Having read the threads on the forum about the GS gearbox conversion for the GT, I decided to try to summarise them all in order to do the upgrade for the project bike. Hope I’ve got this all right!!
As the thread highlights, the reason for doing this is to primarily take advantage of the better ratios of 1st and 2nd on the GS. You also get a modern output shaft sprocket fitting to allow for a larger range of sprockets.
Bit of ebaying turned up a GS750 (1980) gearbox. Although digging through specs, there is some confusion where the GS’s ended and the GSXr’s took over.
Anyway, the ratios for the GT’s, GS and GSXr ratios seem to be similar;
750 A \ GS \ GSX E
1 – 2.84 \ 2.57 \ 2.5
2 – 1.83 \ 1.83 \ 1.75
3 – 1.38 \ 1.38 \ 1.37
4 – 1.13 \ 1.3 \ 1.13
5 - .96 \ .96 \ .96
So there isn’t much between the GS and the GSX, so I’m happy I’ve got the right box without counting all the teeth!
The forumed basic changes are;
Input shaft (clutch) is the same width between bearings as the GT so it drops right in. The GS shaft is longer which means mods to the clutch basket (6mm counter-bore) to make it sit back in the same position on the splines as the GT and remove 6mm off the length of the shaft (clutch side) to stop it clashing the outer case and having done that (and ending up with a shorter thread for the nut), undercutting the thread to allow the nut to go all the way on again.
The GT lubricates the input shaft gears/dogs by passing oil down the shaft through a central hole that is fed by the primary gear (clutch) picking up oil and dropping it into catch trays that then feed it to the end of the shaft via an oil seal in the shaft. This seal is missing in the GS and the shaft blocked so the GT feature has to be replicated in the GS shaft.
Seal shown on output shaft of GT with none on the GS.
I am fitting a dry clutch so I have other issues with lubrication (later).
The output (sprocket) shaft on the GS is 1.6mm longer between the bearings. The easy way to fix this appears to be;
1. Fit the right hand (bike clutch side) bearing of the output shaft in its correct place with the ½ moon retaining clip in place. This allows first gear to rotate without clashing on the case.
2. Now it is easy to see that the right hand bearing (sprocket) is 1.6mm away from the retaining clip groove line-up.
3. Take the sprocket side bearing off and machine 1.6mm of the inner race (bit that touches the shaft). This is easier that machining 1.6mm off the hardened shaft bearing face, which would move the bearing in as well.
4. The output shaft also needs the oil seal fitting in the end opposite the sprocket as in the input shaft mod.
Next, the GT’s output shaft has a larger oil feed hole in the bike clutch side of the shaft. The excellent U tube video on this conversion suggests opening up the GS hole to match the GT as the GS end bearing suffers with a small hole.
My output shaft doesn’t seem to have that hole at all (see questions at end), although there is one under first gear. I assume it because my output shaft has a bearing rather than a lubricated sleeve.
The GT also has an oil feed hole in the input shaft that feeds the clutch basket roller bearing carrier. I cant see any references to modifying this on the GS shaft and my GS shaft has a smaller hole in a different position (within the splines) that doesn’t seem to be under the roller bearing carrier position I am unsure how the GS shaft oils the basket roller bearing carrier?
GS shaft on left showing small hole NOT in-line with GT bearing carrier oil hole position on the right shaft (end bearing removed).
The last noted modification I can find is that when all mods are done and the cases trial assembled, one of the gears (probably first, haven’t checked yet) snags on the oil catch plate in the upper casing. So if the gears won’t spin, check that!
KNOWN MODIFICATIONS SUMMARY
1. Fit oil seal recess to both GS shafts (bike clutch side)
2. Shorten input shaft clutch side by 6mm and undercut screw thread
3. Machine 1.6mm off inside bearing sleeve on sprocket side of output shaft.
4. Open up oil feed hole in output shaft (clutch side) to match GT
Questions.
1. My GS output shaft does not have the shaft lube hole at the bike clutch side under the bearing. How can oil get into the shaft and gears/dogs? Is it really a GSX shaft and if so how does it lubricate? Are the holes (picking up oil in the gearbox along the shaft) enough without an end feed?
2. Do I need to change the input shaft clutch basket bearing carrier oil feed hole? It doesn’t line up with the bearing position will be on the GS shaft.
3. I am tuning to around 100bhp and am concerned about the side loads on the bevel primaries. Should I move to straight primaries?
4. Because of the dry clutch complications, I am going to have to feed lubrication for the clutch shaft from the left hand engine side (rear sprocket). The output shaft can still get its oil using the catch plate system for that shaft in the GT because it is still in the ‘wet’ bit of a dry clutch. I cant see why the input shaft lubrication cant happen naturally as the gears spin because there are oil holes in the shaft anyway but I suppose oil needs to move along it which is why the GT shaft fed the oil in from the engine clutch side to the shaft holes? Not sure how to fix this yet in a dry clutch?
5. Finally, I’ve seen some comments about dogs clashing with this (or the GSX) mod, but I can’t see any evidence yet. Anyone had any experience?