GS400 to GS550 Engine Swap

[Guest]

Apologies if this has been answered before. I am new to the forum but i did hunt pretty hard before asking this question. I have a 1978 GS400 which I like a lot for it's light weight and 70's Japanese bike appeal. Would a GS550 four cylinder engine fit in this frame? I have read several articles which suggest that the frame is the same. My reason for using the 400 instead of just getting a GS550 is because I have modded the look of my GS400 to be a bit more early 70's looking and the spoke wheeled GS550's are pretty hard to find. Thanks for any input from any experienced project folks.

[GregT]

It'd probably fit. The 550 engine is shorter front to back and not as tall as the twin so you'd have room.
Chain offset would probably be very similar as both use skinny rear wheels.

I'd suggest that the frames are similar but not necessarily the same...

[Steve]

I think it would be easier to put your wire wheels on a 550.

.

[Grimly]

I can say for sure the GS550 four cylinder engine fits into the frame of the GSX400/4 and whether your frame (being the twin cylinder) is the same or close enough I couldn't tell you.
The swap meant the airbox was junked and pods fitted, and a couple of the engine plates at front and sides were slightly modded, but it worked well enough. The exhaust was re-used iirc.

[Mrmidcentury]

Thanks all for the input. A GS550 came up on Craigslist in wrecked condition with a good engine. There's a sane argument to replace the rings on the GS400 and leave it alone. Maybe improve the handling a bit. I commute on this bike and I tend to twist it up and change gears a lot. Kinda like a low speed race. Which is part of the fun of a smaller motorcycle!

[Chuck78]

If you are doing a re-ring job, just bore in some Wiseco K844 pistons, Big Jay at APE Race Products will sell you two of these pistons since they normally come as a set of four, but can be bought individually, but he also will sell you a custom Cometic head gasket to fit this big bore configuration.
Then you will have high compression pistons, larger engine displacement size, and deeper valve pockets in the pistons so that you are able to run aftermarket camshafts in more powerful high-performance grinds...

Doing this will add some serious boost to your power and fun factor...
You will also want to have the valves lapped at least, and new valve stem seals installed, if not doing a full on valve job to completely freshen up your engine. When people neglect these bikes and don't do valve adjustments, this can lead too degraded engine performance and lack of valves sealing properly.

I was directed here from another thread 2 months more recent, with nearly the same subject matter. You should read that or I will cut and paste the info here, there is a lot of benefit to be had in upgrading one of these lightweight GS400/425 twins.

the four-cylinder engine swap is just not worth it, too much work, and I do not believe the exhaust will clear the frame. I can try and measure this for you if you really insist, I do have a GS425, and 3 GS550's here. but then you will also need to upgrade the spindly anemic stock 33mm forks & brakes (this is advised anyway even if sticking with the two cylinder engine), swap on a GS550 swingarm and wider back wheel & tire & sprocket carrier, then hope and pray that the chain does not rub the frame since it is outboard about 12mm further than the GS400, etc etc...
And the frame was not really designed for the 50 to 70 horsepower that the 550 / 650 engines put out.

Upgrade the 400... It is a cool bike oh and can be a real sleeper if you do the mods I suggest to the engine. Rapid Ray can do the engine machine work and send the cams out for regrinding for you, he is a member of this forum that builds GS 1100 and Kawasaki race bike engines.

[Chuck78]

Copy and paste and a little bit of weeding out, from my other similar posting:

It would be advisable to buy a spare GS400 '77-'78 cylinders and head/cams (make sure the seller is not an idiot and already sold the cam bearing caps separately, they are factory machined and matched sets specific to that head), have the cylinders bored out for 2 Wiseco K844 pistons (you can buy them individually, but they are generally sold as a set of 4, to punch out a GS750 up to 844cc), & buy those from forum member Big Jay @ APE Racing Products, he does not have them advertised but sells them with a custom Cometic head gasket for the larger than standard bore size.


Then have the cylinders bored and honed and the deck resurfaced. Also send the head out to a proper motorcycle engine machinist like Rapid Ray etc, and have a Serdi type valve job done on it, & if you really want some extra power packed in, have some Web Camshafts grind #110 camshaft profiles ground on your spare cam cores for some extra potent punch...
Install a 2 into 1 performance exhaust MAC? Or Japanese off of Japan.WeBike.net) and re-jet the carburetors accordingly after a full carb cleaning & rebuild.
MegaCycle Cams also sells nice performance grind cams for the GS twins.

