GS tranny in a GT750 box

Suzukidave · Post by **Suzukidave** » Sat Mar 28, 2009 5:45 pm

I have been doing a bit of talking with several other list guys about this swap over , so i pulled out a 77~79 GS750 gear set and a GT750 lower case just to see what would be involved i could see there isnt a whole lot different between the 2 sets ( besides the better gear ratio of the GS gears ) the output shaft of the GS is longer that would allow spaceing the sprocket out for wider rear tires easier and also its splined for the later model front sprockets like come on also the GSXR and that will allow you to use there 520 and 525 and 530 chains . the biggest problem is on the clutch output end of the gear set , the GS shaft is longer ( about 7mm ) as the GS and GT use different methods to disengage the clutch . The GS uses the shaft thru the engine from the left side to push the pressure plate off where the GT uses the lever on the right side to pull the clutch pressure plate off the clutch pack so i dont know if the longer GS shaft will interfere with the GT clutch release http://suzukidave.myphotos.cc/suzukidav ... T%20Gears/ has anyone done this swap before ? .. Dave

Suzukidave · Post by **Suzukidave** » Sun Mar 29, 2009 8:07 pm

This is a surprise that no one here has done this swap

Post by **Suzsmokeyallan** » Sun Mar 29, 2009 8:49 pm

Dont worry Dave, i'm backing you up on this one 100%.

Suzukidave · Post by **Suzukidave** » Sun Mar 29, 2009 8:56 pm

Thanx for the support guys .. at least with this venture i know its been done before . It still would be great for someone whos done it to step up and tell me do this and this and this and its a done deal

Ogri · Post by **Ogri** » Mon Mar 30, 2009 5:21 am

I have always wondered if this would work. But, I wouldn't know what to look for to determine it.

It would definitely help when it comes time to replace the sprocket. IIRC it cost me over $200 to have Kosman make the offset sprocket for me. So, every time I do a chain and sprocket replacement, it is going to be a major expenditure. If one of his standard Suzuki sprockets could be used, it would only cost $85 each time the sprocket needed changed.

My mechanical knowledge is limited, so keep that in mind as you consider this...

What if one were to replace the gearing on the buffalo clutch output shaft with the GS gears and use the GS output shaft for the sprocket? This way the GT clutch could be used and still have the benefit of the GS gearing and the longer output shaft. Would this work?

rngdng · Post by **rngdng** » Mon Mar 30, 2009 7:24 am

There's a blank plug on the left case of the GT where a clutch pushrod might have gone. I figure they planned it that way, but changed to the other type during development. Maybe a hydraulic actuator on the left??

Lane

Post by **Suzsmokeyallan** » Mon Mar 30, 2009 8:05 am

Having some conversations with Dave proved to be rather interesting with regards to the swap over of the shafts.
He was lucky to have both transmissions to hand for direct comparison, thats always a big benefit..
From his detailed pics and what was discussed, the actual transplant is looking pretty straight forward so far. Once Dave gets the shaft bearings swapped over from the GTs shafts to the GSs so they can actually drop into the GT cases he will know better just what further adjustments are to be done.
You will also need the GS clutch inner basket for the upgrade too because the splines on the end of the GS clutch shaft are different than the GTs.
THEN theres the main issue of the slightly longer clutch shaft threaded end thats going to need some minor reworking to shorten it a bit.
The big benefits here are the longer and better designed drive sprocket splines on the output shaft and of course the better shifting and closer ratios between 1st and 2nd.
Gearboxes for the first generation GS750s show up on ebay for about $40.00 including shipping, so its not an expensive retrofit.

tz375 · Post by **tz375** » Mon Mar 30, 2009 8:47 am

Dave has it in hand. As long as the width across the gears is the same, it will fit. The input shaft should not be hard to modify to use a GT lifter and GS hub. As lane suggested, it may be possible to fit a hydraulic lifter, but I couldn't work out an elegant fit for the one I have here.

It's a lot easier than trying to squeeze a 2001 GSXR 6 speed transmission into a set of GT cases. Shafts are good, input end is fine. Shafts in fact are the same diameter as a GT, so the bearings can be swapped. BUT the damn thing is 7mm to wide to fit in the cases and the selector mechanism is in completely the wrong place and would end up outside a set of GT cases!

