GS tranny in a GT750 box

Suzukidave · Post by **Suzukidave** » Fri May 22, 2009 4:42 am

tz375 wrote:Dave

I hope you do a better job that Honda, BMW or Bimota on the EFI system.

Richard , i am hoping with the R1 throttle bodies being CV type they will help smooth out the power .. we will see

pjmcburney · Post by **pjmcburney** » Mon Jun 08, 2009 9:45 pm

tz375 wrote:Paul, Sounds like that might be a good idea. How does teh GS get oil into that needle roller bearing and bush?

I also checked the gear ratios in a GS750 trans and not surprisingly, it's identical to a late A model GT750 apart from a slightly higher first gear. 2 through 5 are identical to a late model GT. It brings the ratios slightly closer overall and reduces first to second jump but will need a few more revs to get it moving off the line.

Primary ratio is much lower though (higher numerically) as the motor revs faster than a GT

Well, (as Dave has done) I got around to doing some comparisons between the two 'boxes on the weekend.

It is actually possible to use the GT input shaft, and all its gears, in combo with the GS output shaft.

The difference between the two different sets of first gears must not be that great, however there does seem to be more lash with the GT/GS combo than with either of the correct shaft combos - makes one wonder if it would be a problem in service?

The other thing that bothers me is the location of the sprocket on the output shaft.
It appears (by my evil eye alone) that the sprocket would be spaced out another six or seven mm's, as the shaft is about that much longer - is this a major drama or does the design of the later sprockets take this into account?

I still want to use the GS 'box I bought as it is far superior to any of the other 'boxes I have, the one I bought must have had a very good life.

Thoughts anyone?
Cheers
Paul

Suzukidave · Post by **Suzukidave** » Tue Jun 30, 2009 8:51 pm

Hi Paul , did you make any progress fitting up the GS gear set to the GT cases ?

pjmcburney · Post by **pjmcburney** » Tue Jun 30, 2009 11:13 pm

Hi Dave,

Not yet...

There is a guy here in Brisbane who has had some experience with this modification. He's a rather grumpy and recalcitrant chap, though, so I may just have to weather his attitude to get the right advice and machining work.

It's a bloody close thing - Suzuki made very few changes to the two different boxes when they went four-stroke. As you've found, the GS box fits straight in and works perfectly with the GT shift drum and forks.
There are no real clearance dramas with any of the gears and the oil pump/kickstart gear meshes quite well with 1st gear on the countershaft.
The GS bearings will have to be changed over to GT bearings as the one inboard of the sprocket is 1mm too wide, and they have pins in them - this is all no big deal as the shafts are the same diameter.

The shafts themselves are the major issue. As mentioned above, the two shafts are just longer than they need to be. I'm sure it's nothing that can't be cured with a bit of fart-arsing about with a lathe.

I'll keep y'all informed of the progress and publish the findings. I might be able to put together a 'GS box in a GT for bodgers' guide when I sort it out.

Don't hold your breath though, I have myriad other projects to start and finish before I can get to this one, including (but not limited to): KTM 495 assembly, RZ250/350 engine, frame swap and rebuild and hopefully an RG500 complete assembly in August (well, it arrives in August), which I'm particularly looking forward to as it will be a realization of requited lust for the best bike I've ever owned.

Cheers
Paul

Suzukidave · Post by **Suzukidave** » Wed Sep 23, 2009 7:53 pm

Ok , finally got around to shortening the GS clutch shaft . I chucked it up in my small lathe , spun it up and because the end is hardened i used my 4" cutoff tool with a metal wheel and removed 4mm off the end .

Suzukidave · Post by **Suzukidave** » Wed Sep 23, 2009 7:57 pm

Then as the GT shaft has a seal in the end i had to cut in the end of the GS shaft a place to put in the seal . This time i used my Dremal with a hard grinder and spun up the lathe the opposite direction of the dremal and ground out a relief for the seal , this took about 30 min or so .

tz375 · Post by **tz375** » Wed Sep 23, 2009 9:47 pm

Dave,

that's great. Are you offering an exchange service on those shafts yet?

Here's the scoop. the difference in gearing is huge between GS and GT but it's only 0.5mm on the diameter to the first gears.

A GS input shaft is too long but can be used if you can shorten it as Dave did. First gear on that shaft is 14t on a GS and 13T on a GT. That is where the improvement in gearing lies - right there. The matching gear is 36 on a GS and 37 on a GT. That's close enough so any combo of GS/GT first gears will work but may make a noise.

