GS750 vs 850 cylinder jugs?

[Chuck78]

Can anyone tell me if the sleeves in the GS750 vs GS850 cylinder jugs/barrels are the same outside diameter, with the 850 just being bored more? or at the actual jugs bored for larger sleeves?

I am wondering about the need to buy a set of 850 jugs for my 750, since the wiseco kit says to just bore your 750 out to 844 with their pistons, which is the same size (843cc) as a GS850.

The 750 should work just fine bored out with factory 850 std pistons, but if I ran a 70mm or 72mm piston, I am pretty certain that I'd need an 850 cylinder block. The max overbore size available from Suzuki was the 70mm (+1.0mm), but I found some high performance discontinued pistons that are 72mm (920cc) and am also wondering how much thickness I'd have left after boring to that...

Anyone with 850 jugs handy to measure!????????!?!?!?! Please!??!?!?

[Nessism]

The 850 sleeves are larger. A good many people have upgraded their 750 with a cylinder and pistons from a 850.

[MisterCinders]

What year is your 750? If it's two-valve, you can bolt in a set of 850 jugs/pistons. I don't think the swap works with the later 16 valve engines.

While I don't have the jugs at hand to measure, check the GS850 service manual for the 850 overbore limits. If they are higher, then there must be more sleeve in the 850 jugs.

[gearhead13]

You can usually bore out factory sleeves 3mm over. So 3mm over whatever the 850 stock bore is. It is generally accepted that 2mm sleeve thickness is the minimum although others have gone a bit thinner, not recommended.

[Chuck78]

Cool, I was going from stock 850 bores at 69mm + 3mm OS, 72mm bores for 920cc. K&N filters, 4 into 1 glasspack, some carb tuning, and the big bore upgrade should really make this bike fly! I've been told to keep the GS750 cams as opposed to the GS850/1000 cam, as the 750 somehow has a "hotter" cam profile. Aftermarket cams are $450-$550+, so I think I'll be liking these mods plenty enough!

[Chuck78]

Since Wiseco advertises their big bore kits as to use the stock 750 jugs bored out to 844cc's, seems to me that you might as well just find a good used set of 850 pistons and do the same, so you will get a brand new fresh cylinder bore surface, as opposed to getting an 850 cylinder with 20,000+ miles of wear on it. Oversized pistons for the 850 are nearly impossible to come by, and no longer available from Suzuki. So if you get a set of 850 jugs for your 750, you'd better hope they need no more than a honing.

I got lucky and found a discontinued big bore kit to go 2mm larger than the 850 +1.0mm os pistons.

The only advantage to going with the Wiseco kit for the 2 valve GS750's is that you go from 8.8:1 factory compression ratio to 10.25:1 (and get a stronger forged piston), which will give you more power, but will put you at the risk of detonation on warm days sitting in traffic unless you run 89 or 91 octane. Also, since very few people buy 89 and 91, the gas in the tanks at the filling station is usually pretty old, and works better for cars than tiny motorcycle carb passages that get gummed up and clogged up.

The 850 cylinders on a 750/850 conversion will give you thicker cylinder walls if you are worried about strength.

[Chuck78]

As you may have read in another thread, I think you can get by running a KZ1000 70mm piston in the GS850 cylinder jugs, as the combustion chamber design is almost identical, and the piston height is apparently as well. The only hangup is that the piston wrist pins are 17mm, so your GS750/850 rods need bored out to 17mm from the standard 16mm.

I have no info on this retrofit, but you might want to look into the piston dome cc's compared to a GS piston or the combustion chamber cc's plus the quench area (gasket thickness and piston clearance issues) to make sure you will have a favorable compression ratio in the end.