but i can use my 550 one so no worries. Now for the cylinder and pistons i found a pair from a german 650G with 22.000 miles. Thats something like 30.000 kms which for our bikes i think is considered very good mileage.

[Kara25]

Anyone???? Need the knowledge truly.

[Chuck78]

I clearanced my wife's 550 case when they were split and empty, but on one set of 750 cases I'd done pretty early on in my mechanical endeavors, I cleaned all oil residue off the areas around each cylinder/crank journal, & used blue masking tape to seal off the crank bearing & crankcase areas, & also stuffed a rag on top of that to collect aluminum shavings, & set cases up on an angle as well to shed shavings to the side. That worked quite well.
My 2nd 750 + 850 cylinders & the first 550/650, I also used a carbide burr bit with a rounded end in a die grinder, but had the cases split for a total rebuild and full inspection of crank, shift dogs, gears, etc.

[Chuck78]

I don't remember hearing about using the 650 ignition advancer mechanism if using 650 cams, that's news to me! I thought they were all the same (Either ND or KD of same specs).

[Chuck78]

High reving cams are definitely fun, but when I switched to 850/1000 cams (more midrange) on my big bore 750, I was quite pleased at the extra midrange grunt... Revving to the moon riding like a maniac everywhere definitely is fun but definitely draws attention from everyone within a mile...
Having a more midrange cam on the street makes you seem a slight bit closer to a sane and civil person when riding aggressively...lol
On my 2nd 550/650, I'll be going for Megacycle Cams and either VM28's or maybe CR 26 smoothbores.
I'm spending most of my time these days ripping through the woods on 2-stroke trail bikes on rugged trails, so that project got pushed back a ways, but I may sell all my 750 stuff this next year in favor of building my $320 score big bore 550 up, and a big bore $300 GS1000 + 1100G D-port top end + CR33 carbs...
A big bore 844cc/920cc GS750 is an awesome all-rounder, but a GS1000 weighs the same, and a big bore 550 is way lighter and narrower, so I can get way lower in corners and burn through the twisties even better... That's really what I'm most interested in.

As far as your 550 carbs, they're the same bodies, but the air jets are different - which are pressed into the carb bodies.
You'll have to experiment a lot more with jetting until you get it right, versus the 650 carbs which will be fairly close with the basic tweaks of shimming the needle and 4+ sizes bigger on the mains etc. Slide cutaway is different also from what I recall. That will also alter the mixture as it changes the density and velocity of the air going through the venturi under the slide, which changes the velocity of the fuel delivery through the metering circuits which changes the quantity of fuel being metered through the jets etc.
To sum it up, you'll have to spend a lot more time swapping jets needles and possibly even slides to get the 550 carbs dialed on a 673cc, but they'll work for sure if you'll put the work into them.

[Chuck78]

On your 550 pipes, you'll need to fabricate some shim stock to wrap as a centering bushing in the 650 head, as it's larger i.d. that the 550 head in the exhaust area.
I've got a MAC 650 exhaust for when I build the 550/650 for myself, but the other one is shimmed with 550 pipes.


On the 550 vs 650 cams, if you were to use 650 cams, the shaft drive GS650G cam sprockets ARE NOT TO BE USED on a chain drive GS550 or GS650E crankshaft...NEVER. Valvetrain destruction will ensue on the first engine revolution, they are a different tooth count, and immediately will not keep the cams in time with the pistons.

Also as you've probably read, you need a thicker head gasket preferably, or else a thicker base gasket (cannot get thicker cylinder base oil pressure o-rings but they may still work if going the bare minimum additional thickness on the base gasket, or squishing them substantially more with a 650G base gasket pressing against them - stock design has the egg shaped oil pressure o-rings obviously not sealing against a paper gasket, but rather metal to viton to metal.)

Cometic can custom make you a head gasket or base gasket to your specs. GS650E base is what you'd want if going for a thicker custom head gasket.

[Chuck78]

On the used cylinders, 40,000 can still look like new, but I've seen 30,000 mile cylinders severely worn from overheating or running low quality air filters or even running K&N's in dusty conditions (the goofy looking Uni pods will filter far better,by the way, protecting your engine substantially more).

[Chuck78]

Also, I've honed scoring & rust spots out of cylinders that looked trashed but had very little bore wear otherwise, and even after extensive honing to get rid of the scoring, they still measured WELL WITHIN SPECS with a dial bore guage throughout the cylinder... Just need to make sure you file fit the rings for the proper end gap spec and keep them unique to each cylinder when custom filing the end gaps. Very crucial step. And MAKE CERTAIN your piston ring clocking is set up per spec in the workshop manual.

[Kara25]

Wow chuck you are the man! I have read about the exhaust fitment from a post of a user called suzuki don basicaly from what i have got is that you need exhaust gaskets of a particular gsxr750 model and some sleeves around the header inlets to the exhaust ports to pick up the additional play and seal well. This doesnt seem hard so i am feeling confident. The cam debate i want the 550 cams so i am not concerned about the 650 sprockets grenading my motor. Also when the cylinders arrive i will take a closer look maybe even post pics of them here you guys can tell even by looking at the parts if there is something wrong LOL. As for the carbs now that you mentioned that even THE SLIDE is different i am getting worried because the 550 Cvs of mine have all 4 slides+diaphgrams brand new 1 year old. Looks like a nightmare to throw all that away + DJ3 needles and start figeting with completely different carb internals. But what got me most puzzled is the comment about a thicker base or head gasket. Cant i buy OEM gaskets for the whole top end and assemble just like i did on my 550 during the rebuild 3 years ago?? Why the need for custom made ones? Unfortunately i didnt pick this crucial info about the gaskets when i was reading all the 550/673 conversion posts..Ok dug around more on suzuki dons thread about the thicker gasket. Turns out in the end that the noise he was having in his motor had nothing to do with the thickness of the gaskets. It was his clutch hub all along that needed replacing plus in his conversion he was running oversize pistons.

[dorkburger]

An older thread on the topic.

https://www.thegsresources.com/_forum/forum/technical-forums/projects-rebuilds/60784-finally-getting-around-to-the-550-699-project/page3

[Kara25]

dorkburger yeap i am reading all day various threads again and again. Head pistons and barrels are on their way as we speak. Another thing that popped in my mind is that i remember back when i did my 550 top end i remember using all the new gaskets and orings in particular some egg shaped ones on the corners of the 550 crankcase on the oil galleries. Now when i am looking at the 650 barrels that i bought it seems that they dont have the same egg shaped oring cavity as the 550 upper crankcase has. What oring is supposed to go there now with tbe 673 upgrade?? A 650 oring?