If you want to keep the stock GT output shaft for the stock sprocket location, strip all the gears off the GT shaft and fit all the gears off the GS shaft. Use a modified GS input shaft and all GS gears and you should be good to go.

If you want to use a GS sprocket and space the sprocket out, use both GS shafts, but machine the right ends of both to take oil seals for the oil feed.

There are several other combos that will work, but you must match gears for dog type and teeth. The GS has a different dog design on several gears just to make life more challenging. You could use ALL GT gears but use the shortened GS input shaft with GT gears on it (apart from first) and all GT output shaft and gears.

As a bonus, the pair of selector forks for the output shaft are a carryover part from GT to GS and the bearings off a GS fit the GT. A nice upgrade are 2001-5 GSXR needle rollers in a new outer adapter - Even sexier would be a 2006-7 GSXR closed end needle roller in a steel adapter and dump teh end cap.

Suzukidave · Post by **Suzukidave** » Thu Sep 24, 2009 9:26 am

I got back to work on the output shaft this morning to grind in the relief for the oil seal , i used the same setup as the clutch shaft and it also came out as planned

so once i put on bearings the shafts are ready to go .

tz375 · Post by **tz375** » Thu Sep 24, 2009 12:34 pm

Dave,

Does the output shaft on yours fit between the half moon clips? I couldn't work out why mine seemed so tight, so i measured it and the GS is 1.6mm wider from groove to groove.

I pulled all the gears, shims clips etc off and the difference is in the middle of the shaft. That is to allow for the different and longer dog design on second gear. That means that the clip groove nees to be moved approx 1.6mm.

The GS is interesting in that the design of gears on the input shaft is the same as the GT750B, but every gear on the output shaft is different and they use hardened steel bushes ILO bronze in the rotating gears. That's how they still do it, so it is much better than a GT, if we can make it fit.

Suzukidave · Post by **Suzukidave** » Thu Sep 24, 2009 8:47 pm

Richard , tomorrow when i get off work i will put on some GT bearings and check it out .

tz375 · Post by **tz375** » Fri Sep 25, 2009 12:46 pm

Dave, I should have noticed this earlier, but that GS output shaft has gears with castellated type dogs. My 81 trans has rounded dogs on the output shaft gears - completely different.

Yours is like a GT750 and it's quite possible that it will slot straight in.

On the later shaft, second gear is slightly wider and so is the shaft.

Don't tell me this is another example where you buy a part and it fits and I buy one from something similar and it won't fit?

From now on I get you to buy all my parts

What I failed to notice was that the so called 81 GS750 was actually one of the 16 valve GSX models and amazingly used the same gear ratios. I guess it was time to fill in that gap in my knowledge...

So the trans I am looking at is a GSX and Dave has a GS trans. It will be interesting to see if that's the only difference in dimensions. If so we may end up with more than one solution to squeezing a GS trans into a GT crankcase.

tz375 · Post by **tz375** » Fri Sep 25, 2009 12:54 pm

Just out of idle curiosity, I discovered that the following bikes share the same clutch hub splines and clutch nut

GT750
GS750
GSXR600K1
GSXR750K7

The ends of the shafts are all the same except for the 2007 750 which is larger so bearings are interchangeable. Pity they are all different lengths though.

Suzukidave · Post by **Suzukidave** » Sun Sep 27, 2009 10:39 pm

It turns out that beside cutting 4mm off the input shaft and grinding in the ends of the shafts for the seals also the output shaft has to be modified as it turns out to be too wide between the bearings as Teaser posted so the gear retainer that is part of the shaft has to be thinned so the bearing at the sprocket end can slide further on the shaft . I put the shaft back in the lathe and set up the dremal again to grind down the flange .

Suzukidave · Post by **Suzukidave** » Sun Sep 27, 2009 10:43 pm

Completed grinding and the flange is much thinner and with the bearings back on it now drops right into place .

Post by **Suzsmokeyallan** » Sun Sep 27, 2009 11:01 pm

Daves got it all figured out so far for a transplant, the only thing left to do is to cross drill a hole for 1st gear in the shaft but its hardened so that may be a bit difficult with common tools.
We also talked about the possibility of grinding a portion off the bearings inner race instead of thinning the shafts flange so much but its really a moot point being the gears are straight cut.

GS tranny in a GT750 box

Redux...