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4.0L Head Swap onto 4.2L block - TECH ARTICLE
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August 22, 2017, 12:41:41 PM

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4.0L Head Swap
Out with the old 4.2L head and in the new 4.0L head

Product Overview:
In this Tech article I'm going to explain the basic fundamentals of a shade tree mechanic head conversion on a 4.2L 258ci. block. As I'm sure any Jeeper realizes these are the factory engines that came in some CJ's and early YJ's mainly. After several months of research I came upon some discoveries about installing a 4.0L head on a 4.2L block. Apparently the block on a 4.2L and 4.0L are very similar except the 4.2L has a longer stroke and a few minor design differences. After I sent several e-mails trying to find out more information about this conversion, I was informed that this conversion is possible and can provide a 40hp gain. That's right 40hp! I was a little surprised myself. Not only does this conversion add more hp, but I have read that people have experienced a substantial increase in Mpg. Approx. 17-18mpg.

I began searching for a 4.0L head and found just what I was looking for at Koller Dodge (check there website for more details). They provided me with a re-manufactured 4.0L head, completely rebuilt and basically new. From here their are a couple Retro-fit modifications needed to make this work correctly and efficiently. Don't worry they don't require any difficult procedures or special tools.   

- 4.0L Head Provided by Koller Dodge
- 4.0L Head Gasket
- 4.0L Exhaust Manifold or Header
- Exhaust Manifold Gasket
- 4.0L Valve Cover
- 4.0L Valve Cover Gasket
- Tubes of QuikSteel Epoxy Putty
- Sand Paper (fine)


This part was almost a dream come true. I have wanted to remove the old head and intake system for as long as I've owned the Jeep. I started by removing the air cleaner, valve cover, power steering pump, radiator hoses, etc. NOTE: I didn't remove the Carter carb. and intake because I had other plans involved with the intake system. (Check next months Issue for more details) Anyways after removing the Rocker-retaining bolts and removing all the rocker arms and rods, I then went ahead and removed the Head Bolts. Now the old 4.2L head is ready for removal.    


After The 4.2L Head, intake and Carb. was removed I was really surprised to see how clean the cylinder walls and pistons where. This engine currently has 98,000 miles on it and appears to be in really good condition. I was afraid I would discover some problems upon removing the head, like excessive carbon build-up or scratched cylinder walls. Well anyways I went ahead and cleaned what ever I could while the head was off. I scrapped off the old head gasket using a combination of a flat head screw driver and putty knife. This concluded the 4.2L Block preparation.


Since the holes in the head expand inside it makes it much easier if you use foam-packing peanuts to fill in the holes. I used on packing peanut for each hole. This makes applying the epoxy putty much easier. Don't worry the foam peanuts will dissolve. After filling in the holes, make sure there isn't any rough edges or pieces of dried epoxy sticking up above the heads surface. I used some fine sandpaper so ensure the head would match perfectly to the head gasket. It's very important that the head is completely smooth to ensure no head leaks, compression loss, or blown head gaskets, etc. 



After preparation of the head is complete your ready for the installation of the new 4.0L head. I suggest having a friend help place the new head on top of the 4.2L block. The head alone weighs 77lbs. and when you trying to line up the head on the block while also trying to align the head gasket all at the same time it would be very difficult as a one-man job. I started by placing the new Head Gasket on the 4.2L block, Then with the help of my friend Nate; we lowered the Head down onto the block. It was surprising how well the head lines up, It appeared to fit as perfect as the factory 4.2L Head.


After placing the head onto the 4.2L block its time to mount the head down. It's recommended that you use new Head-Bolts because of accuracy but the old head bolts will still work. For my application I used the old head bolts. A new set of Cylinder Head Bolts are about $17.00 from Mopar Performance. (Part #: 4529204) I personally was unable to find new head bolts in time for the installation. It's very important that you torque the head bolts down in order and to the right torque spec. I created a little Diagram below to view. A similar diagram can be obtained in a Haynes or Chilton's Repair manual. The first round you want to torque the Head bolts down to 22lbs. of pressure. (I recommend using a quality torque wrench for better accuracy). On the second round tightening the head bolts torque them to 45 lbs. of pressure. The last round tighten all the head bolts to 110 lbs. of pressure except head bolt #11, you must torque this bolt to 100 lbs. of pressure. NOTE: Before installing the Number# 11 Head Bolt put some Loctite 592 sealant on the bolt. All these torque ratios aren't custom or created specifically for this installation, these are basic head bolt tightening spec.'s, and can be obtained in your Jeep repair manual as well.


Now that the head is in position and mounted you can go ahead and place the valve cover on. Be sure to use a new cork valve cover gasket. I'm not sure of the torque spec.'s for the valve cover but its specified in your Haynes repair manual. Check you Haynes or Chilton repair manual for exact specs.


Now that the 4.0L head is mounted and ready your going to need a 4.0L header or exhaust manifold. For my application I had a friend that had a spare 4.0L header form an 88' Jeep Cherokee lying around. Since my project is on a budget I went ahead and put this header on for now. I highly recommend using a Borla header for both hp performance and lifetime warranty. Be aware that the intake manifold and exhaust use a couple of the same mounting bolts. There is no need to customize anything, except you'll need to mount the intake manifold and exhaust at the same time. There is also a specific torque sequence and lb. amount for these bolts as well. I took the liberty of creating another chart to specify these torque sequences. Torque the Intake/Exhaust Manifold to 23 ft/lbs. (Click on Diagram 1.2 for printable size)

Diagram 1.2 Torque Sequence




The overall conclusion of my 4.0L head conversion was definitely two thumbs up. The cost was fairly inexpensive compared to the amount of power gained. Although I've done minor engine repairs and modifications before, I have never done anything even remotely close to this and to my surprise it was fairly easy. The overall preparation, old head removal, and new head installation took about 5 - 6 hours. This may vary depending on the tools used. I can't say exactly my overall results of the power since I'm currently adding additional engine modification. (See next months Issue for more details) I can say that from speaking with Hesco and other Fellow Jeepers on the Internet that have performed the same conversion the overall performance increase is substantial and well worth the time.


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