This is much more straightforward work than attempting to re-engineer an entire bike and then risk potentially leaving it sit in pieces forever, eventually destined for the scrap yard... but this will set you back the cost of another used bike without a doubt ($1200-1500), but will get you about the fastest GS twin that you can reasonably attain without dropping a fortune on it, and it will be a GS twin that dreams are made of... And you will still have the same bike that you know and love, and it will be a prized trophy piece at that point.

You will then require some very grippy upgraded tires, and upgraded front brakes at least, I would go for upgraded forks to GS500 models mounted in 83 GS1100E triple clamps, & the GS500 brakes with a 98-99 CBR900RR rotor with the bolt holes enlarged from 6mm to 8mm, using a drill press. GS850/1000/1100 axle, axle spacers, speedometer drive.
Hagon or YSS shocks for the rear, & RaceTech gold valve cartridge emulators for the GS500 forks.
Get the GS500 OEM (or aftermarket) fork brace and use it to mount a fender to (old Honda VF750C I believe can be modded to work well in this configuration).

That will be the recipe for you to keep your first bike that you love so much, but make it significantly faster & improved in nearly every way.


Also, it really benefits the GS400 frames to get some spare frame tubing and weld in some braces from the center spine down to the top cradle tubes, directly behind the ignition coils.
If you find photos of other GS frames on the internet, anything other than the early GS550, 400, and 425, all other GS models have frame bracing in this area, this is highly beneficial to add to the GS 400, and cures the 400 / 550 handling issues where you make get a little bit of a weave when hitting irregularities in the road at higher speeds while leaning over and going around a slight curve or tighter curve...



As you can see, shopping for a GS650, GS550, KZ550, GPz550, KZ650 would definitely be the easier route, but then that bike likely will still need better tires, a lot of maintenance caught up on, and upgraded brakes... And probably a carburetor rebuild and valve adjustment. And regulator rectifier upgrade, ignition relay added, wiring harness issues investigated with a few upgrades made....


You know what you have now, but the 448cc (449cc?) upgrade I recommended is the most power you'll get out of it, however you could have the cylinder head ported as well, that will give you a bit more extra power with a mild street porting job... It will really surprise you how much that will wake it up boring it out 50cc, adding high-compression pistons, and adding high performance aftermarket camshaft profiles. But still, the 73hp of the GS650 will leave it in the dust on the wide open roads.... BUT... 30lbs bike weight difference is significantly noticeable when leaning the thing over in corners, so this is where the small twin will really excel with all of the modifications that I recommended for you...


I am building a similar bike, but going to great lengths and putting a lot more money and major parts upgrades into it. I would not recommend this for the faint of heart, you need to be a really serious tinkerer and have some more in-depth mechanical skills and knowledge. Mine will be near 500cc with more chassis mods and GS1000 hubs with aftermarket wider rims...among other mods.

[Chuck78]

back to your original question, it really just is not worth it because you would need the engine, carburetors, exhaust, swingarm, rear wheel, forks, brakes, triple clamps, and some axle and spacer swapping if you want to keep the 18 inch front wheel for crisper steering on the bike that has one degree more rake than the 550 frame.
So you are needing over half of the 550 bike to do this swap properly, and then you still will have to do some custom fabrication, and hope and pray that the exhaust and chain all clear the frame.


so if you are really looking for a bike that will be more suited for high-speed highway cruising oh, but still want something fairly light for a bike of this style, and agile with nimble handling, you may want to look at a GS 550 / 650 four cylinder, or a kz550, gpz550, or KZ650. These are all awesome bikes. The GS engines are a lot tougher, the Kawasakis are a little faster stock, but a Suzuki GS550-650 hybrid or a GS650E will definitely have the most performance potential of all.

[Rob S.]

I did mods to my '71 CB350 twin in '78. Stock cc was 325; largest possible was 435. "Do it," I said. I had a good summer job and my goal was to spend every penny I had earned. Electric guitar, "Fuzz Buster" radar detector and custom seat for the CB. But I had more money, so I inquired about cams.

"Three different stages available. One is a mild street upgrade, two is more extreme."

"What about stage 3?"

"Not for the street - too extreme." I was immediately hooked.

"Why?" I asked.

"Poor low end performance. Rough idle, poor throttle response."

"Won't the bigger pistons mitigate that?"

"Only a little. We advise against it." I was 21. If there was a stage 4, I would have gone for it. As is, they had to grind the inside of the cam covers so the stage 3's would physically fit.

Poor throttle response at low rpms was right. Give it a twist, nothing. More twist, hardly anything. But when the tach hit 6k, hold on!

A few weeks after I let my 300lb boss try to pop clutch-less wheelies around the parking lot, the bearings went.

Long distance phone call: "Mom, I need $800 to get my motorcycle fixed."

"Who is this? Sorry, wrong number!"

