The inner workings of a YZF 5 Speed transmission

Hey gang,

For those knowledge junkies out there wondering how our beasts shift thru the gears here is another dissertation:

Where’s it all stack up at

It all happens on two shaft assemblies. In between the two case halves right behind the crankshaft. The first shaft Yamaha calls the “main axle” we’ll call it the clutch shaft, as it is the same shaft that the clutch hub stacks up on drives power into from the crankshaft. The second shaft Yamaha calls the “drive axle” but we’ll call it the output shaft as it has the sprocket attached to it and this is where the power exits or “output” to the chain.

How the gears stack-up

The gears stack up on each shaft in a kind of reverse order. One shaft starts with the biggest gear and works it’s way to a small gear. The mating shaft does it in opposite order. When a big gear drives a little gear it can spin the little gears shaft several times for every one time it rotates. The opposite is true for a small gear driving a big gear, it has to rotate several times for the big gear to go around once. We call these differences in rotation “ratios” The first I mentioned would be called a “fifth gear ratio” where as the last I mentioned would be called a “first gear ratio”. A ratio is the rate at which one shaft turns relative to another.

Some basics

Before we got into detail let’s understand a few basic things about our transmissions. We have shafts that are “splined” and on these “splines” we have “sliding gears” that slide left to right but cannot rotate because they have teeth on their inside diameter (ID) that locks them to the shaft. Each of these sliding gears is controlled in it’s action by what is called a shift fork. Shifting forks are shaped like the letter “C” they wrap around grooves or collars on the sliding gears so that as the gears rotate these forks can still hold the gear and slide left to right or side to side. These forks are controlled by a shift drum which we will discuss later. We also have gears mixed in with these slide gears that do not slide along the shaft they are fixed from going side to side with snap rings and washers, but they can do something different than the sliders, they can rotate freely around the shafts diameter. They do NOT have splines on their ID. All the gears that rotate or spin freely on a shaft have what we call drive lugs on the sides of them. Drive lugs are kind of like protrusions sticking out from the side of the gear so that when a nearby gear “slides” right up tight they engage with each other through their sides and lock both of them to the shaft. The spinning gear can’t spin anymore because it is “Lugged” to the sliding gear which remember what we said about all sliding gears – they can slide because they are splined to the shaft.

The stack-up

This is where it gets confusing, remember what we’ve discussed so far and follow closely. It helped me to even draw a picture as I went along. Got get a picture of the transmission page at Yamahas website for a 98 YZ400F. Print it out and follow along as I show the flow (via the numbers on the picture) of power from the clutch shaft thru to the output shaft when the transmission is shifted into each of the various gears. It might help to write down next to the gear what its name is from the part description as well (IE first gear second gear etc…).

Power Flow

In neutral all sliding gears are turning on their perspective shafts and mating with their perspective free spinning gears on the mating shaft. For any gear to be selected a slide gear must move over and “lug” a free spinning gear to it’s shaft thus making power connection from the clutch shaft thru a gear set to the output shaft.

First = 5th gear (#15) slides to the right and lugs 1st gear (#14)(free spinning gear), to output shaft (#13). Power comes in the clutch shaft (#1), goes thru the 5th gear ratio (#14), into the lugs of the 5th gear slide gear (#15), which is splined to the output shaft (#13), thus transmitting power out. (Power flows from item number1-14-15-13)

Second = 4th gear (#21) slides to the left and lugs 2nd gear (#23)(free spinning gear) to output shaft. Power comes in the clutch shaft (#1), goes thru the 2nd gear (#9 & #23) ratio, into the lugs of the 4th gear slide gear (#21), which is splined to the output shaft (#13), thus transmitting power out. Power flows from item number 1-9-23-21-13).

Third = 5th gear (#15) slides left and lugs 3rd gear (#19) (free spinning gear) to the output shaft (#13). Power comes in thru the clutch shaft (#1), up into the 3rd gear slider (#6), thru its splines, thru the 3rd gear ratio (#6 & #19), into the lugs of the 5th gear slider (#15), which is splined to the output shaft (#13), thus transmitting power out. (Power flows from item number 1-6-19-15-13).

Fourth = 3rd gear (#6) slides left and lugs 4th gear (#7) (free spinning gear) to clutch shaft (#1). Power comes thru clutch shaft (#1), up thru splines in 3rd gear slide gear (#6), thru its lugs into 4th gear (#7), thru the 4th gear ratio (#7 & #21), which the 4th mating gear (#21) is splined to the output shaft thus transmitting power out. (Power flows from item number 1-6-7-21-13).

Fifth = 3rd gear (#6) slides to the right and lugs 5th gear (#3) (free spinning gear) to the clutch shaft (#1). Power comes thru clutch shaft (#1), up thru splines in 3rd gear slide gear (#6), thru its lugs into 5th gear (#3), thru the 5th gear ratio (#3 & #15), which the 5th mating gear (#15) is splined to the output shaft (#13) thus transmitting power out. (Power flows from item number 1-6-3-15-13).


