GS550 Bored 740cc Officially Done!

[Chuck78]

Attached are 2 photos. The single gs750 piston shows the flat area around the edges of the top and the more mellow dome. the shots with multiple pistons are gs550 pistons, notice there is no flat area on the top outside edges like the 750, and it immediately goes up into an aggressive steep dome with a large flat top.

I looked up photos on eBay of 550 combustion chambers vs 750, and they looked identical to the naked eye. the 550 and 650 shared the same valves, which are different from the 750/850/400/425/450 valves. I assume they are 1-2mm smaller on the 550/650. comparing the valves in the head photos, if they are smaller valves, then the chambers are still very close in size 550 vs 750. If the chambers are slightly smaller, that would work out in our favor, but I am thinking they are either the same size or very slightly smaller. Smaller would be better as long as the piston dome edges clear. With the big flat edge on the 750 pistons and none on the 550, that leads me to believe that they may very well be the exact same combustion chamber size and shape.

this basically tells me that you should be able to use the 750 pistons with the 77-82 gs550 2 valve head no problem (80-82 550 head with larger intakes that fit bs32 carbs are better imo, w/gs650 bs32 cv carbs OR Keihin CR29 smoothbore carbs ($749 new) if they can adjust narrower like the 550/650 carbs), but I would lean towards milling .010" off of each the head and the cylinder deck to get the compression to about 9.5:1 I am guessing, and then degreeing in the cam sprockets (requires slotting the bolt holes and use of special tools) to correct for the shorter cam chain length crankshaft to cam shaft, & also because simply degreeing in the cam can give you much more accurate timing on the stock spec than what it left the factory with, and you can also advance or retard them, as well as change the lobe separation angles, all of which will drastically alter the powerband and peak power rpms, which is very useful in tuning them for optimized power and power band range.

[gmansyz]

Hiya,
Chuck you are on the right track, but it would be better to actually measure everything. Measure diameter of the combustion chambers. And the diameter of the dome on the piston and then the actual valve diameters. Plus CC the combustion chambers and the piston domes. No guessing! Remember, with the larger bore, your swept volume goes up, changing the compression ratio. With this combo, I would actually try for 9.25:1 CR as you are compromising the shape of the combustion space. Poor flame travel and more. And clay up the valve pockets as well. I am thinking the CV carb 550 head may be the hot ticket. Unless you can fit the special 650 head to the 750/850 pistons.
Just my thoughts all. Not ready to get into camshafts.
Laters
G

[Chuck78]

I wouldn't want to try to run 750 pistons on a 650 head, although the 650 head and piston technology us vastly superior. Custom pistons for that would be a one off, $700, and a lot of measuring and mocking up just to get Wiseco or JE the info and molds they need to make the pistons.

Shame, Wiseco used to make a 65mm 741cc gs650 piston. I emailed them about posibilities of them being able to pull up drawings and reproduce a small batch, but no reply. Maybe a phone call would solve that.

Still the 789cc hemi setup seems quite a hot ticket if I can get some custom sleeves & use these 11:1 hemi pistons.
I plan to pull a spare 550 head off this winter and measure the chambers. 550/650 valves are a different part number than 750/850/400/425/450 valves, so I assume slightly smaller.

A worked over big valve 650 head and Wiseco 741cc gs650 pistons and a megacycle came would be an amazing combo, probably nearly as fast as my 894cc gs750 option just less mid torque.

[tkent02]

Shame, Wiseco used to make a 65mm 741cc gs650 piston. I emailed them about posibilities of them being able to pull up drawings and reproduce a small batch, but no reply. Maybe a phone call would solve that.

If a larger order would help convince them, I'm down for two or three sets.

[Chuck78]

Yes, another member had Carillo make a one off set of CP GR650 Tempter 10.25:1 pistons because he said Wiseco would only do a minimum run of 12 pistons I believe it was. 789cc in a 550 would be equally awesome but the resleeving may be a little trickier than usual. 741cc closed chamber design with higher compression would undoubtedly make as much or more hp than the Hemi chambered 789cc, just slightly less torque.

I'm still considering an 894cc gs750 since I need 2 of those pistons for the 475cc GS425 and have 3 spare 750's, but the lighter 550 with a serious punch in the 740-790cc range put in a modded out Rickman chassis would be an a very impressive road handler no doubt about it.