Good luck with your project. Keep us advised.

[Chuck78]

I did mods to my '71 CB350 twin in '78. Stock cc was 325; largest possible was 435. "Do it," I said. I had a good summer job and my goal was to spend every penny I had earned. Electric guitar, "Fuzz Buster" radar detector and custom seat for the CB. But I had more money, so I inquired about cams.

"Three different stages available. One is a mild street upgrade, two is more extreme."

"What about stage 3?"

"Not for the street - too extreme." I was immediately hooked.

"Why?" I asked.

"Poor low end performance. Rough idle, poor throttle response."

"Won't the bigger pistons mitigate that?"

"Only a little. We advise against it." I was 21. If there was a stage 4, I would have gone for it. As is, they had to grind the inside of the cam covers so the stage 3's would physically fit.

Poor throttle response at low rpms was right. Give it a twist, nothing. More twist, hardly anything. But when the tach hit 6k, hold on!

A few weeks after I let my 300lb boss try to pop clutch-less wheelies around the parking lot, the bearings went.

Long distance phone call: "Mom, I need $800 to get my motorcycle fixed."

"Who is this? Sorry, wrong number!"

Good luck with your project. Keep us advised.

Yes those are popular bikes to build into vintage racers, but are a very weak base to start with - poor frames compared to the GS's, inferior head design, weak cylinder/head studs, weak plain bearing crank...

And anything beyond a Stage 2 cam is ALWAYS a bad idea on the street...If you are looking for a fun, responsive, all around good riding bike...
I have witnessed a few builds like this, no fun... Blown head gaskets on air cooled engines?!? Due to the studs stretching! Have to upgrade the weak thin CB studs to XS650 studs, then since that blocks off most of the oil flow capacity to the top end, you have to invest in an external top end oil feed kit... Just how far down the rabbit hole can you go? Pretty far when you don't choose a better starting point &/or don't recognize the limitations of your starting point!

[Chuck78]

I did mods to my '71 CB350 twin in '78. Stock cc was 325; largest possible was 435. "Do it," I said.
But I had more money (summer job), so I inquired about cams.
"What about stage 3?"
"Poor low end performance. Rough idle, poor throttle response."


Poor throttle response at low rpms was right. Give it a twist, nothing. More twist, hardly anything. But when the tach hit 6k, hold on!

A few weeks after I let my 300lb boss try to pop clutch-less wheelies around the parking lot, the bearings went.

Long distance phone call: "Mom, I need $800 to get my motorcycle fixed."

"Who is this? Sorry, wrong number!"

Good luck with your project. Keep us advised.

The GS's are a MUCH stronger starting point than the 1971+ CB designs, with a much better frame and a significantly tougher roller bearing crankshaft engine...
Just don't go too crazy with radical cams and huge carbs, & don't drop a ton of cash on re-sleeving the cylinder block to fit 72mm or even perhaps 73mm custom pistons (502cc, the MAX for resleeved big bore on the GS400/425 inner 4 cylinder studs' spacing, in my opinion; not an issue on the later GS450's - different inner stud spacing) unless you want to be in REAL DEEP $$$$...
I am resleeving mine but that is because I have 3 spare 72mm GS750 (for 850 cylinder block transplants) pistons from an engine that blew up due to detonation on cylinder #4 caused by a poor valve job from an automotive machine shop that I WILL NOT be using be again! Very receeded/tuliped(?) valve faces that wore to a razor sharp edge that heated up red hot & pre-ignited the mixture, ruining #4 piston & combustion chamber...
So I'll be utilizing these 3 pistons to bore out my twin with GS450 or GS850 sleeves installed to 489cc & ~10.4:1 compression (mandatory high octane in hot summer weather)

It will be an advantage over 449cc high compression Wiseco's bored in, but even with already having the pistons, I'll be spending similar $$$ as the 449cc Wiseco K844 to get 40cc's more.
It will be a BLAST though, especially on a diet to 330lbs with the susoension/brakes/wheels I've got here for it...


FYI...
This build I suggested with a pair of 2 Wiseco K844's & Web Cams #110 regrind profiles, re-jetted rebuilt carbs, K&N filter(s), & performance exhaust will be nothing like the radical, unreliable prrformance build with unstreetable cams as described above... Some of us have dome our homework for a long time now to know how far we can reasonably take our builds...

Hope the previous posts help assist someone in building a very fun and reliable corner burning rocket out of a GS400/425 twin!

FYI these pistons DO NOT fit GS400 4 valve per cylinder TSCC models, nor do they fit GS450's. They only fit the 77-79 GS400/425 2 valve per cylinder models, which are essentially half of a GS750 top end and crankshaft