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Old 09-04-2015, 12:25 PM
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4.0 in a 1990 yj ?

I have search and search on google and jeep forums and keep getting mixed answers..
I have a 4.2 with tf999 3 speed trans(I believe). I also have a '93 4.0 with 5 speed (whole jeep).
Can I join the 4.0 to my 3speed trans? I keep seeing things about the pilot bearing ,, and flex plates.. are these parts on the 4.2 able to be switched to the 4.0?
I know I'll need all the wiring and computers as well as the gauges and fuel tank, this is why I bought the whole jeep(frame toast).
Or would it be easier to swap the whole thing motor-trans-transcase,,, ?
I know its a lot of work either way , but body, frame, axles are better on the 1990, and I like the auto trans when of road. please help thnx.

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Old 09-04-2015, 02:41 PM   #2
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The flex plate doesn't bolt to the crank on your 4.0L so without a flex plate for a 4.0L auto you won't be able to make the swap. The TF999 will bolt to the back of the 4.0L motor.

I would think that this will be the easier route than swapping the entire drivetrain, since you would have to worry about different clutch and brake pedals inside plus the clutch master and brake master are different between 90 and 93, and the driveshafts are a different length between a standard and an auto. Plus switching to a 5spd requires re-wiring your neutral safety switch as well.

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Old 09-04-2015, 07:15 PM
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o.. I will need a flex plate for a 4.0 auto.. and then should work?.. what about the pilot bearing? or does the flex plate take care of that issue too?.. sorry ,, for all the questions
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Old 09-04-2015, 07:16 PM
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what about the flex plate off the 4.2?
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Old 09-04-2015, 07:50 PM
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disregard the last 2 post,, sry
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Old 09-15-2015, 05:04 PM   #6
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I have a 89 with 4.0 out of a 1995. but I also upgraded the transmission to the AX-15 and put a dana 300 behind it. it is well worth the time and effort...

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