5-speed gearbox: rebuild, strengthening, tips (WIP)

[Growlerbearnz]

Most 1987-2000 L300 Delicas have a cable-shift 5-speed gearbox. It was originally a 4-speed until Mitsubishi slapped a 5th gear on the back as a temporary fix, and like most temporary fixes it stayed that way for a couple of decades. The gearbox has a few weak spots, and was continually revised and upgraded over its lifetime to try and fix these. Many of these improvements can be fitted to older gearboxes.

Much of the info in this FAQ is ganked from these threads:

Clutch Issues

hi y’all. Got a new noise, rattling until clutch is depressed. Also started to get a whiff of that scorched clutch smell. Happened really fast, no warning leading up to it unless I just didn’t notice. I know these have a weak 5th gear, hope I didn’t really mess this up. I have the gas g63b with...

delicaforum.com

5 speed rebuild

I finally got a little time to start fixing my new Delica. I bought it with a bad transmission, it would only move in 4th gear, which made it a bit of a challenge driving it into the garage. I was pleased that the transmission came out fairly quickly and I only needed 12 and 14mm wrenches and...

delicaforum.com

Looking for guidance fixing V5M21 5 speed

It's looking like rebuild time for my 5sp, V5M21, 1996. It wants new synchros for 1 and 2. No other problems are obvious right now, other then oil residue from the engine/trans interface. 1) Per GBNZ, I should replace the input shaft bearing; and check/replace as needed/locktite the fifth...

delicaforum.com

Converting from Automatic to Manual: Manual gearbox rebuild - Delica Canada | Mitsubishi Van Club

delica.ca

Your first stop should be to download a copy of the Mitsubishi 5-speed Transmission workshop manual. You want book 22A- V5M21. As you'll see, there are quite a few variations and changes to the internals over the years- I strongly recommend disassembling and inspecting your gearbox before ordering any new parts. Genuine Mitsubishi parts aren't expensive if you go through Amayama.com.

 

[Growlerbearnz]

First up and easiest upgrade: Gearbox flex.
The layshaft originally ran on normal ball bearings, which can take a lot of load but have slop in the axial direction. A layshaft that moves fore and aft makes the whole gearbox a bit sloppy, so Mitsubishi switched to taper roller bearings (like your front hubs). This solved the slop problem, but now the driving load tries to force the end walls of the gearbox case apart: a gearbox case that wasn't designed to handle these loads.

A flexing gearbox allows the countershaft to move away from the mainshaft, changing how the gear teeth mesh just when they're trying to handle a lot of torque. Mitsubishi's solution to this never made it onto the L300, but you can retrofit it:



A cast aluminium lower cover, which replaces the pressed steel cover. Fitted to the Mitsubishi Starion for only two years (1988-89) with original part number MD727553, no longer available, and rare as hell. Starion and 4WD guys have been making their own versions out of alloy plate at enormous expense. (http://projectzerog.com/forum/viewtopic ... 6c461631ff , http://www.starquestclub.com/forum/inde ... pic=100703)

...except... they're not rare or unavailable. Mitsubishi put them back into production in 2000, albeit with a different part number.

Part 2502A008. If you're feeling fancy you can also fit the O-ring seal MD727531, but I use Permatex grey high-strength silicone instead.

 

[Growlerbearnz]

The input shaft bearing is a known weak spot- they tend to collect metal particles and fail. Something to do with the flow of oil in the case, I suspect.

Early gearboxes (and cheap rebuild kits) have open bearings, but Mitsubishi upgraded the input bearing to have felt seals, which let the oil in but keep the swarf out. Compare the upgraded bearing to a cheap replacement:



Genuine part MD731727 is the snap-ring style input bearing with the felt seals. (Check that your gearbox uses a snap-ring style bearing first, as some had a flanged bearing.)

 

[Growlerbearnz]

5th gear failure modes:

In the before times there was a 4-speed gearbox. Torque from the clutch came into the input shaft, was transferred to the layshaft via the layshaft reduction gears, and then went out to the driveshaft via whichever gear was selected (pay no attention to 4th gear!). All fairly normal so far.

Mitsubishi made an ordinary little 4-speed gearbox, but since it was designed during the fuel crisis they made some optimisations to make it strong, quiet, efficient, and cheap.

The layshaft reduction, for example: the input shaft had 17 teeth, the layshaft had 29 teeth (17/29) for an overall gear reduction of 1.706. This is quite a high reduction for a layshaft, but it meant the 3 other gear sets (1st through to 3rd) could be closer in size, making them stronger and cheaper to manufacture.

Things went well for a while and many other manufacturers used or licensed Mitsubishi's gearboxes, but then everyone decided they wanted an overdrive 5th gear. Mitsubishi's clever gear ratios weren't so clever any more. Slapping a 5th gear on the end of the gearbox wasn't difficult, but it needed a high gear ratio to overcome the quite high 1.706 layshaft ratio. This meant the layshaft 5th gear had to be large, and the mainshaft 5th gear quite small. Inappropriately small.

Result?

Om nom nom.

Later gearboxes have revised gear ratios to fix this problem, which largely works, but in doing so they introduced a *new* issue: the 5th gear nut comes loose.

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.:

Inside the 5th gear are caged needle rollers. These let the 5th gear spin freely when it's not being used.

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.

You can see the sleeve just slips over the mainshaft so it's replaceable.

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):

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):


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.

Eventually the sleeve chews into the mainshaft, destroys the needle rollers, and breaks 5th gear. A new mainshaft is *not* cheap.

The easiest thing is to remove the transfer case and check that the mainshaft nut is still tight. If it's loose you'll be able to move it with your fingers.

 

[Growlerbearnz]

Rebuild tip/warning:

BEWARE! The workshop manual correctly states in the "technical data" table that the countershaft should have zero to 0.05mm preload:



...but the step-by-step instructions say only "obtain the standard value" of 0 to 0.05mm




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...