So I have finally found the time to assemble my bottom end now that my connecting rod bearings showed up. So that means I finally got to fully assemble the 6-speed transmission and seal the cases so its time to document what I did.
First what I needed for the swap:
1. zr/kz/gpz550 6-speed transmission. (They all have the same gear ratios. I had two ’84 gpz550 transmissions in my basement so that is the only reason I used that particular model. The zr550 has a 10mm longer output shaft in case you have a really wide rear tire.)
2. Shift drum and selector forks and rod from corresponding 6-speed transmission.
3. 550 style sprocket and splined retaining ring and bolts. ( I used a 530 17-Tooth 13mm offset sprocket. I have a non stock swingarm and rear wheel that required the offset sprocket)
What I also needed but can be reused from the 750:
1. Transmission bearing cap with clutch pushrod port. ( The gpz transmission I used is not a pushrod clutch, so the bearing cap was the wrong style. If you are doing this on a later model 750 with the lever type clutch release mechanism, then this not needed.)
2. Gear selector linkage.
3. Transmission cover.
4. Clutch assembly
5. Neutral spring and plunger
6. Shift drum alignment bolt
The transmission is simply installed in place just like the 750 transmission, only making sure that you have the correct style bearing cap installed for your particular clutch release type.
The 550 shift drum installs in exactly the same way as the original, BUT TAKE NOTE OF THE ORIENTATION OF THE SHIFT FORKS! The 550 shift forks install facing different directions from the 750 ones, so get a 550 manual to make sure you are installing the forks properly!!
Assemble the upper and lower case halves (making sure the shift drum is in the neutral position) and make sure that the transmission spins freely. There are shims that can be swapped out on the input shaft in the event that the transmission binds when the case is bolted together. The directions for this are in the service manual.
I luckily had no issues.
Install the gear shift mechanism to the shift drum. I then spun the input shaft with a cordless drill so I could get past the neutral lock out and made sure I could select all six gears before going any further.
Install the transmission cover as normal. On the kz750e I installed this on, the neutral indicator lamp switch and contact on the shift drum lined up perfectly and no modifications were needed to make it work. ( It is possible that other models with out the push rod clutch may need to have the shift drum contact plate unscrewed and re-indexed to align with the neutral switch on the transmission cover. Again…Possibly.)
The clutch for the 550 transmissions are much smaller and as a result the input shafts are shorter and there aren’t enough threads to fully bolt on the 750 clutch hub.
The 550 clutches will work but they have a totally different primary gear ratio (2.934) than the 750 (2.55)
So different that it is would negate and benefits of swapping to a 6-speed.
The nut is 3mm short from fully threading onto the input shaft. So I had a machinist remove 3mm from the inside of the hub. There was a washer originally under the clutch nut, so for some reason I got it into my head I should also make room for a washer, so I had the machinist remove another 2mm to make room for a 47x16x2mm stainless washer for a total of 5mm of material removed from the inside of the hub. I used such a wide washer as I didn’t want to recess the washer and nut into a hole that I couldn’t get an impact socket into.
If I was to do this again, I would not make room for a washer and only remove the 3mm and not use a washer at all.
I then bolted the clutch inner and outer hub onto the input shaft using loctite. Only time will tell if this holds up to the rigors of normal usage.
Note that nut is not fully able to thread on shaft.
The only issue I had with my sprocket is that the offset sprocket had a much wider splined area and it would not fit onto the output shaft. There is a collar that is on the shaft that the transmission cover seal rides on that is taking up to much space.
On the 750 transmissions this collar is held on by the sprocket which is bolted in place and once the sprocket is removed, the collar slides right off. But the 550 collar is pressed in place as the sprocket is held on by a splined collar that allows the sprocket to “float” on the output shaft. The 550 collar is designed to be removed by pulling off the outer bearing. Instead of doing this, I wedged the collar away from the bearing with a jewelers screwdriver just enough that I could fit a bearing puller on the collar and pull it off. I then had to have 3mm of the collar machined off to make room for the new sprocket and give it room to float. Then I pressed it back on and it sits just flush with the outer edge of the seal.
Note gap between sprocket and cover
When I was researching this swap I got on gearingcommander.com and started running all the numbers to see if what kind of RPM’s and speed benefits could be made.
This is the factory kz750e gearing stats:
This is what ended up using to keep close to the factory gearing yet adding a usable 6th gear:
1. 6- speed transmission
2. 530 pitch chain
3. 750 clutch with 2.55 primary ratio
4. 17 tooth front sprocket
5. 39 tooth rear sprocket
6. 140/70-18 rear tire
For a final result of:
I need to adjust chain slack yet.