My Transmission Teardown

Loosen transmission top cap and shift mechanism


Remove top cap


Loosen and remove tailhousing


Once the tailhousing has been removed, remove the staked nut from the mainshaft. A hammer, cold chisel and a pipe wrench will come in handy here.



With the tailhousing removed, tap here to remove mainshaft end bearing


An alternative method is to use a prybar and a hammer with a brass or aluminum drift, or a custom gear puller, or possibly a slide hammer to pull the mainshaft end bearing. This technique worked for me, although it is easy to chip gear teeth by prying against them.

Continued

With the mainshaft gear and bearing removed, you can move on to the 5th/reverse assembly on the countershaft.

First, remove the staked nut with again, a chisel and pipe wrench, then the countershaft end bearing should slide off.

Unbolt the reverse idler shaft from the outside of the case...I believe it's a 17mm bolt and remove the entire assembly.

The best thing to do now is to unbolt the fork guide from the outside of the case and slide it out with the collar and gear to avoid reassembly of the detent device, but you can remove the fork alone as I did, but be sure to catch the ball and the spring that work with the detent.



Now everything in the back half of the transmission will simply slide out.



Now you're left with an empty rear case once you remove the 5th/reverse shift fork.

Continued

Loosen and remove the rear bearing retainer.


With the slave cylinder previously removed, loosen and remove the front bearing retainer from inside the bellhousing. The input shaft should be removable at this point, depending on whether it's broken or not.



Notice how the fourth gear has broken off my input shaft, hence my transmission rebuild. I got new one on ebay from cityauto for $65.


Now just take out the guts by sliding the shafts around and tilting them for removal.





Now if you'd like...clean that sucker. Be sure to get all your gasket and RTV surfaces as clean as possible.

Comments inserted in RED below.

But one other note first... in the first picture viewing the 5th gear
synchro hub this is retained by a two piece split lock.
you MUST keep these cresent shaped pieces as a "pair"
because this the fourth "Select fit" component inside
the transmission.

The input bearing spacer (inside the front bearing retainer) is the first
the second is the cluster bearing spacer is the second and
the splined "cap" that sits over them is the third.

Langdog said:

With the mainshaft gear and bearing removed, you can move on to the 5th/reverse assembly on the countershaft.

First, remove the staked nut with again, a chisel and pipe wrench, then the countershaft end bearing should slide off.

or a 32mm Socket and an impact gun

Unbolt the reverse idler shaft from the outside of the case...I believe it's a 17mm bolt and remove the entire assembly.
17mm is correct

The best thing to do now is to unbolt the fork guide from the outside of the case and slide it out with the collar and gear to avoid reassembly of the detent device, but you can remove the fork alone as I did, but be sure to catch the ball and the spring that work with the detent.

I have never in ten years slid the fork off the detent shaft.
I ALWAYS remove the retaining bolt and liftht he fork, detent shaft and 5th/rev syncro assembly out o the trans as an assembly.

Because reinstalling the fork onto the detent shaft with the shaft mounted in the case as you reassemble the 5th reverse assembly is an absolute
BASTARD with the gears in the way

Now everything in the back half of the transmission will simply slide out.

Now you're left with an empty rear case once you remove the 5th/reverse shift fork.

Click to expand...

Excellent writeup... so far

That is a "different" failure of an input gear

I've seen 3rd fail the same way in 4.0 transmissions several times

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AWESOME!! I just bought a $50 M50 the other day. Even though I know the fellow I bought it from and was told it's all good I still want to go through it. Thanks for the write up and the time you took to do it. Much appreciated.

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Concerning the 5th/reverse shift fork, I actualy re-assembled the unit with all the parts outside the case and didn't really have any problems. I still would recommend just removing the whole assembly by unbolting it from the case

Be sure during disassembly to keep all your parts in the same orientation/order in which they came out of the housing. There are many parts that can go back in both ways and cause some pretty serious issues if installed backwards. Also, It's a great idea to disassemble all the sub-assemblies and clean parts individually at this point. This is where you have the best chance of finding any otherwise unnoticed damage that may have occurred to your transmission.

Check out what I found in my 5th/reverse hub assembly. There was severe pitting on both the hub and inside of the gear, plus the split needle bearings had some severe abrasions on the roller surfaces. The needles actually fell right out when I removed the cage. Along with two or three huge cracks on the synchro surface, I'd say this took some pretty good abuse.



Here's also a picture of how my input shaft bearing looked upon removal. The rollers fell right out of this one as well.

THE thing about manual transmissions is NOT "getting them apart"
or getting the parts back together in the correct order...

THE thing is recognizing an "almost bad" part.

Not to offend any idiots present, (or lurking idiots), but any idiot
can recognize parts that are completely "gerfukt", it's the parts
that look good until you look REAL CLOSE under GOOD light.

Like a faint silvery line on a gear tooth face that is actually
micro spalling from oil cavitation and most often happens on the
"coast" side of the gears (particularly on 2nd and 3rd on the cluster gear)

From what you've already shown I'd keep the case and toss most of the rest

Some of those parts are parts I've never actually seen "bad" like
the spacer/inner journal for the 5th gear countershaft gear

The burned up rollers for 5th gear? not common, in point of fact I've never seen that before. need another 5th gear with bearings?
I've got a dozen of them.

