Can I swap in a Chrysler 4.0?
So you want to swap your Eagle's engine!
The 4.0L Chrysler and the 258 share bell housing bolt patterns. The engine mounts are also identical. The Eagles require an axle bracket on the drivers side rear of the engine block. The 258's have bosses on the block that allow for this bracket to be attached at this location. When Chrysler bought AMC they kept intack so much of the original tooling for these engines that those bosses are still present on the 4.0 long into its production run (if not till the bitter end). Some 4.0's may not be tapped for threads, but the bosses are still present. I personally have a 93 HO 4.0 that not only has these bosses but they are also factory tapped for threads.
When swapping in a 4.0 there are a few things to consider. Because of the interchangeability of these engines and the vast amount of years they were produced there are near endless possibilities for mixing and matching parts depending on your application and need. You must plan ahead and decided upon what parts and configurations you wish to use. Things to be considered are belt set ups, carburation or fuel injection, exhaust, standard or manual transmission, flexplates and flywheels, pilot bushings, wiring harness, starters, water pumps, and weather or not you want to build a 4.6L stroker.
CARBURATION vs FUEL INJECTION:
CARB IT!!! - Your choice of fueling your 4.0's fire is base upon practicality, expense, and your own skill and ingenuity. You can re-use your factory carburated intake on the 4.0 head as with a 4.0 head swap. You may need to ding the two center exhuast manifold pipes in with a ball-peen hammer ever so slightly to clear your intake (may also need to do this with the front pipe to clear the powersteering bracket). But I assure you, the amount of pipe you hammer in to gain clearance is tiny, and will not interfere with the flow of the exhuast in the slightest. The carbed intake will work on the 4.0 head. Also, you may need to mix and match gaskets. When I did a non-HO 4.0 head swap on a 258 I used the 4.0 exhuast manifold, and the 258 intake. The intake/exhuast gasket set I purchased for the 4.0 wouldn't work for me as is. The intake port cut outs on the gasket were too large for my carbed intake to cover. I had to trace, and cut out the intake portion of the gasket and use the exhuast portion left over for my exhuast manifold in conjunction with a 258 intake gasket on my carbed intake manifold. I have recently been in contact with a Eagle nest member that is involved with a 4.0 HO head swap on his Eagle and I was informed this was not necessary on his application. He said the intake covered the ports just fine. Maybe this is due to a difference in gaskets, or ports I due not know. But it is clear this step may not be crucial for all assemblies. In addition the space between the intake and exhaust manifold mounting tabs may be too large for the factory washers to cover the gap. You may need to buy or fabricate some larger washers to bridge this gap. When I did my 4.0 head swap I encountered this problem twice, and just inserted a large washer under the factory retainer washer and it worked fine. Clifford also makes an assortment of carbed and fuel injected maniflods for the 258 and 4.0 as well.
INJECT THE MUTHA!!! - If you wish to inject your 258 or 4.0 you can buy any number of aftermarket kits produced by Hesco, Clifford, or Mopar (possibly others as well). Or, you can shade tree mechanic it buy using the stock 4.0 injection set. Earlier 4.0 use a Renix system with a throttle body, and the High Output 4.0 uses a redesigned mulitport injection set up. If you choose to inject and you use the stock intake weather it be Renix or HO (both use different harnesses and computers) you will need to know that the automatic transmissions that came on 4.0 vehicles were computer controlled. And unless you swap in the entire tranny and harness set from that vehicle into your Eagle these computers and wire sets will not work for you, at all. You must obtian a wire harness set and computer from a manual 4.0 Cherokee or Wrangler. Now you must consider that you will need to incorperate this computer/wire harness set into your Eagle's under hood wiring. Also with injection you will need to work around a problem with mounting a Crank Position Sensor (also called a crank trigger). On all the 4.0's there is a CPS on the bell housing of both the automatic and manual transmissions. This reads the position of the crank by detecting flywheel/flexplate position. As that implies the other half of this sensor is engrained into the flexplate/flywheel. This is a real problem if you are mating up to your factory Eagle transmission as the Eagles do not have the CPS mounts on the bellhousing. Not only that, but the other half of the CPS system ingrained into the flywheel/flexplates on the 4.0's does not exist on the 258's counter parts. However Advance Adapters and Hesco both come to the rescue on this one as well. Wich ever 4.0 incarnation you have chosen to install in your Eagle, there is a CPS relocation kit available from both manufactures. These kits relocate the CPS to the harmonic balancer on the front of the engine. Adding better service-ability to the CPS and solving the need to replace the transmission as well just to ad the fuel injection you so desperately wish to have. Advance Adapters also makes an adapter bell housing with the mounts for the CPS. This bell housing will bolt to a T-5 (I'm not sure of what other manuals if any, this adapter bell will work on). With this adapter you can use the stock 4.0 CPS and flywheel. You will however need to use a clutch disc for an Eagle as the spline is different from the the manuals use on the 4.0. And this kit will not work for you automatic tranny guys, sorry. All of these kits run about $300 weather it be the CPS relocation kits, or the bell housing from Advance Adapters.
