Has anyone done 69mm Wiseco GS750 10.25:1 844cc pistons in a GS400/425?

[Chuck78]

I am really curious about building a lightweight twin based on the kickstart engines on the GS400 and GS425, with upgraded brakes, spoked wheels, sticky tires, stiffer springs, emulators, and drag bars or clip-ons. Not going for the chopped frame/seat look of the trendy cafe racer style however.

since the GS 400 and GS 750 use the exact same pistons, putting the wiseco 844 kit into one of these bikes seems like an obvious answer to building a performance engine on one of these lightweight machines.

as tempting as it sounds to use some GS 500 sleeves in the 400 block and use custom made big bore pistons, I doubt that much in performance gains would be there to have had without upgrading to larger valves, which aren't very possible to go much bigger on, & would be way more costly than one of these bikes could justify.

simply boring out the cylinders and putting in these pistons to make a 449cc 10.25:1 compression mini-powerhouse using a custom bore Cometic head gasket would be pretty simple, but would be pushing it as far as resale value on the bike vs the investment.

I have read on here before about someone hot rodding one of these old twins and saying that the bike had such an impressive power to weight ratio that it would just about throw people off of the back on sudden low speed WOT twists because it was a wheelie machine! A longer/stiffer GS1100E allow swingarm would be a good investment, but would also require going to rear disc instead of drums due to brake stay placement.

I was wondering if many people had done this mod before, and how much performance potential the bike has with this, pods, jets, and free flowing mufflers or two into one?

[rapidray]

The 400/425 uses a 16mm wrist pin & the 750 is either 17 or 18, I can't remember which. I just know they aren't the same. Ray.

[Chuck78]

I am pretty positive that the GS400 & GS750 use the EXACT same piston, and you can even buy the Wiseco's in a pair of 2 as well as a set of 4. 400's just use a longer stroke crankshaft to make the additional 12+ cc's per cylinder

[Chuck78]

Ray, I believe you must be thinking of the later 8 valve 750's. I confirmed that the wiseco 844 kit comes with 16mm pins for the GS 750

[rapidray]

Thanks Chuck, I stand corrected. Ray.

[Chuck78]

Ray, I believe you must be thinking of the later 8 valve 750's. I confirmed that the wiseco 844 kit comes with 16mm pins for the GS 750

looks like I stand corrected as well, that should have said the later (1980+)16-valve GS750's, or 4 valves per cylinder model. the 1976 through 1979 (2 valves per cylinder) models use the exact same piston as the GS400. 1979 & 1980+ GS850's retained the same 2 valve design as the original GS750 engine, & I believe kept the same piston through all years (16mm pin) despite the gs750 engine changing to the tscc design in 1980 with 4 valves per cylinder and a totally different bore and stroke as the older gs750's.

The Wiseco GS750 big bore" 69mm pistons are basically just a cnc machined high compression version of the GS850 standard bore piston, sonce the 850 was just a gs750 engine with bigger bore cylinders (&shaft drive transmission & gs1000 cams)

[pete]

I haven't done it but I'm keen to see if you do.

When I eventually get my Kat and the 450 gets a bit tired I'd be keen to investigate doing something fun with it... not sure how much is common with the 400/425/450 in that regard though...

[Chuck78]

The 450's use the bigger piston pins, & you can use larger big bire pistons for the gs1100 or 1150 I believe. You would probably have to use the 4 valve head from some 450's to use the 1100/1150 pistons, as I "think" those are 4 valve per cylinder tscc engines ("twin swirl combustion chamber").

If you want to do some custom exhaust fitting, you can swap a gs500 cylinder bloxk, head, & pistons on as well. the gs500 exhaust will hit the frame when it comes out of the head.

There is a guy on here who built a beast of a 450 using this type of setup. Search for his "Frankentwin" build (540cc's or more if I remember correctly).

Due to the heads not flowing the best, going bigger with custom pistons and sleeves isnt really woryh it, but to some extent, you can get a whole lot more torque out of these engines, but there is a cutoff of diminishing returns with the hp gains based on the head's flow. You cant go much bigger on the valves either.

[pete]

Yeah that's Mekanix with the Frankentwin.

I'm not keen to change the head after spending $800 on my custom pipe, so whatever I do I want to retain that... it's a nice free flowing pipe that I believe can be adjusted to flow even freer again too...

[Chuck78]

Find a 4 valve/cylinder 450 head if yours isnt already, exhaust would swap. Then find some 1100/1150 aftetmarket big bore pistons that are 4mm larger diameter than stock, as that is typically the max overbore on gs sleeves.

[Chuck78]

450's use a 71mm piston and a short stroke crank. The 816cc wiseco tscc gs750 kit is only 70mm, so you could go with 74 or 75mm gs1100 10.25:1 compression pistons from wiseco and a custom sized Cometic mls or copper head gasket. They offer both. Wiseco have two larger size pistons but they are racing-only, like 13:1 compression.

[Chuck78]

74mm bore x 56.6mm stroke = 10.25:1 487cc GS450

75mm bore x 56.6mm stroke = 10.25:1 500cc GS450

that's AWESOME! shedding 30 lbs and adding that power with k&n pods & rejetying the carbs would make it keep up with 650's & 750's easily and be very very fun to throw around in the tight twisty country roads!

[hardline_42]

74mm bore x 56.6mm stroke = 10.25:1 487cc GS450

75mm bore x 56.6mm stroke = 10.25:1 500cc GS450

that's AWESOME! shedding 30 lbs and adding that power with k&n pods & rejetying the carbs would make it keep up with 650's & 750's easily and be very very fun to throw around in the tight twisty country roads!

The 74x56.6 combo is the same bore and stroke as the GS500. Wouldn't it be easier/cheaper to just swap GS500 cylinders in? And you can get +1mm and +2mm ring and pistons for the GS500 from Suzuki.

[Chuck78]

You would need the gs500 head, as the piston domes are matched in shape to the combustion chamber. I believe you would be waaaayyy low on compression without the gs500 head, and then with the gs500 head, you would have the exhaust to frame problem to deal with and only 9.0:1 vs 10.25:1. Not really worth it for the performance tradeoff unless you get the gs500 stuff for really cheap. High compression makes a BIIIGGG difference in performance.

[Chuck78]

The 74x56.6 combo is the same bore and stroke as the GS500. Wouldn't it be easier/cheaper to just swap GS500 cylinders in? And you can get +1mm and +2mm ring and pistons for the GS500 from Suzuki.

I think you could possibly swap the gs500 cylinders in to avoid boring the gs450 cylinders to 74mm, but with the cost of boring & honing only two cylinders ($80-$150) it's not really worth it to purchase a set of used cylinders and check out deck height & oil passage alignment and everything else. Plus the gs500 cylinder cooling fins are more squared off and modern looking.

So for the newer 450, I think the easiest way is the 75mm gs1100 big bore wiseco's and the tscc 4 valve per cylinder head if the gs1100 was a tscc (pretty sure all 80+ non-"G" models were tscc) to get 500cc's. A gs500 head gasket may be an easy off the shelf drop in as long as the thicknesses are in the correct range for the older engines.

For the older kickstart 76-79 engines, looks like the wiseco 844 pistons are the best reasonable option to get 449cc's and 10.25:1.



On the more rare 1979-only GS425, I assume it uses 4mm thick sleeves and not just bored out gs400 sleeves, seems that you could get some custom 71mm pistons made if you wanted to shell out some more cash.That would be 475cc's at 10.25:1 or whatever you choose