Those rollers can be pushed out of the cage with a fingernail
BTW, that cage is supposed to be in two pieces.




The other "thing" about transmission work is understanding what problems to ignore.

Yeah everyone wants to be a purist and use ALL new bearings in their trans
but some of those bearings can actually outlive several sets of gears...

Another word of advice NEVER disassemble a top cover
(remove the forks and shafts), for ANY reason

as for that trans? getting another transmission is likely a VERY good idea

Lastly, fter you remove the circlip that retains the "Bellcrank" that works the 5th/Rev fork remove the retaining bolt and slide the pivot shaft out of the trans
and put the bellcrank, washer and circlip back on it.

But keep that shaft with that particular transmission case.

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It's already reassembled, resealed, reinstalled and working great. I was fortunate to find a local M50D-R1 guru who had all the replacement parts and know-how to check everything. I'm not so sure about the idiot comment, but at least you can back up your statements with some knowledge. I do have to agree that having as much knowledge as possible about a project before starting is a wonderful advantage, even though you have to pay for it sometimes. Thanks for the tips AlanD. Here's a brewski

wow, I'd never expect an input shaft to do THAT! Seems like there are weaker points in the shaft that would have broken before that happened.

Mine just stripped all the teeth from the gear. I didn't want to pay for someone's knowledge so I pulled an M5OD out of a '92 Explorer that I drove before I took the trans out so I know it works.

LittleHorse said:

wow, I'd never expect an input shaft to do THAT! Seems like there are weaker points in the shaft that would have broken before that happened.

Mine just stripped all the teeth from the gear. I didn't want to pay for someone's knowledge so I pulled an M5OD out of a '92 Explorer that I drove before I took the trans out so I know it works.

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Actually yours losing all the input gear teeth was FAR more likely
to have been "melted off" rather than "broken off"

that synchro dog is retained to the gear with one TIG weld
on the part even a tiny crack leads to a propagating crack
until the remaining metal isn't sufficient to take the load.

I broke an output shaft and you can see the "wake marks" of
a progression of crack growth over time until you get to the
shattered metal of the final catastrophic failure.

Langdog said:

It's already reassembled, resealed, reinstalled and working great. I was fortunate to find a local M50D-R1 guru who had all the replacement parts and know-how to check everything. I'm not so sure about the idiot comment, but at least you can back up your statements with some knowledge. I do have to agree that having as much knowledge as possible about a project before starting is a wonderful advantage, even though you have to pay for it sometimes. Thanks for the tips AlanD. Here's a brewski

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I was definatly not calling you an idiot but I see the results of their labors all the time.

the guy who replaces the expensive parts and all the bearings and misses something that is obvious (to me) that would have been EASY to fix while the trans was apart.... But they just didn't see the problem.

Like the guy whp replaced the input and cluster gear but missed ithe crack in the 3rd gear he reinstalled in the trans that shed two teeth and RUINED the shiny new cluster gear when the broken chunks gouged the hell out of that shiny new cluster.

Or the other guy who installed a new second gear on a new output shaft abd reused the 2nd gear float bearing and wrecked BOTH the shaft and the new gear.... (I had to torch that gear off the shaft to retrieve the equally shiny new 3rd gear and 3-4
synchro assembly

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Manual transmissions like everything else respond well to exposure to a trained eye.

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AllanD said:

Actually yours losing all the input gear teeth was FAR more likely
to have been "melted off" rather than "broken off"

Click to expand...

funny you say that, I made the comment to a buddy of mine that the teeth looked like they'd been melted. Could this be the results of three rubber plugs leaking the oil out of the transmission? I had originally figured it was a shock loading thing from a 4k RPM clutch dump or something, but when I looked closely the deformation definitely looks like the metal was in a softened state rather than clean solid break. There are also some blued spots indicating that it had gotten hotter than a transmission should...

alright ....... if I ever need to disassemble an M5odR1 I will set up a video link with AllenD so he can "help" me check my parts before they go back in...... the above right up is so far over my head that I was lost reading it......... but I can admit it so that immediately removes me from the idiot column.

LittleHorse said:

funny you say that, I made the comment to a buddy of mine that the teeth looked like they'd been melted. Could this be the results of three rubber plugs leaking the oil out of the transmission? I had originally figured it was a shock loading thing from a 4k RPM clutch dump or something, but when I looked closely the deformation definitely looks like the metal was in a softened state rather than clean solid break. There are also some blued spots indicating that it had gotten hotter than a transmission should...

Click to expand...

I've gone one shaft here where th gear is burned dowm beblw the minor diameter of the teeth yet the "tips" of the teath out of the contact path
wiht the mating spur on the clutster ger are still intact.

I lookes literally like the cluster gear was a tire spinning in the
wet "mud" of the input gear...

Oddly both the input bearing and "pocket" bearing are intact.
Burned as black as a lump of coal but other than their obvious color
and ruined heat treating, mechanically perfect.. "Undistorted" is
probably the best description.

AND the trans still had oil in it, burned to a watery black "liquid smoke" I still don't understand why that particular trans "died"

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