EXHUAST - You must use a manifold from a 4.0 as the ports are different. A 258 manifold will not work as far as I know. Now the 4.0 manifold flange will interfere with the front axle differential on the Eagle, and you will have to alter the manifold at the flange to make it work. An Eagle nest member recently created a unique method of using a piece of U-channel and some spacers to make a new axle bracket that clears the manifold with no alteration to the manifold at all. I will post in more detail on this later.
BELTS - The 258 came with both a serpentine belt set up and a V-belt set. Brackets and accessories are interchangeable with little or no modifications depending on what you are using. You can use the Eagles V-belt set even on a fuel injected 4.0. However you may have to cut the top off the power steering bracket as with the 4.0 head swap. This is necessary even if you use your original carburated intake, this is due to the difference in port height on the 4.0 head verses the 4.2. Also the 4.0 in most applications uses an offset fan that will not be Eagle friendly. You can solve this issue by finding a 4.0 center fan equiped wrangler for parts, or equip your Eagle with an electric fan.
STARTERS - The starters between these engines are fully interchangable. One thing to rememeber is you must match flywheels/flexplates to starters. If you use a 4.0L starter you must use a 4.0L flywheel/flexplate and visa versa. This is due to a difference in the ring gearing.
FLYWHEELS/FLEXPLATES - Flywheels and flexplates are also fully interchangeable between these engines. The mounting holes are clocked in such a way as they will only bolt onto the crank shaft one way. So if your holes are not lining up, your not clocking them to match correctly, or you have the flexplate/flywheel turned around and are trying to mount it backwards (tranny side facing engine). Flywheel/flexplates must be considered when choosing a transmission. If you are planning on using the stock Eagle tranny with your 4.0 swap you must also remember to use its flywheel/flexplate counterpart. Remember, match starter to flywheel/flexplate. If you plan to make use of your stock transmission and flywheel/flexplate during your 4.0 swap, also use the 258 starter.
TRANSMISSION - If you choose to use the 4.0 transmission please remember there was never a 4.0 installed from the factory in an Eagle. The transmissions equipped on 4.0's are not designed for these cars and will take fabrication to install. You will need to fabricate cross-members and drive shafts. If you choose to stay with a stock Eagle transmission you will have no difficulty attaching it to the 4.0. However, this brings us to Pilot bushings for standards, and CPS switches for fuel injected applications (see: INJECT THE MUTHA!!! above).
PILOT BUSHING - If you are placing your 4.0L in front of a stock Eagle standard transmission you must note that the input shaft on these transmissions were never destined to see the crank shaft of a 4.0. If you are using a 4.0 that was previously equipped with an automatic transmission you will see that the machined pilot bushing hole is way to big to fit the 258 pilot bushing. If you are using a 4.0 that was used in a standard transmission application you will see a spacer bushing in place with a pilot bearing that will not fit the input shaft on your transmission. This spacer bushing will also not accept the pilot bearing for the 258. Advance Adapters produce a one piece adapter bushing for Jeeps that will adapt the T-5 and other manual transmission input shafts to the 4.0 crank. I orderd this part recently and it was approximately $30 with shipping. Advance Adapters have no information about Eagles so don't ask. Tell them the model of your transmission, the year of your Eagle, and tell them it is for a JEEP Wrangler. The part they send you will work.
WATER PUMPS - Because of the nature of serpentine belt, these engines have a water pump that spins in the opposite direction as the V-belt engines. This pump still pushes water in the same direction as the V-belt pumps (if you try and pump water backwards through an engine you can destroy the engine). This backwards rotation is accomplished by reversing the orientation of the impeller fins inside the pump. The 4.0's have a much higher flowing pump as these engines need better cooling capabilities. If you try and use your 4.2L V-belt pump on the 4.0 you may cause an overheating condition, especially if you build the 4.0 as a stroker engine. To solve this Hesco (possibly others) make a high flow 4.2L V-belt type water pump. If you decide to keep your V-belts I highly recommend getting a higher flow performance water pump.
Oil Pan - You will need to use the 258 oil pan from an Eagle. This pan was made to fit the front axle of the Eagle and is different then the other 258 and 4.0 pans available.
This article written by 80SpiritGT If I forgot anything, if I need to be corrected, or if you have any questions please contact me.
Edit: Jan 19, 2010 - added oil pan info and corrected multiple spelling errors. Special thanks to StayFrosty for bringing the oil pan issue to my attention. When I originally wrote this article I completely overlooked the oil pan as a potential incompatibility.