I probably won't do anything until late winter as far as ordering any pistons are coming to any conclusion on which engine I will build, but I might just do both the 750 and the 550 builds. I could always build up another 750 in a stock Suzuki frame that I have here in case I am with that highly impressed with the 741cc GS 550 engine build in the Rickman. Plus that would be another excuse to start collecting extra GS550 and 650 engines and parts bikes! I love the old 2v/cyl first generation GS chain drive stuff - 400/425, 550 (&650E), 750, 1000). the 16v GS1100E is also a total masterpiece, but that is a whole lot of engine, some serious power, I couldn't help myself if I had that, would get me in too much trouble!

[gmansyz]

I would talk to Arias, Ross, or CP pistons. But a bunch of measuring before hand. See Carolina Cycle about Sleeves. Maybe Arias still has some and you would not need custom ones. But first check how much material you have between the bores on the 650 cylinder block. There is the trick of boring the block seriously oversize and pressing big aluminum sleeves in. Weld top and bottom. Surface, bore, and then press in iron sleeves and go from there. See if you can use stock 850 pistons.
Now if you have pistons made, get five. Then you have a spare for down the road in case something happens. Or one to always show you what you have in the engine..
Laters
Greg

[Chuck78]

The benefit of trying for 741cc gs650 Wiseco pistons is that they likely already have 30 year old drawings in their archives on the piston that all of us 550/650 owners want, so less leg work is involved. If I can run some 67mm gs750 pistons, they are high quality NOS pieces that I got for less than OEM stockers, so the only real work would be the custom sleeve machining (of a 750 sleeve) or ordering some new custom spec sleeves.

If 850 pistons would work with the 550 head, I'd surely just run the Wiseco 844's but I like the idea of thicker walls for cooling, & the 67mm 798cc 750 pistons for 789cc in the 550 crankcase may still be my preferred route for longevity (cooler cylinders than a very thin walled 69mm 830-ish cc build.

[Chuck78]

The 789cc would also likely work better with Keihin CR smoothbore carbs, as I'd go with CR26's or CR29's for 741-789cc, but the CR31's like my buddy runs on his 1135cc KZ1000 may be a better fit with keeping the intake tract similar diameter all the way through, as opposed to a cr26 on a 741cc with a larger port head designed for a bs32... is that completely acceptable & giving the carbs a better jet signal than a BIG throat CR31 carb, or will it all work out just fine with fuel delivery on a medium engine with large carbs? Keihin makes adapters to go to any size intake boot so I'm good there.

[Ace07]

Wow guys that is a lot of information. A lot of it is still over my head. I just started tearing down my bike last week, and I eventually plan on making a thread for it. I know I am late to this party, but if you can get those pistons from wisco again, please mark me down for a couple sets. I purchased the cylinder and head from gentleman who started this thread, and he seemed convinced that this project would work, but I would have to take out the ridges in the head so the 750 pistons will properly match up.

[Chuck78]

I purchased the cylinder and head from gentleman who started this thread, and he seemed convinced that this project would work, but I would have to take out the ridges in the head so the 750 pistons will properly match up.

If you do that, the combustion chambers will still be far too small for the big hemi domed 750 pistons, resulting in too high of compression and overheating, potential engine failure due to detonation.

AND you really don't want to take those ridges oyt, because they are 1/3 of what makes the advanced 673cc head make 73hp vs the old scool hemi head gs750 748cc making only 72hp!!!!! Get some GS650 pistons, or else send rapid ray a set of gs650 cylinders and tell him you want them specially resleeved to run Wiseco gs750-844cc pistons, and then run the 80-82 550 head that looks to match those pistons.this will give you the biggest power in the end. The 673cc will be quite powerful especially with a very slightly milled head (.010"), freshly honed cylinders and new piston rings, and the hot street cams at .354" lift that Megacycle regrinds.

Or you can try and sculpt a 750 piston into the shape of a gs650 piston. The piston shape, chamber shape, and offset staggered intake-exhaust port flow with raised intake runners is why the gs650 makes 1hp more with 75cc less, and better fuel mileage and less pollution...

[Ace07]

Hey Chuck,

Here is a list of the items that I have:

GS550 head
GS650 bored to fit GS750 pistons
GS750 Pistons and Rings
I also have the head I bought from the other gentleman, but I cant remember if its a GS550 or Gs650

If I have a GS650 head, should wait to see if you are able to get a set of GS650 big bore pistons from Wiseco?
If you cannot get the pistons from wisco, then should I able to use my 750 piston and 550 head together?
Or should I order new 750 pistons from wisco?

[Chuck78]

The new 750 pistons from Wiseco are +4mm oversize, 69mm, & would require a more involved than usual resleeving of the gs650 block, but would make the biggest most powerful gs550 to date!

