2.5 Liter to 4.2 liter swap - JeepForum.com
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post #1 of 7 Old 02-01-2012, 10:21 AM Thread Starter
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1985 CJ7 
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2.5 Liter to 4.2 liter swap

I bought a 1985 cj7 right when I graduated high school and have owned it for about a year now. It has some outer modifications (diamond plate, lift, lights, bumpers etc.) but the powertrain is factory stock. 2.5 AMC engine mated to the borg t5. dana 30 up front amc 20 rear and the dana 300 transfercase of course. I would like to swap in a 4.2 liter out of a cj7, cj5 whatever. What would I have to change besides the radiator, bell housing, and motor mounts? If anybody has done this swap and knows an approximate cost I would greatly apprieciate the advice! thanks!

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post #2 of 7 Old 02-01-2012, 11:49 AM
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I would find another CJ with a 6 in it and compare the other little things that can stop you like clutch linkage parts, wiring harness, air breather, throttle cable, fuel lines, etc. Used, Ive seen the 258 go from free to about 400 bucks. Another show stopper is going to be the input shaft of your T-5. You have a 14 spline now and need a 10 spline. You can swap the whole transmission or disassemble it and change just the input shaft. I was was searching for a donor 6 cyl Jeep motor and possible having to address the engine side of the harness as well, I would swap in a 91 to 95 4.0 and get the benefit of multi port fuel injection.

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post #3 of 7 Old 02-01-2012, 12:46 PM
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Not sure of cost, but like said wiring would be the biggest headache to me. A couple of things come to mind, starter and distributor. Should not be to hard to add on the fender mounted solenoid and what distributor are you going to use? The stock Jeep Motorcraft you would have to wire in the control module or go with a HEI and you could get around that. I think the alternator gm to gm style should not be much to deal with. The T5 should be 10 spline, I think that the SR4 was the last 4cyl trans that had a 14 spline input, easy to check out and if 14 just use the 4cyl disc.
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post #4 of 7 Old 02-01-2012, 03:50 PM
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Easy people. most everything is the same from the 2.5 to the 4.2. I did this swap about 6 years ago.Outside of new bellhousing, input shaft on the trans, clutch slave and line, mounts and frame buckets and exhaust, it went together fine. Wiring will all work on the 2.5 or 4.2.

jduress - shoot me a pm and I will give you as much info as I can. One word of advice, collect as much as you can of the new 258, like all the accessory brackets.

It's just a Jeep, and if you don't wheel it once in a while, it's not even that.
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post #5 of 7 Old 02-01-2012, 08:34 PM Thread Starter
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The engine I have in mind is complete, it is in a 1970s cj5 that was parked years ago but ran fine when it was parked.
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post #6 of 7 Old 02-02-2012, 01:47 AM
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some good advice here but for a painless swap I would suggest a motor you have seen running and that you will not have to rebuild to fit.

If you get yourself a copy of the parts manual you will see the different part numbers between a 2.5 and a 4.2 for this year. Some of them will make no difference whilst others are completely different.

For instance the bellhousing part numbers are different, does this make a difference?

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post #7 of 7 Old 02-02-2012, 02:40 PM Thread Starter
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The Bell housing that is on the 4.2 that was in the cj5 is a t150 three speed housing
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