Dave, I forgot about the oil pump. Will the GS gear drive that OK or will you have to swap over one pair of GT cogs?

Post by **Suzsmokeyallan** » Mon Mar 30, 2009 12:05 pm

Richard lets see that 6 speed transmission in the GT cases and where the selector will fall, do you have pics?????

tz375 · Post by **tz375** » Mon Mar 30, 2009 1:36 pm

Allan,

It's about 2 inches below the GT cases. The GSXR gears are stacked and not on the same plane as a GT, and that makes the whole thing shorter. Later designs are even shorter by placing one shaft above the other.

But to fit a GSXR tranny in, needs fairly extensive machining, welding and then more machining to move the left wall of the transmission out. If the GSXR had a proper (casette) trans, it would be much easier.

And there's still the oil pump issue to resolve. Water pump is easy to replace with an electric pulse pump from world leaders Davis Craig in Australia, but the oil pump would be slightly more difficult to arrange. I already looked at Polaris triple oil pumps which are cheap and available, but not so easy to arrange the drive.

I think Ogri Tim has the right answer if the Gt input shaft will work.

tz375 · Post by **tz375** » Mon Mar 30, 2009 4:29 pm

Overall the GS cluster is closer than any of the GT gear sets. GT has a 68.57% gap on early bikes and 66.22% on M/A/B)

First is taller and fifth is lower. 1-2 gap is reduced (30.96% v's 37.54% on late A's and 38.98% on JKLM), 2-3 and 3-4 are almost identical gaps on both bikes and 4-5 gap is smaller on the GS (7.56% v's 14.53%) .

The gear pairs are as follows:
GT750 L GT750A/B GS750
First 13:37 13:37 14:36
Second 19:33 18:32 13:32
Third 22:30 22:30 21:29
Fourth 24:27 24:27 24:27
Fifth 26:24 26:25 25:26

On the Gt750, top was changed from 26:24 to 26:25 with the M and late in the A production, seconds changed from 19:33 to 18:32.

A couple of years ago, I sat down and worked out an ideal set of ratios for a GT and they are the same as the GS except for 3rd where I chose 22:30 and Suzuki chose 21:29, a difference of less than 2%.

So Dave, I like the ratios and it looks like the oil pump will probably work with that 1 tooth larger first gear

Post by **Suzsmokeyallan** » Mon Mar 30, 2009 5:03 pm

Pardon my question here Richard but what other GS models from the same time period had a 6 speed transmission....

tz375 · Post by **tz375** » Mon Mar 30, 2009 7:30 pm

Allan, are you trying to tell us that there's a large bore GS from the era that had a 6 speed box that we are all forgetting about?

The 250/400/425/500 series are most likely to be inadequate for the power/torque we're looking at here.

When did Suzuki introduce a 6 speed trans on the 750 and larger bikes?

85 Slabside GSXR750 had 6 speeds. GSX750F katana had 6 speeds in 88 I believe but I'm no expert on those. The GSX750E from 83 on had only 5 speeds. The 1100 had 5 speeds up to 97.

So are you thinking Katana perhaps?

I have never been good at guessing games.

Suzukidave · Post by **Suzukidave** » Mon Mar 30, 2009 8:16 pm

There is a post on Smokeriders from Gary who races a GT750 and this is his comment on this as he has the GS tranny in his racer " I am running a GS input shaft and 1st gear in my GT. From memory the main problem is that the splines on the GS shaft are relatively further outboard (because the shaft is longer) and this would force the clutch hub outboard unless you put a 1/4" counterbore in the back of the hub and I wasn't too happy about loosing that much spline.

In the end I used a combination of a smaller counterbore and a spacer with 3mm taken off the threaded end of the shaft and it's been working fine in the vintage race bike for 3 or 4 years now.

Post by **Suzsmokeyallan** » Mon Mar 30, 2009 8:33 pm

Richard i was thinking along the lines of the GS550 transmission if it was a six speed.
It could take the power of a STOCK GT750 quite easily, unless it was designed poorly, but Suzuki always overbuilt these things anyways.