6-speed Transmission Swap Instructions

This is a summary of the 6-speed transmission swap I did on my 81 kz750e. Because I could not find any instructions to do this modification, I researched the subject for months. These are the final results of my efforts and are not the definitive instructions, but simply the way I went about it.

** These steps are for informational and purposes only, and I am not responsible for any injuries, damages or mistakes you incur if you decide to follow these steps!**
**If you do attempt this conversion, follow any related disassembly and assembly directions and torque specs in your specific service manual for your specific bike.**

**It is possible to use these instruction to swap a 6-speed into any four cylinder kz750 model, Zephyr zr750 and possibly a zr-7. This can also be done on a kz650 but the kick start mechanism will have to be deleted and a later style non-kick kz650 clutch cover used. **

To preface this, my entire goal of this transmission swap was to be able to keep the stock gearing on 1st through 5th gear, and yet add a usable overdrive to keep revs down while on the interstate. So 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 by using the 550 6-speed transmission.

This is the factory kz750e gearing stats:





This is what I used for my particular build.
(Your results will vary depending on your rear wheel and tire sizes.)

1. 6- speed transmission from a 84 zx550
2. 530 pitch chain
3. 750 clutch with 2.55 primary ratio
4. 17 tooth front sprocket with 13mm offset
5. 39 tooth rear sprocket
6. 140/70-18 rear tire and rim from a zr550

For a final result of:






What is needed to be sourced for the swap:

1. kz/zx/gpz550 (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 Zephyr zr550 and 1985-1997 zx600a/b/c 6-speed transmissions have a ~10mm longer output shaft than the kz/gpz550’s. Thus requiring a smaller offset sprocket which is very important when doing this swap on a Zephyr zr750 or zr-7. Those bikes have longer output shafts to accommodate wider swingarms, rims and tires.**

2. Shift drum and selector forks and rod from the corresponding 6-speed transmission.

3. zr-7 shift return spring. This can also be found in zr550/750’s, zx600’s and other bikes (part # 92144-1074)

4. 550 front sprocket with mounting holes tapped in it.
Your particular wheel and tire size and chain pitch will dictate how many teeth and what offset the sprocket will need.

5. Front sprocket splined keeper plate and bolts. Part # 13270-1060

6. Rear sprocket and corresponding pitch chain. (530 pitch preferably due to availability of offset sprocket choices. Also order a 120+ link chain and cut it to length.)

7. Heavy duty or hardened washer with 20mm ID (optional)

6-speed bits


Picture shows a spacer that came with the sprocket but was not needed.





What will also be needed but can be reused from the original 650/750 engine:

1. Transmission bearing cap with clutch pushrod port/guide. ( The gpz transmission I used was 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 pull-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!
On the 550 models, the longer side of the fork boss faces the RIGHT-hand side of the shift drum.
On the 750 models, the longer side of the fork boss faces the left-hand side of the shift drum. THIS IS WRONG!

CORRECT!





WRONG!

Assemble the upper and lower case halves following the service manuals instructions. (making sure the shift drum is in the neutral position)
Once the case halves are bolted together, make sure that the transmission spins freely. There are factory shims that can be swapped out on between the gears in the event that the transmission binds when the case is bolted together. The directions for this are in the service manual. (But it you disassembled the gear stacks from the shafts for inspection, you may have reassembled the gear sets improperly or with the wrong shims. Hence the binding.)

Case halves bolted together for testing.



Install the gear shift mechanism to the shift drum. Be sure to use the lighter style zr-7 shift return spring before installing the shift mechanism. The heavier kz750 spring will work, but if you intend on using rearsets with heim linkage, the heavy spring will cause a very heavy pedal feel and may stress the rearset linkage causing clunky shifts. ( I found this out the hard way.)
Then spin the input shaft with a cordless drill so you can get past the neutral lock out and made sure that you can select all six gears before going any further.
It may be necessary to put a small amount of drag on the output shaft when doing this. Under normal operation, the tire and chain propelling the bike forward provides tension on the output shaft. Without this tension, the transmission shafts rattle and oscillate at different speeds and will not let the dogs engage properly when testing. I lightly put drag on the output shaft with one hand and operated the shift mechanism with the other all while a helper spun the input shaft with a cordless drill.

Return springs





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 without 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 as the neutral switch is in a slightly different location. Again…possibly. As of this writing, I have not verified this.)

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 any benefits of swapping to a 6-speed.
(Using the 550 also requires use of the corresponding secondary shaft gear from a 550 and shortening the clutch pushrod. On bikes with a pull-style clutch, the pull lever would have to be custom made in order to reach the smaller clutch hub.)




Note that nut is not fully able to thread on shaft.



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 belleville (warp) washer originally under the clutch nut to keep it from backing off, but without the washer, the contact patch between the clutch nut and clutch hub barely 1.5mm wide. (The hub has a deeply chamfered area at the splines that greatly reduces the contact area between the hub and nut.) So the warp washer not only provides tension, but creates a larger surface area for the nut to distribute the clamping force.

I did not feel comfortable with such a narrow contact patch holding such an important component together that also sees so much abuse. So I had the machinist remove another 2.5mm to make room for a 47x20x2.5mm stainless washer for a total of 5.5mm of material removed from the inside of the hub.
I used such a wide and thick washer because:
1. When the clutch hub was machined, the splined steel core of the hub was exposed. So I wanted to use the washer to also provide what ever structural support that I could to make up for the material
that was removed. The wider washer would also help distribute the clamping force over a wider area.
2. The beefy 30mm impact socket I used to remove the nut was 47mm in diameter. So I didn’t want to recess the washer and nut into a narrow hole that I couldn’t get an impact socket into.
3. I could not find a hardened washer with a 20mm ID that had the OD that was even close to what I wanted, so I went with a beefy 2.5mm thickness instead.

