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post #1 of (permalink) Old 02-12-2001, 02:09 AM
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4.2 -> 4.0 swap questions

OK, my 4.2L is getting tired in my 89 wrangler. I am thinking about swapping in a 4.0L, maybe this summer if I can scrape up enough cash. I have a few questions.

1) Do I need an adapter kit to mount the engine to my existing AX15??

2) Did the AX15 change at all after they started using 4.0L engines (I.E. Changes that would help it cope with the newfound HP of the 4.0L)

3) Can I use the same engine mounts that were used by the 4.2L?

4) Where is a good place to get a 4.0L? (Where have others gotten theirs for their conversions)

5) Were there any "really good" years for the 4.0L, or were they all the same?

6) Is a 4.2 -> 4.0 swap actually doable by someone without 20 years of Jeep experience?

Sorry if these questions are strange, but I need to be able to think this through thoroughly before I drop the cash for this. If there are any important things I should know, let me know. Thanks in advance.

89YJ 4.2 5"lft 33"Firehawk ATX
Damn you, Firestone!! Now I have drag slicks!!
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post #2 of (permalink) Old 02-12-2001, 02:29 AM
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Re: 4.2 -> 4.0 swap questions

I'm gonna take a shot at some of your questions.

First off, I do not believe your 89 YJ has an AX15 in it. It's got a BA10(is that right?) Peugout(spelling?) in it. That tranny is nothing but a hugh waste of crap! So you'll need to get an new tranny with the 4.0. I'd suggest doing that anyway even if you stuck with the 4.2.

I don't believe the AX15 changed at all. It may of changed in some small ways with the newer TJs.

I'm not sure about the motor mounts. I don't see why you couldn't just use them. It's pretty much the same engine block. But make sure they don't need to be replaced first.

As for where to find a 4.0, I don't really know. I haven't done the swap. Look in junkyards for wrecked YJs and TJs. I'm no help in this department

Really good years? I don't know about that. I'd say grab one from a newer TJ. New is always good IMOP. I've also heard somewhere that the newer 4.0s in the TJs have a few extra HP. Not sure though.

Is it do-able without 20 years experience, I don't see why not. How much mechanic skill do you have? Any friends that'll help? Also, how ambitious are you?

Another thought, have you considered building up your 4.2? Maybe slap a 4.0 head on it and put MPI on it later or at the same time. Just a thought. When my engine blows or gets too tired to work anymore, I'm going to do this. Of course I already have the MPI sitting on my 4.0 in my YJ Good luck!

"James and the Giant Jeep"
1993 Wrangler 4.0/5spd, Herculined Interior, D44s Soon
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post #3 of (permalink) Old 02-12-2001, 03:50 AM
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Re: 4.2 -> 4.0 swap questions

It is possible to have an AX15 in the '89 model year YJ's. But very few got out of the factory with this tranny. You need to verify which trans you have. If the trans is split lenghtwise down the middle then you have the dreaded BA-10. If it is split cross ways then you have an AX15, a tranny extremely hard to find in that model year.

Your bellhousing is different from the 4.0 to a 4.2 engine. The 4.0 has a fancy smanchy flywheel sensor on the top of the bellhousing. You can take the 4.2 bellhousing and get the top machined out in the right place allowing clearance for the sensor. But I have no idea where that machineing is to be done.

I like the idea of doing the engine buildup of a 4.2 as well. A bored, bluprinted, balanced, 4.9 headed, stroked and MPI'ed 4.2(actually 4.7 after stroking) is a strong engine. The really cool thing is how low the torque peaks in the 4.2, much lower than the 4.0. All of those mods on a 4.2 will make it scream, almost as good a factory 302 V8(some will tell you even better).


