Grabbing two GS twins for cheap... Q's

[Jedz123]

Hey Folks
I'm looking to pick up 2 GS twins for CHEAP and am being sold by the current owner take 2 GS twins make 1 GS twin. Well I'm pretty familiar with the rock solid GS450 this poor bike burned up when the current owner openly admitted he caught it on fire after trying to force start and fire it... Motor is in good shape other than the harness being pretty burned up. The second bike is a GS300, motor turns but doesn't run (no carbs, no tank) I was interested in the micro GS for I believe you could run a bigger twin in that frame (I believe). Tiny bike with a what looks to be a traditional GS twin 300 cc 8V motor. I also will be getting a GS500 motor with no cylinder head... I figured the parts alone are more than worth the purchase price but it would be cool to fab some kind of Franken-Twin up (thinking Scrambler or back road brawler).
Anyway more interested in the 300 and if that would be a good candidate for a big motor. I would also be interested to see if the 300 head would work on the 500. I know the 450 head does work... Don't know much about the 300's I'm kinda curious on just buying it to look at it.
Likely will buy the Twin lot and play around with making something for winter blasting. Any insight on the 300 would be stellar!
Thanks guys!

[bwringer]

The GS300 is an 8-valve engine and the GS400-500 are 4-valve. They're very different engines.

The GS250 twin and the GS300 have a lot in common (same head gasket, for example), but I don't think the smaller 8V engines share much with the 4V GS400-500 engines.

As to whether the engine mounting points are the same or close enough to adapt, I have no idea; someone else may be able to answer that. However, it wouldn't surprise me much given Suzuki's predilection for parts bin engineering.

And of course, there's a ton of the usual Suzuki parts commonality between all of them as far as electrics and other running gear (for one example, my GS850 is wearing a stator from a 2001 GS500). Depending on how bad the fire was, bringing the 450 back from the dead might be a better course. Or transplanting that engine into the 300. But then you'd need to scare up a tank and carbs, and...

The GS500 engine is a bit of a pig in a poke, depending on why it's headless -- if the cylinder head was damaged by lack of oil, then the bottom end is certainly trashed as well. So you definitely don't want to pay much if anything for it. But there are certainly some good parts to be had and sold if you're so inclined.

Hard to say what's best to do here. Winters are long up thataway, so you certainly will have time for a major project...

[Chuck78]

I would go for fully resurrecting the 450, as there is probably a more solid frame bolted to that engine currently than what the 300 could offer you without extensive bracing. It probably has thicker walled tubing, more bracing, and possibly larger diameter tubing as well. The 450 frame is quite short wheel base as it is! The steering angle is a bit lazy (28 degrees?), but if you notice, the triples have a significant amount of offset, which reduces the trail significantly and makes it steer very snappy.
I pulled a 2 into 1 header off of a parts bike in the junkyard that I believe was a GS 250 or maybe GS300, and it appears to fit a GS 400 or 450 as well. That's about all I can tell you as far as interchange.
I would be looking at 300 head to see if it has the same stud spacing as the 450 and 500, but I am inclined to say no, probably not. If it did, that would be cool, but the valves probably aren't size big enough for a GS450 or 472 or whatever the GS500 capacity is. The 8-valve twins have much better low lift flow as any for valve combustion chamber does, so it would be definitely more efficient, but would probably run out of steam early on top if it did even fit.

[Chuck78]

GS500 Forks make a really nice upgrade, and then you can run a 98 - 99 CBR900RR front brake rotor and the GS500 brakes, and have some pretty darn good stopping power with a 310 mm rotor. GS wheels will bolt in with axle spacers off of an 850+ & GS500 axle. GS1000 chain drive & GS850 non-L triples will swap over fairly well with perhaps more offset than the nice alumimum (lower & upper) 82-83 GS1100E triples ( I'm going to assume the GS 1100 G also has similar offset to make steering more stable for a bigger less twisty road oriented machine).

As you probably already know, the GS500 head would not do you any good with an earlier GS frame anyways, so that's no big deal at all. Basically all of the internal parts would swap over to a 450, the only big difference is the engine case mounts are not compatible, and the shift drum drops right in but is a reverse shift pattern because the 500 has a rear set orientation shifter. And of course the cylinder block and pistons, but those together will swap over onto a 450 as well, but I believe you need a substantially thicker head or base gasket. Cometic can fix you up with just that.

If you put bigger camshafts in, you would need to have some machining done to the valve pockets on the GS500 pistons in order to work with a 450 head. 500 valve Pockets do not quite line up with where they should be on a 450, but I believe with stock camshaft timing and stock camshaft, this is not an interference issue whatsoever.

If the engine is apart, and you are trying to build something really fun and special, I would use that GS500 cylinder block and have it bored out 1, 1.5, or 2mm (75mm, 75.5mm, 76mm) & look into getting custom pistons made or compare a GS1000/1100G big bore piston to the 450 pistons. I believe the compression height is the same, the pin size is definitely the same, the big question lies in the dome size and clearance on the 450 head, and the valve pocket locations & depths.

