Written / Photographs by: Tony Carricaburu


In this article I am going to explain the basic fundamentals of a not so hard shadetree mechanic Head conversion on a 4.2L 258ci. Block. As I'm sure any Jeeper knows these are the engines that came in the CJ's and early YJ's mainly. After several months of research I came upon some discoveries about putting 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 different 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 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 Mileage. 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 a remanufactured head, completely rebuilt and basically new. From here there are a couple Retro-fit modifications needed to make this work right. Don't worry they don't require any difficult procedures or special tools. Below is a picture of my new 4.0L head upon arrival via UPS.



- 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 Ive 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) Anyway 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. The pictures below show the Head removal process.

(Click on images for large view)



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. I guess it make since the engine never burned oil, just didn't run good, that was probably do to the Carter carb and poor adjustments. Well anyways I went ahead and cleaned whatever I could while the head was off. I scrapped off the entire old head gasket and cleaned the new mounting surface. This concluded the 4.2L Block preperation.


The first step is to prepare the Head for installation, after you've obtained all the parts from the parts list you will need to perform the following task. In order for the 4.0L head to work on the 4.2L I had to fill in a couple water jackets in the 4.0L Head. They are the small triangular water jackets on the same side as the Intake. These must be filled because on the 4.2L block these water jackets don't exist. I made a couple calls and came to the conclusion that the best way to fill these holes in is to use a high temp. Epoxy Putty. I bought two tubes of QuikSteel. This is a 500 degree max temp. Super strength epoxy putty found at most auto parts stores.

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 much easier. Don't worry the foam peanut will dissolve. After filling in the holes, make sure there isn't any burs or pieces of 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. Below are some pictures of the holes filled in.

(Click on images for large view)






This Diagram shows exactly which holes need to be filled on the new 4.0L head in order for the conversion to work. NOTE: Its very important that you plug only the holes that are shown in the Diagram. If you accidentally plug additional holes in the head it could cause your engine to overheat or cause other problems. See Diagram below for details.


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 4.2L Head.

(Click on images for large view)



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. New set of Cylinder Head Bolts are $17.00 from Mopar Performance. (Part #: 4529204) I 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 for this installation, these are basic head bolt tightening spec.'s, and can be obtained in your Jeep repair manual as well. Click on Diagram 1.1 for printable size.

(Click on images for large view)

Diagram 1.1 Torque Sequence


Next thing to do is install all the rods and rockers. When doing so, you will want to set the engine at TDC (Top Dead Center) that means the number one piston must be at the top of the cylinder. To obtain this you must move the harmonic damper until the mark is directly pointed at the 0 position on the timing settings. After the motor is verified to be at TDC go ahead and place the rods into position as you took them out. After placing all the rods back into position place, mount the rockers back on as you took them off. When bolting the rockers down it is very important that you don't under or over tighten them. The dealership recommended torque spec.'s say to torque them to 19 lbs.

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 spec.'s. Below are some pictures of my Jeep with its new 4.0L head and valve cover mounted in position.

(Click on images for large view)




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. Be aware that the intake manifold and exhaust use a lot 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 created 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



After completing the installation of the new Head its recommended that you use different spark plugs. I read a very informative similar article that Tim Weaver produced and he states that for this conversion you will need to use Part Number: RC-9YC4 spark Plugs. Gap at .035". For my Jeep I bought Bosch Platinum platinum +4 plugs. There a little more expensive and I'm not sure if they will even make a difference but I figured I would go ahead and give them a try. Torque Spark Plugs to 27 ft/lbs.


From here go ahead and put your carburetor back onto the intake manifold including all the vacuum lines, fuel lines, etc. as you took it off. Hopefully you marked all the hoses and lines so that they can easily put back in there proper locations. If not the vacuum diagrams can be obtained in the Chilton or Haynes repair manual. Everything should bolt right back into position except the rear power steering pump-mounting bracket. This needed a little modification, which is pretty self-explanatory upon installation.




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 pretty 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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