I have fitted 3YF 5th speed gears to my 2KF, tested it for a short ride and found no issues yet. Time will tell if there are faults in the execution and what the life will be.

This is the second repair to 5th gear. 32000 km ago, before i had 'cracked the code', i fitted early gears 5Y1 and 1VJ. Not only did they last poorly, they wrecked the new 3AJ oil pump, put fine scratches on many working surfaces.

I have learned so much since then, thanks to many posts here, especially XTKlaus, and to guys like martyn biker who sent the 1st proper parts list i have.

1st, a little history: the 1983 XT600 34L was a development of the 550, code 5Y1. By 1986 the next significant model change was the 1VJ, and already there was a new 5th gear with that code. 1987, '88 and 1989 saw more changes including revised primary ratio and pressure oil feed . 1990 brought paired 2JT gears with fewer teeth.

A combination of these, but i suspect predominantly different use, dampened the 5th gear issue. Hard riders migrated to other brands. Yamaha 'milked the cow'.

The 1st XT 660 3YF developed from 600 got new gears. I can't find were they were used in a 600 before the 4PTA of 2000. The 600 4PTB ended 2002? Odd, maybe that engine lives on in another model. Belgradia?

The 3YF 5th gears are 1.5 mm wider than all previous gears. The casing of 4PTA has a changed part number suffix, '00' to '11', indicating changes.

I believe the bearing housings in left hand casing were machined to accommodate wider gears on longer shafts.

Modifying the earlier housings similarly is possible, but there may be an easier, cheaper way:

I have yet to figure out how to add text after placing photos, so may have to post this and add to it afterwards.

p

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The photos posted earlier are of 2nd gear. We have to make space for new wider 5th gears and must do so without detriment by thinning and moving 2nd gears.

Tip: if this is new to you, open a 600 parts list, the item numbers are the same or close.

2nd gear on input shaft is 2 mm wider at centre than at teeth, 1.5 mm on 5th side and 0.5 mm on bearing side. Machine the 1.5 mm on 5th side off and we have space for new wider 5th gear. Just remember to record and press it back to the same dimension it was from end of shaft after fitting new 5th pinion.

Output shaft 2nd gear has no boss. Instead a 1 mm shim is used between gear and bearing. Item 20 or 21. Remove the shim, gain 1 mm.
To replace the roll of the shim and prevent the gear fouling the bearing next to it, machine 0.5 mm off that face of the gear but leave inner diameter of 32 mm. That get you 1 mm and in my case was sufficient. 2nd is not a stressed gear so taking a full 1.5 mm prolly won't hurt.

More difficult: i made 2 rings to fit output shaft, photo below. To correct for end play roll played by removed shim, i made a hard press-fit ring can be seen, click to enlarge. Also supports moved 2nd gear.

Also a loose spacer ring to prevent 2nd gear from wandering towards 5th and blunting selector dog teeth.

Oy vey, if you don't understand i can understand, better at doing than teaching.

More, and critical 1st aid for your engine coming.
p

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Be interested in seeing what everything looks like at say 5k miles to see if any galling happens from shim removal ect. I think sometimes those shims act like a thrust bearing allowing 2 sides for movement in case one side doesn't want to spin. Maybe no problem, hardness and smoothness of parts helps.

I don't think wider gears were needed for strength in a bikes, just proper heat treatment to stop chipping, maybe the wider contact reduced contact pressures but if thickness was an issue they would be breaking instead of just chipping. Hopfully this will solve it by having better gears in them.

Greetings jjrider,

Agreed, the shims have a function. Straight-cut gears have very little need for axial support, the small 2nd gear has a machined boss, no shim. The fact that it is a press fit is the difference. Engineering principles.

I have to consider another principle; the lesser of two evils :-)

I'm applying that to the larger gear, admit it can wobble around to an extent being free on its shaft, but is held up by the bearing next to it. Hard surface to hard surface in oil without side thrust is not a problem.

Reducing the pressure between mating surfaces by increasing contact area works for me. Also, when you have held most of the gears Yamaha have tried, different materials and surfaces, you may think differently.

Thank you for your comments and yes, time will tell.

p

I have been appalled at what i found inside this engine.

I've seen many photos posted of the failed pistons, cylinders, and wondered why. This is such a beautifully made engine, why do they fail?

The new oil pump fitted just 32000 km ago is trashed. There are fine scratches in the cylinder, on the piston.

The only thing could have done that is dirt or the bits off 5th gears. Looked for but found no metal in the strainer, a little on the magnet fitted to the drain plug recently, when the noise started.

The conclusion is that the oil strainer is unable to protect the pump, and 5th gear does more than just wear out, it can be the catalyst that destroys the engine.