Or you could cheat and contact Rich Graver on here if he is still poking around on this forum, WERA90EX or WERA90X? He is a veteran GS twin racer, and I believe he has past order numbers that he may or may not share with you for custom-made Big Bore pistons to fit a 450. I believe he has gone as large as 550cc, the Formula 500 class limit in WERA.
It would be interesting to compare the bore and stroke on a 450 or 500 to that of a GS1000 or GS 1100 G, and compare a big bore piston for those bikes.
With a high-compression piston, I think it might just work out for you compression ratio wise to get a decent streetable compression figure. The question is the dome shape and valve pocket location.
GS500 is 74mm x 56.6mm.
GS1000E is 70mm x 64.8mm.
1115cc GS1000 pistons = 74mm. MTC has some old school NOS sets of these for $250 (set of 4). They bore in to a true 74.000mm cylinder to give the recommended clearance, unlike many where the actual bore size is much smaller. This would allow you a slight bore or hone job to clean up the standard GS500 bores. MTC could also machine the valve pockets differently for you.

MTC also makes an 1146cc GS1000 piston, 1.0mm larger than those NOS old school 74mm pistons on clearance.
KS1146 part number on the APE - GS Zone website.

1146cc

75MM

10.5:1

KS1146

MTC

Cylinder boring only for GS1100 2 valve. 1000 requires KA629 sleeves

Heck, you could probably talk to Big Jay @ APE & see if he has these in stock, as if he did, they could probably have you send them a 450 piston to compare valve pocket locations and dome height to see if they could machine the pockets to fit, and if so, send him the head to make sure the dome clears.

The GS450 is 71mm stock bore, so going up to 75mm would be an AWESOME upgrade! As they are bone stock, they are very fast for a 450, but still don't really have the gusto to do much to impress at highway speeds and above.

78 GS400C cams have the biggest duration of the GS twins, and the GR650 cams have tge biggest lift (& mild duration). 400 cams will give you a screaming top-end. The 650 cams will give you a tourqey mid-range for the size of the engine, and make a grrat street cam. Plus they are hollow core like the Yoshimura factory team cams!

GS500 carbs will bolt on and offer some added performance. There are a few different generations of 500 carbs, you will have to ask Big Rich on here or someone else fluent with the 1980s twins, on which version is preferred. I believe it is the first or second version, as the last uses too many plastic parts if I recall. I could be mistaken.


I believe it was a Vance & Hines Big Bore Kit for the GPZ 1100 that people used to use for making a big bore GS500 that was close to 550cc. Unfortunately the dome design changed and still works with a GPZ 1100, but no longer fits the GS500 head. This may still work out okay as the GS 450 likely has a larger combustion chamber than the GS500. I am not an 80s GS twin guy so much, so I don't know all of the exact details here. Aftermarket Pistons are made for aftermarket cams, however, so the valve pocket should be cut a good bit deeper to accommodate some serious cams, so I think there is some wiggle room there.

There you go, Justin, way more information than you bargained for, but enough to build a really sweet and severely potent GS 450 engine! GO BIG! Cometic can make any thickness or bore size change to a 450 or 500 base and head gasket set!
Talk to Rich Graver if you can reach him. He has raced since the 1980's on GS twins. He builds them himself. He will have some piston insights. WERA90EX.


Darth Graver, GS twins wizard, Grattan, 1991:

[Big T]

Back in the early 90s, there was a guy from Seattle area that ran a GS 500 using a GSXR cylinder head with the two outer cylinders chopped off.
That thing flew, but reliability?

[Chuck78]

Also, it is possible to bolt on the much more efficiently designed gr650 head onto a gs450, but I believe one area of the cam chain tunnel just barely matches up between two different applications. And the GR650 pistons are designed for a very long stroke engine, so they will give you fairly low compression without milling the head significantly to get it up to a stock compression ratio. This gives you 527cc and a much better cylinder head. Nothing to do here would be have some custom pistons made up. I believe a member on here did have some made up for a costume set up GS 400 bottom end with custom respace cylinder studs to fit a GR650 head. He shared the Carillo or JE order #. All you would need to do would be contact them and make any alterations to the pin size and compression height, and use the same dome or have them add a few cc's to it to accomodate the difference in stroke between the 400 & 450.
Or Rich Graver may have his own custom GR->GS450 piston design that he could share with a reorder for you. I know he has built at least one of thise 527cc 450's before.

The GS1000 1146cc or GPZ1100 piston option may be the most accessible without any outside help.

Best of luck... I love lightweight old vintage hot rod projects built from GS's!

[Jedz123]

I was Soo hoping you'd respond Chuck!
Thanks man! Definitely have the gears spinning!
I just locked the deal in and threw down not allot of cash for 2 machines. I will likely build something that may not look great but will run good. I want something light and quick to go up into the mountain dirt roads with and if it wrecks I won't be too upset or can walk away from.

I will have 3 vlocks to choose from and I will likely make some monster twin to put in once I burn out whatever motor I decide to use to romp on...