<font color=green>----------------</font color=green>
<font color=green>[]llll[] '90 YJ</font color=green>
<font color=green>"There are no shortcuts to any place worth going."-Anon.</font color=green>
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post #4 of (permalink) Old 02-14-2001, 02:45 AM
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Re: 4.2 -> 4.0 swap questions

I have an AX-15, I know this because I asked the guy who rebuilt it. I had it rebuilt by a place that does nothing but tranny's, so I i'm sure it is an AX-15. As far as building up my 258, that is a no-go. If I'm gonna spend that kinda money, I would rather just get a 4.0. If my pistons dont seal good anymore, or my crank bearings are going or something like that, then that just adds to the work I would already need to do on the 4.2

89YJ 4.2 5"lft 33"Firehawk ATX
Damn you, Firestone!! Now I have drag slicks!!
post #5 of (permalink) Old 02-14-2001, 07:46 AM
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Re: 4.2 -> 4.0 swap questions

If you look at the top of your bellhousing on the Driverside, you may see a small square hole. This is common, since the AX-15 was also out in the XJ's with the 4.0L of the same model year.
How does this help you?
assuming you have this hole, you can use a 4.0L injection set up from a 87-90 Jeep Cherokee(91-present won't work). You won't need the 'spensive A-A bellhousing or the Hesco Crank sensor. both of which cost almost $300. This is not the HO set-up,(190hp), but it is basically the same (177hp) and is still worlds better than your stock 258 engine. I've done a few 4.0L swaps before and I can help walk you through the wiring, plumbing, and finding parts.


post #6 of (permalink) Old 02-14-2001, 01:44 PM
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Re: 4.2 -> 4.0 swap questions

Your questions are not strange but all you need to do is look around a little bit to answer them for your self. This swap has been done hundreds of times and discussed more than that. Just 20 or so posts back was the same subject. Use the 'Search' facility for the value '4.0' or '4.0 swap' and you should find more information than you will want.

To answer your questions specifically,
1, You do not need an adapter but you will need the crank position sensor from Hesco.
2, Do not know the answer to this.
3, Yes, but you will have to move the mounts from the old 258 block to the 4.0 block. One hole on the drivers side has to be enlarged to 12mm.
4. Salvage yards or find someone who is upgrading to a V8. There are many who really diss the I6 and they are suckers for selling it cheap. You need to buy it complete including starter, alt, computer, PS pump, fan blade, etc. If you buy one from an XJ, you will have to install a YJ water pump and fan blade.
5, I believe you should stick to the 91 HO and up. Some believe their are even improvements after that but I do not know what they would be. Go for low mileage engines, they are out there. I think it is too much work for the older style FI.
6. I do not think it has to do with Jeep experience as much as overall mechanical skils, tools, workplace, friends, self confidence, etc. It is not a one week end event for most of us. Some have done it in a week end. I used it as an opportunity to basically rebuild/replace many things while I was at it. I took about 4 months but went clear down to the frame. Working on my Jeep is much of the reward of owning it. I wouldn't have it any other way. But my Jeep is not a daily driver, it is all hobby.

Later, ken.

83 CJ8, 4.0 HO/AW4, 2.5 Superlift, 1" Daystar body, 33x12.50 GY MTs, 4.10, Lockers front & rear, on-board air, Durabak cab and bed, Trailquest tank, lots of small stuff.
post #7 of (permalink) Old 02-14-2001, 02:21 PM
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Re: 4.2 -> 4.0 swap questions

I have done this swap and may be able to help.
1. No adapter is needed to bolt the 4.0 to the 4.2 bellhousing. It is true that the 4.0's ignition is triggered by a crankshaft position sensor which is mounted on the flywheel. The sensor is in the bellhousing and yours may or may not have the provision for it. I doubt it as the '89 MPI used in Cherokees was different than the current MPI. Hesco makes a harmonic balancer which you can purchase to take over the function of the flywheel mounted CPS. Machining the bellhousing can be avoided with this balancer.

2. Don't know for sure. Most likely and AX-15 is an AX-15.

3. Pull the old mounts from your 4.2 and they will bolt up to the 4.0. Like was mentioned earlier, one bolt is a 12mm. All the others from the 4.2 will work.

4. Get a 4.0 wherever you can find one. My friend that I helped scoured the newspaper classified for months before he found one. His came from a ZJ. Get everything that you can including the wiring harness, computer, and accesory pulleys.

5. The 4.0 to look for is any from '91-'95. Prior to '91 is a different injection system and isn't as powerful. After '95 the emmissions equipment became more complicated (OBD II) and isn't as easy to swap if you have questions while doing it (less people to ask with experience).

6. Jeep experience isn't necessarily what you need. Being comfortable with wiring and troubleshooting is more important along with good wrenchin' skills.

'75 CJ5,258 w/Howell EFI,T18a,4.27's,33's,On-board air,Warn 8274
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