I haven't been in contact with Wiseco as I have several other projects going currently, but here is a 1981 Great State Performance catalog with the Wiseco K740 part number we need to reference when contacting Wiseco. It needs to be stated to Wiseco that we potentially have a great deal of interest in 24, 36, maybe 40+ pistons if we give it enough time to get the word out on here and get enough people decided upon them to get our collective group bargaining finances together enough to get them to do a big run at a bigger than cystom/1-off price break.
Maybe Rapid Ray may be the better person to contact Wiseco about looking for their old design drawings for the K740 pistons to reproduce a run of them? He has a Wiseco dealer account I believe.


Probably a good idea to start a new thread about "Potential Group Purchase - Wiseco GS650 741cc pistons?" as the subject. See how many people are interested. Myself x1 or 2 sets (4-8 pistons ), I believe tkent x2, drey6 x1, and certainly others.
I did email Wiseco once maybe 6 months ago and got no reply, but I did not have this k740 part number back then. Of course there is always a chance that the design drawings of the pistons we are wanting are long lost and forgotten, and they would have to do a fresh design completely from scratch. that would make it much more difficult. we are talking about the most advanced to valve for cylinder that Suzuki made, however. The best power to weight ratio and the best candidate for all around best handling of any 2 valve GS due to the lighter weight of the 550 and 650 2-valvers.

This would also require a batch of 65mm bore Cometic head gaskets. There are a few NOS Wiseco gaskets still floating around on the Web however, I have 1, new in the package still!

Wiseco W3169 GS650 65mm head gasket $44.95:

http://www.ebay.com/itm/281796374599

"stock photo, actual item may vary." photo shows a multi-layer steel gasket that does not look like a GS 650 gasket, but they have the right part number and bore size and model size.
I think you still need these used on the 4 outside corner studs when using that Wiseco gasket (oil pressure passage seals):


Ace, since you are the only one that is already set up with the parts to do at least the 550 head 750 pistons 650 cylinders, you might as well buy that vintage big bore head gasket as it is the only one I see for sale right now! When I make it back over to my shop at the house I just moved out of, I will take a photograph of the gs650 65mm Wiseco gasket still in the package that I have there.

[Chuck78]

Otherwise, with what you have, you would want to put a fresh hone on the cylinders and drop in 750 pistons on a GS550 head, preferably the 80-82 version for bs32 carbs. Run BS32 GS650 carbs + dyno jet kit of 80+ GS750 carbs rejetted for your pods&pipe and needle shimmed up slightly.
Or go all out and talk to a Sudco dealer about getting a set of Keihin CR29 smoothbores with the right adapters to make them work in bs32 boots. Maybe CR31's but that's more for a high compression big cam high revving engine, low end response would not be as great I suspect on 31's.
The CR smoothbores are adjustable spacing width, fyi. They make a kit to fit CR 26 smoothbores onto cb550's in their 22mm carb oem intake boots, but I'd be afraid that'd choke down the flow too much. the head could be ported bigger, but the intake boot is the limiting factor then. Unless they offer a special boot. even with the adjustable spacing, I was worried that the CR 31 smoothbores would be too wide even at their most inboard narrowest spacing.

[tkent02]

The new 750 pistons from Wiseco are +4mm oversize, 69mm, & would require a more involved than usual resleeving of the gs650 block, but would male the biggest mist powerful gs550 to date!

I haven't been in contact with Wiseco as I have several other projects going currently, but here is a 1981 Great State Performance catalog with the Wiseco K740 part number we need to reference when contacting Wiseco. It needs to be stated to Wiseco that we potentially have a great deal of interest in 24, 36, maybe 40+ pistons if we give it enough time to get the word out on here and get enough people decided upon them to get our collective group bargaining finances together enough to get them to do a big run at a bigger than cystom/1-off price break.
Maybe Rapid Ray may be the better person to contact Wiseco about looking for their old design drawings for the K740 pistons to reproduce a run of them? He has a Wiseco dealer account I believe.


Probably a good idea to start a new thread about "Potential Group Purchase - Wiseco GS650 741cc pistons?" as the subject. See how many people are interested. Myself x1 or 2 sets (4-8 pistons ), I believe tkent x2, drey6 x1, and certainly others.

Hell yah I'm in for a set or three! Maybe more if necessary to make the deal go and I can sell them later. Make it happen.

[boontonmike]

This post gets my blood flowing. If we could just get those drawings!!! add me to the buy a set or two crowd.