Suzuki GS 400-500 Top End Swap

[t500racer]

Hi guys. I am new to this board. I came across a 1977 GS 400X a few years ago for an excellent price (free!), wire wheels, drum front brake, points ignition, the cheap model. The bike was a cosmetic disaster, rotted wheels, a barn had actually collapsed on this thing and you could tell!

It sat outside my garage for months until one night I had too much time on my hands and decided to see if the thing would run. A carb cleaning and battery theft from one of my other bikes had the beast running about an hour later. I was so impressed with how well it ran (only 3k miles), I decided to chop it for use in the dirt. A GS550 front end with GT750 triple clamps went on, with a T500 drum brake front wheel to replace the rotted one, XR honda plastic and headlight, 1975 Yamaha aluminum YZ Tank, 1974 TS 185 seat, 15" rear shocks and some knobby tires turned it into a heavy dirt bike. Pitched the stock mufflers, rejetted the carbs, K&N Filters. Good for the fire roads, but really not good for jumping, ask me how I know that! You might have seen the beast in the road race pits at Mid-Ohio this year.

Fast forward to this spring. I used the price of gas as an excuse to put it on the road, but quickly tired of it in dirt bike trim, so I changed it into a 50s style cafe racer. Polished aluminum tank, humped seat, clubman's, ect. The only problem I've had with the bike so far was a slipping clutch, fixed with some new metal plates and springs from a GS550. But I just can't leave well enough alone.

I've been told that the top ends from the other Suzuki GS twins will bolt directly on to my 400. I'd love to make this thing a 500, but I currently do not have a 500 or 450 to compare with. I have also been told to find a Canadian 4 valve 450 model GS. Can anybody confirm which top ends will fit my 77 GS 400? Would the 4 valve 450 or the 2 valve 500 be better. Or if the other, larger displacement motors are a direct bolt up?

I know it's silly looking for more power from an anemic old air-cooled twin tiddler like this, but I just can't help myself. I appreciate any information you can give. Thanks in advance.

Kris Larrivee
AHRMA#473
t500racer@yahoo.com

[Nerobro]

Well the plan with KDC67's GS400X is to find a GS500 top end to put on it. The top ends should be compatable. The 4 valve top end won't likely fit.

[focus frenzy]

the 8 valve engine is a 400, they never built a 8 valve 450.

unfortunatly the 400 8 valve, 450 4 valve, and 500 4 valve, are of a differant family from the original 400 (4 valve) and 425 engines.

the original GS400/(1979)425 is a low pressure roller bearing crank engine, Suzuki re-designed the engine when they upped the displacement, and came out with the 8 valve engine, and changed over to a high pressure plain bearing crank shaft.

I have a classic motorcycle mechanics magazine issue here someplace that has a complete article on the 400-450 GS twins and if I remember right suzuki also changed the bore and stroke so changing top ends is not possible.

you can upgrade to 425 pistons and rings as it was a bore increase.

I did some compairing (fische diagrams) and it looks like the engine mounts are in the same locations so you might be able to bolt in a 450 or early 500 (they neutered the later 500's)

PS: the 500 top end is a dead on bolt on for 450's as the 500 is just a 3mm over bored 450 with modern looking cooling fins.

[t500racer]

Thank you for the information. I knew it couldn't be as direct as a bolt on top end swap.

I am considering the 500 motor if it will bolt into the frame. I know you said later 500s were neutered. What year 500 should I be looking for?

If that fails, I was thinking about the 8 valve 400, what years was that produced? I am quite close to Canada, so I might just find one there. Would that motor bolt into the 77 GS 400 frame?

Thanks again

Kris

[Nerobro]

I still think it would be a direct bolt on. http://www.eurospares.com/gsfaq.htm Different bottom end or not.

[t500racer]

1989 GS 500 Help

Hey guys, I've just been offered a 1989 GS500 Canadian model at a reasonable price. There are few others sniffing around the bike, so if anybody out there can confirm whether or not the 1989 500 motor will bolt up to a 1977 GS 400 frame, please let me know as soon as you can. Or if the top end will bolt on the 400?

It seems too easy to me that an 89 motor would bolt into a 77, but I suppose stranger things have happened. I anxiously await a reply from anyone in the know. Thanks.

Kris

[focus frenzy]

it should bolt up, you may have to fabricate a spacer or two or more, but by looking at the diagrams the mount locations are in the same places.
I can not say with 100% it is a perfect drop in as I don't have the bikes here to measure.

I do know for a fact you will need the wiring harness for the 500 as well as the ignition module as the 400 has points and the 500 is electronic ignition.

suzukicycles.org list the 77 400 as having 36 HP, a 81 450L has 43HP (450 E or S have 44HP) and that 500 has 52hp (more than the early GS550!!)

[t500racer]

Thank you for that. I actually spoke to another Suzuki guy this evening and he told me almost exactly what you wrote. I don't mind a bit of tweaking, but I didn't want to get into a huge fabrication project. I've already been down that path with stuffing a T500 Suzuki motor into an RD400 frame for my AHRMA racebike, I want something a little simpler this time.

I think I will go ahead and grab the GS500. It will be nice not worrying about setting the points anymore and if those horsepower figures are even remotely correct, this is well worth the effort and expenditure for the gain. Of course 52hp in a 'flexi-flyer' frame ought to be fun. Suppose I will really have to start paying some attention the the handling. On that note I was offered a Dresda box section swingarm that was supposedly made for a racing GS400. I saw a picture of it and it's beautiful, as soon as I take some measurements I will know if it's a bolt on or not.

The bike currently has a GT750 front end with GT550 springs, drilled GS750 single disc and GT750 caliper with Ferodo race pads, a 2.15X18" rim laced up front (rear is the same size) and last season's Avon race tires from my racebike. Shocks are Suzuki from some motocrosser (14.75"), that originally had reservoirs, but were converted to oil only, reservoirs removed. I know that sounds fishy, and I was a bit skeptical, but they have worked well for a many hard miles, and I can change the damping by changing the weight of the oil in the shock. Not Ohlins by any means, but this is supposed to be a "budget" project. I don't know what the stocker weighed, but this is at least 40-50 pounds lighter without the stock mufflers, gauges, brackets, turn signals, fenders, etc..

The best part of the whole bike is the "What the f@#$" factor. Nobody knows what it is. I pull up to a stoplight and people are looking over trying to figure out what I'm riding. A guy at a gas station the other day asked me if it was a BMW, I told him no, suppressing a giggle, It's a Suzuki. He stalked off in a huff, commenting that at least my buddy (on a Harley) had a real bike. That was worth a million bucks. If anybody is interested I can post a picture, if there is a place to do that on this board.

Kris

[bikechump]

interested?

stop being modest, lets see it!:-D