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Why did my transmission break?

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mini zilo

15+ Year Contributor
783
16
Jul 17, 2006
Chicago, Illinois
I posted this on another board but didnt get any answers so I am copy pasting it to here;

I was driving around all day yesterday, everything was completely fine. The transmission was rebuilt a few months ago, but only has about 1,000 miles on it. The parts that were replaced in the rebuild were:

All synchros
1-2 hub and slider
3-4 hub and slider
new 2nd gear
new 3rd gear
new 4th gear

I run Redline MT-90 with a touch of heavy shock-proof in it. If I had to take my best guess, the car is somewhere around 350 whp.

I was at a stop light, and when it turned green I rode out first and then went WOT to 6750 rpms. I shifted into second no problems (this was FULLY lifting off the throttle, and fully pushing the clutch to the floor. I took 2nd gear up to 6750 rpms, and then fully pressed the clutch, and shifted intro 3rd. As soon as I moved the shifter, I felt a grinding in shifter, and then it stopped and just fell into the 3rd gear position, but 3rd and 4th were gone. I drove home using 1st, 2nd, and 5th.

Now, I was pretty certain that I had broken the synchro slider. Well, I was right.

I took the transmission out today, and my 3rd-4th slider is broken into peices, the shift fork also broke, the hub has serious wear, the center diff ring gear is damaged, and that's about all I have noticed so far.

Now when the transmission was rebuilt, I did not replace the 3-4 fork. Is this a possibility of why this happened?

I don't understand how a ~350 hp car can break a rebuilt trans with only 1000 miles on it, while fully lifting to shift, and fully depressing the clutch, and only shifting at 6700 rpms!!!!

I wasn't even ripping the shifter, I was just pushing it! This is ridiculous.

What reasons could cause the transmission to break this easily? The only thing I can think of is it was rebuilt by a local transmission shop, and they might not have done the best job. But I can't think of any other reason. Why would this happen? Thanks!

Looks like it's time to call shep. Which is both :( and :D
 
The 3/4 hub and slider on the 2G is pretty weak in comparison to the late 1991-early 92 (or Evo 3) 3/4 hub and slider. Though the hub and slider is larger in diameter, and incorporates larger synchros than the earlier design, it also incorporates a much thinner inner-diameter slider. These do have a tendency to shatter. It just happens. Heck, it can happen at the stock power levels with abuse, or at moderate power levels through spirited driving.

Though the shift fork for the 2G is a "better" design than the late 91-early 92 shift fork, it also still can break.

What we do to make the 3/4 hub and slider stronger, is to use a late 1991-early 1992 3rd gear and 4th gear assembly, along with the corresponding hub and slider, synchros, keys and springs. You will also need the corresponding shift rail and rail end, along with a new 3/4 shift fork. Though the shift fork is "weaker", it can be modified by milling a hole on the top of the shift fork to accept an additional roll pin to further reinforce the 3/4 shift fork on both sides so it reduces the potential of shattering the lower pin locator or snapping an ear due to shear force, or due to the wrong shimming of the input shaft.

As well, you can go one step further and use an Evo 3 3rd/4th gear assembly, along with the corresponding double synchros. Be sure to also use the Evo 3 3rd gear subgear. You will still need the late 91-early 92 3/4 shift rail and rail end, along with the 3/4 hub and slider, keys and springs.

The late 91-early 92 3/4 hub and slider are a much stronger design, and are identical to the Evo 3 hub and slider; along with the shift rail. The only thing that the Evo 3 comes with factory are steel shift forks which have plastic pads that meet up with the hub and slider. Unfortunately, though these forks are stronger, the plastic pads wear quickly, and then cause sloppyness or excessive wear to the slider; which will further cause damage down the long road. This is why most builders stick with the factory aluminum ones, and modify them.



So you said that your 3/4 slider shattered -- is there any discoloration along the shift fork riding surface? Was it overheated? Galling? As for the 3/4 hub itself, was there any galling of the sliding surface that mates with the slider?

As well, you mentioned your 3/4 shift fork snapped. Where specifically did it fail? Photo?

You also said you were using HEAVY shockproof with MT-90. I would not mix two very different viscosity gear oils with each other. The heavy is definitely made to be used in our transfer case and rear end. As for the transmission, I would suggest running Redline Lightweight Shockproof Gear Oil (blue) completely. It has been proven (in my car at least) to handle the shockload of our drivetrain decently, along with provide excellent adhesion to the gears, along with getting enough oiling to the passages through the input shaft, intermediate shaft, etc. to properly oil the hub and sliders along with the needle bearings on the gearsets.

Unfortunately, if your center differential case drive gear was damaged, you will need to purchase a new one -- they aren't cheap by any means. As well, I would strongly suggest inspecting, and possibly replacing your intermediate shaft as they are mating surfaces and can easily unevenly wear if only one piece is replaced.

I am unsure why you didn't replace the 3/4 shift fork along with the 1/2 shift fork, as it would have only costed you $40.00 for both of them during the rebuild. They are cheap insurance.

I guess you will be looking for some parts. FYI -- you will have $800-1000 in just a center diff alone (4-spider NEW), along with another $225 for an intermediate shaft, and another $350-600 for the 3/4 gearset, hub and slider, shift fork and shift rail. It will likely cost you around $2100 at the least.

On a final note...I would reccomend NOT using NLTS (no lift to shift), as you will continue to chew up transmissions -- the DSM transmission just cannot take it and live for long periods of time. If you want to do this, you will need to purchase a dogbox and a twin disk clutch.
 
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