Since the rebuild I have watched how the oil tank, always full in the morning started to be low, then empty. The one way valve in the side cover is not the only culprit. Not even the main culprit.

When the oil pump loses capacity thru' wear, thing start going south... a little less oil to the piston... bit more wear to rings... more carbon in the oil. Etc.

The oil circuit starts at the strainer, goes to the pump then to the filter...all those fine bits of gear go thru' the pump before being filtered...and maybe some thru' the filter.

So i suggest setting a trap at the strainer and under the oil tank, with magnets harvested from old computer hard drives.

See photos below.
1 pump rotors.
2 strainer with magnets JB Welded in and edge sealed.
3 added magnet holding itself against strainer.

That's it,

p

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magnet are always good , I have one on the side cover of my new motor, should've put one down there also. I've had a bunch of these pumps apart and your was the second worst I've seen, the only worse one was a 660 quad motor(same motor family) that had a destroyed transmission also and a bad crank bearing. Obviously those tranny issue filter down to everything else.

A strong rare earth magnet glued to the outside side of the oil tank can hold particles up there also.

Any update on this? It was quite awhile ago. I am new xt600 owner and have to crack the case and would fix this if it works. Since I will be in there already.

all the 660 motors i opened had f$%^&d up 5th gears too, so i dont think that 3YF parts are going to solve the basic wear issue of the XT.
as for a magnetic catch before debris do damage to the pump, this is what i use as the engine drain plug instead of that glued on magnet pete is suggesting. at least you can pull it out and see if it's collecting any crap.



https://www.ebay.com/itm/Black-Magne...cAAOSwcj1aBnPF

I'd rather weld the oil-pick-up-filter instead of using some sort of liquid gasket.
Unfortunately I didn't see this topic until after my engine halves were together.

Hi all,
Interesting comment that 660s also wear their 5th gears.
The mods i did have done 10 000km now without any issues at all. There is no metal on the drain plug at services, but there never was much, because it is not in the oil stream. Now i have magnets attached were the oil passes i am confident i catch any particles.
The strainer gause is sealed around the edge with gasket silicon. But it is a poor barrier against the fine stuff the comes of the gears.
The other 2 XT600s i modified in a similar way have not reported any problems.
Best,
p

Good to hear. How did you secure the 3rd magnet btw? Surely press-fit is quite a risk with all the vibrations of a offroad thumper.

besides the noted 5th, also 3rd is another weak pair....


my theory is:
5th gear - very long ratio, people logging the engine at low revs while cruising, gear set gets hammered steadily...


3rd gear, sits in middle of the shafts where deflection due to load is greatest, so when used for strong acceleration ends up working not fully meshed.


BTW, also the honda xr600 suffers form 3rd gear issues, so common problem in singles....

3rd and 5th seems to fail the most on wr450f aswell, they changed the internal ratio in 2007, where the problems almost stopped.

I've never seen og heard about failures on the 660's. A friend got a few high mileage motors which burned oil, but never any transmission problems.

Hi Doubleyoupee,
You can see the 2 magnets on the strainer mouth? Found in old computer hard drives, they are shaped and affixed with high temp silicon. The 3rd partially fits under and attached itself to the bottom of the strainer cover (iron). These are incredibly powerful magnets, you don't want to catch your finger under one of them. They lie in and around the passage of oil to the pump. BTW, you can't weld the strainer. Aluminium with ferric cover, it was rolled closed on manufacture. If you have to open it, you stretch it as little as possible. Closing is not perfectly strong and leaves small gaps hence the sealer.

Greetings turboguzzi,
Agree with your theory. Most often, the longest ratio gear operates highest percentage of time and inside high torque range, therefor will wear faster.
As for 3rd gear; i recently renewed my chain an sprockets. On the first ride other that down to the shops, a large oil leak developed from the output shaft seal. Dismayed and angry with myself and expecting the worst, i stripped off the bits to find nothing cracked or broken. After refitting and adjusting the chain with more slack the oil leak disappeared. I suspect that indicates how much the output shaft can flex. And straight cut gears wear rapidly when run away from their designed centres. But i suspect you see racing bike issues and i don't. Racing may use 3rd gear a lot more than 5th.

Hi Jens,
Yes, as you will recall, the change of internal ratio was used with later XT as part of the evolution of 5th gear remedies.
For the less tech savvy; torque and speed in transmissions are related where increasing speed reduces torque imposed on components. You raise the input speed at crankshaft/clutch end and reduce speed at chain and sprockets end.
Power in and out remain equal, gearbox runs at higher speed but with less load on all gear teeth.

Cheers,
p

Thanks. What (type of) magnet did you put under the strainer?