Machined hub




I then bolted the clutch hub onto the input shaft using blue loctite on the threads.

Finished product

FRONT SPROCKET
You will first need to decide if you want to use a 520 or 530 pitch chain. I chose the 530 as there is a much greater selection of aftermarket offset sprockets for the 530’s.
** Zephyr zr750 and zr-7 transmission output shafts are a 1/2” and 5/8” longer than the kz650/750. So you will need to use the longer zr550 or zx600 transmission and an aftermarket sprocket in order to retain a straight chain alignment. The offset can’t be determined until the motor is back in the frame. And you will have to verify sprocket chain alignment using a used/cheap 550 sprocket to measure off of.**
** A 520 or 530 17-tooth front sprocket is the largest that will fit without forgoing the use of the front chain guard **
**550 sprockets are designed to float around on the output shaft and are NOT supposed to be bolted rigid like the 650/750 sprockets are.**
** Kawasaki and some Suzuki’s share the same output shaft splines. The sprocket I used was from japan.webike.net and was for a gsx1100 Katana but has the Kawasaki keeper holes drilled and tapped in it already.**


The other issue I encountered, is that the 550 transmission is designed to use a 520 sprocket and the 530 sprocket I used had a wider splined area and it would not fit onto the output shaft fully. There is a collar that is on the shaft that the transmission cover seal rides on, that kept the wider sprocket from seating properly.
*If using the longer zr550 or zx600 transmissions, this step will not be needed.*
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 transmission cover.

Difference between stock and modified collar


Pressing the modified collar back on


Finished product.




Note gap between sprocket and cover.



You will probably need to custom fit your chain depending on the variables of what pitch you chose and what sprocket tooth count you used, combined with any swingarm modifications. I bought a 120 link chain and trimmed off the extra links to fit my specific setup.




Having done this swap, I simply cannot imagine how it did not come this way from the factory. I was worried that in sixth gear and at a steady cruise that I would be out of the torque band of the motor and it would bog and have no ability to accelerate. These fears proved to be completely unfounded. And as long as I don’t let the rpm’s dip below ~2750 in sixth, the engines low end torque can easily accelerate the bike as if Kawasaki designed it that way.
So I got exactly what I wanted, totally stock gearing and a true over drive gear that did not sacrifice my low speed performance the way that a sprocket only upgrade would have done.

Great job. I have always wanted an additional gear on the freeway.
I wish you were my neighbor...LOL

That's an amazing write up on a project that has been rumored to work around here for years. I'm not sure anyone ever really had a 100% confirmed-to-work and ride-able example before.

Since it is pretty important, based on your write-up, you may want to make a note that most (all?) Kz550's came with 530 chain/sprockets. The Zx550 came with the 520. I don't know if that bushing on the output shaft would then have been shorter on the Kz550's. But it would be worth looking into. If a simple photo from the trans cover area would show it, I might have a photo, or I can take one, or make a measurement if you need it

Thanks guys!

I just wanted there to be some sort of information out there to dispel the urban legend of the mythical "6-speed in a kz650/750."
When researching this, I read a lot of comments like: "I did this on my bike years ago" or " Its real easy. Just bolts right up."
But I never found anybody that would provide any specifics. So since I had to split the engine cases on my 750 anyway, while it was apart I test fit the zx550 transmission I had laying around. And since it fit so easily I took a stab at solving the clutch hub issue and once that was sorted, I was was pretty committed to going all the way.

loudhvx - You bring up another detail that I missed. I didn't realize that the early 550's used 530 chains.
But the output shaft collar (part# 92027-1106 25X32X23.5) is shared by the early kz550's to the zx550's, Zephyr 550 and even the zx600's. So the extra wide offset sprocket I used was to blame for the fitment issue and not it just being a 530 sprocket.
These are the kind of important details that need to be pointed out!

I just did a rough measure on a Kz550C (Ltd) motor. The output shaft sticks out about 14.5 mm from the edge of the collar. I can get it more precise if you need,

That corresponds with the roughly ~15mm on the zx550 shaft.

I thought about doing this when i swapped a 650 into my 550 (it blew a rod) but eh i didn't feel like splitting the case.... I calculated the ratios and I get 5.8mph more (62.7mph) at 4500rpm in 5th with the 650 over the 550 in 6th so thats nice.

I am curious though, can the case be split and the 6 speed installed without removing the crank/pistons/head etc? I can't remember how that part went when i took apart the 550 motor.

Having done this swap, I simply cannot imagine how it did not come this way from the factory. I was worried that in sixth gear and at a steady cruise that I would be out of the torque band of the motor and it would bog and have no ability to accelerate. These fears proved to be completely unfounded. And as long as I don’t let the rpm’s dip below ~2750 in sixth, the engines low end torque can easily accelerate the bike as if Kawasaki designed it that way.
So I got exactly what I wanted, totally stock gearing and a true over drive gear that did not sacrifice my low speed performance the way that a sprocket only upgrade would have done.

Great write up, I was planning to do this too sometime. I'm puzzled why they didn't make the 750 six speed, could it be the tourque loading increasing by a 120 percent ( 6 /5) put it outside their 'bulletproof' safety margins? The GPZ600 water cooled shared the same 550 transmiission and had more power but less tourque.