Three shifting forks that are slid side to side in the transmission by what is called a “shift drum” control all gear changing. The shift drum is rotated a small amount every time you depress the shifter. Fro example; as you depress the shifter down into first the drum rotates about 30 counterclockwise, where as shifting into second rotates the shift drum clockwise the 30 degrees coming back from first gear and an additional 30 degrees to get into second gear. Along about now your saying OK but how does this rotating shift drum make the shift forks slide side to side as you noted above to change gears? Simple: Imagine cutting grooves around the OD of the drum. Now place pins (that are part of the shift forks into these said “grooves”. As I rotate the drum around and around nothing would happen to the forks because the are just setting there with their respective pins resting in the grooves. But what would happen if I took and put an “S” curve in one of the grooves, meaning that it wasn’t a straight line all the way around the shaft, it wiggled side to side as it went around the shaft. Now as I rotate the shift drum the respective fork would be forced to slide side to side as it’s pin passes thru the “S” curve. The shift drum has three grooves in it, one for each of the three shift forks. Each groove has a precise path that it related fork has to follow as the drum goes around. Each groove has it’s “S” curves precisely placed so that the three forks are sliding left or right at the right time.

Bored yet!


sorry dude i did not get past the first paragraph :)

WOW, someone asked you to please post the gear workings and you did. Thanks I appreciate it, it is hard to understand as its much more complicated than most people realize I read it 3 times. I know alot of people don't care until it gets broke, and they they get frustrated how to diagnose it when they dont fully understand how it works. This is where a video could help with your excellent explanation. This and your explanation of the oiling system should go into a tech article. :D:)

WOW. Did you write that up or was it from another source??

No. I wrote it. I was asked after posting the one about the oiling system to post one about the transmission. Being an Engineer and having just gone thru mine, I found it awesome, and wanted to share the knowledge. The problem is I need to create some pics / schematics to go with the write up. Obviously, it's far to detailed to get unless you have been inside one yourself and can picture the parts.


and all I thoughtit was was step on this little thingy that sticks out of the engine case

down 1 up 4 :)

Can you elaborate on

First = 5th gear (#15) slides to the right and lugs 1st gear

If 1st gear lugs then the powerband needs adjustment or tightend Right :D

Im confused :D

It'll need adjusting only if the muffler bearing isn't interfacing with the cannutent valve properly. It all has to do with the multiplicative conjugative inverse of the protensious shift fork :D:)

What about those footpeg bearings?

Just put a big bore kit in my wr400/430 and upgraded the crank to the 426 crank while I was it - so I had to split the case. Anyway, your write-up makes perfect sense, as I had both tranny shafts completely dismantled to change out two pitted gears. Was a bit of a challenge putting the shift drum/cam and forks back in, but everything worked out fine. Good write-up! :)

What about those footpeg bearings?

Never heard of pivot pegs ?

What about those footpeg bearings?

Never heard of pivot pegs ?

Oh damn. Forgot about those


You aint kiddin, the shafts drop in real nice but trying to snake the shift drum and shift forks in was another story. They were one of those things (once you got them in you didn't know how but you were glad they were in) :D

Which gears were pitted on yours, I lost both 4th gears and changed the 5th wheel gear out becasue it's lugs looked a little rough edged. Clutch shaft shift fork (the one by itself) looked a little rough as well :)

Here and I thought you were going to detail the 5sp conversion for a 450.

The 4th pinion gear was pitted (on the main axle/clutch shaft). It was kinda strange wear, almost like a casting defect - hard to believe who ever assembled the tranny missed that. Anyway, I changed that gear and the gear it engages with on the drive shaft. I showed the shift forks to the head mechanic at Long Beach Yamaha here in Ca, and his comments were: "if it were my bike I'd put them back in, but my labor is free, if it were from my customers bike I'd replace them"!!! I ended up replacing all three. As I was already in the motor, I replaced all nine engine bearings and assorted seals and o-rings.

It was a great learning experience and I'm glad I did it. This forum (TT) was an invaluable asset, and between the collective wisdom/experience of this group and the Yam service manual I was able to successfully complete the job and save a bunch of money.

Anybody try converting a yz450 4 speed into a 5 speed :) 4 speed sucks in the mountain trails :D

Joe, interesting that you posted teh tranny info. My son and I have just gone through splitting the case on a 99 400F. Have you looked closely at or ahd to replace the kick-start shaft and related pawl/ratchet? This was a source for a small bit of aluminum breakout, from underneath the stop plate, which eventually lodged into the large clutch drive gear and caused a "clunking" sound that brought to mind a bottom bearnig problem. I know others have had issues with the starting shaft and ratchet. I was able to fabricate a better stop plate and we have had no trouble since. Thanks for your posts. Regards, BD

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