5th gear failure modes:
KM145 gearboxes (up until about 1990) had a very weak 5th gear. Because the gearbox was originally designed as a 4-speed, Mitsubishi had to make the 5th gear very small to get the desired overdrive ratio, but that didn't leave much metal to hold the gear together. Older 5th gears just broke like this:
- Old5th.JPG (101.57 KiB) Viewed 16658 times
When they revised the gearbox (changing the name to the V5M21) they increased the countershaft gear ratio to enable a larger 5th gear to be used, while reducing all the other gear ratios to keep the overall ratios the same as previous gearboxes. The 5th gear no longer split as often, but now there was another problem: the 5th gear bearing sleeve comes loose. The gearbox I'm currently rebuilding has come loose, but I've caught it just in time to be repairable.
If you remove the transfer case and look inside the gearbox extension casing you'll see the retaining nut holding the thrust washer, keeping the 5th gear and bronze synchro ring against the shifter hub.:
- 1.5thAssy.JPG (64.15 KiB) Viewed 16658 times
Inside the 5th gear are caged needle rollers. These let the 5th gear spin freely when it's not being used.
- 2.5thGrear.JPG (83.98 KiB) Viewed 16658 times
Those needle rollers run on a sleeve which is clamped in place by the big retaining nut: here's the previous assembly without the 5th gear in place.
- 3.Sleeve.JPG (61.94 KiB) Viewed 16658 times
You can see the sleeve just slips over the mainshaft so it's replaceable.
- 4.SleeveOut.JPG (68.12 KiB) Viewed 16658 times
5th gear begins to fail when the retaining nut comes loose. The mainshaft twists as it transfers torque to the wheels, but the thrust washer and bearing sleeve are more rigid and don't twist as much. Eventually the retaining nut frets into the thrust washer and comes loose. Here you can *just* see the hexagonal marks where the nut has chewed into the thrust washer (I've ground both surfaces back a little before reassembly):
- 5.LooseNut.JPG (72.26 KiB) Viewed 16658 times
Once the nut is loose the bearing sleeve is no longer clamped in place, and when you're in any gear but 5th it will spin on the mainshaft (rather than staying still while the bearings rotate around it):
- 6.Burns.JPG (69.64 KiB) Viewed 16658 times
- 7.SleeveBurns.JPG (50.36 KiB) Viewed 16658 times
If the sleeve is free to spin, the needle rollers have no incentive to rotate and tend to rest in one position, brinelling (denting) the sleeve and making the sleeve spin even more.
- 8.brinelling.JPG (58 KiB) Viewed 16658 times
Eventually the sleeve chews into the mainshaft, destroys the needle rollers, and breaks 5th gear. A new mainshaft is *not* cheap.
The proper way to prevent this would have been for Mitsubishi to grind a notch in the sleeve which engaged with a peg on the mainshaft, so even if the sleeve came loose it couldn't spin (like they did on the transfer case mainshaft). I guess you could add one by grinding a notch into the sleeve and mainshaft, but that would also create a stress-raiser, and I'm not brave enough to risk it. Instead I'm using retaining compound (Loctite for bearing sleeves) to glue the sleeve to the mainshaft, and regular loctite on the retaining nut.
I'll also be periodically removing the transfer case and checking that the mainshaft nut is still tight.
Edit/Update:
BEWARE! The workshop manual correctly states in the "technical data" table that the countershaft should have zero to 0.05mm preload:
- Screen Shot 2019-01-04 at 19.36.53.png (61.95 KiB) Viewed 14469 times
The step-by-step instructions say only "obtain the standard value" of 0 to 0.05mm
- Screen Shot 2019-01-04 at 19.36.29.png (78.14 KiB) Viewed 14469 times
You'll be checking the *clearance* of a whole bunch of components while you're rebuilding the gearbox. If you're not paying attention (and I wasn't) you might end up giving your countershaft 0.05 clearance instead of preload. Whoops.
Guess what I'm working on right now...
- IMG_20190104_173149.jpg (74.34 KiB) Viewed 14469 times