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4.0 Head Swap 

After researching and gathering parts for the 4.0 head swap I finally got started the other day. This modification is supposed to add 40 hp to the 4.2 engine. I've never seen a dyno result but most who have done it claim it makes a big difference.  Being that my 4.2 is in a full size jeep cherokee (5000+ lbs.) I'll take any gain in hp I can get. 

The two articles below cover most if not all of the steps involved in this process and are very much recommended reading.  I'm simply going to cover my experience doing this swap on a 1983 full size jeep cherokee. It seems that everyone I've talked to has had a slightly different experience doing this swap due to different year blocks, manifolds, etc. so buy your parts, plan carefully and expect the unexpected. 

http://monsterslayer.com/jeep/40Head/40Head.htm

http://www.off-road.com/jeep/new/40conv.html 

Since I have to pass local emissions testing I planned to keep the stock intake w/ egr valve as well as the tubes for the pulse air system. I've already passed the test with the weber 32/36 and the computer bypassed so I'll be reusing it and possibly upgrading to the weber 38 in the near future.   

The first step was finding a used head which I was able to locate on ebay. Next I had the water jackets welded and a valve job.  If I had to do it over again I would have test fitted the head before having it welded. All of my research on the web led me to believe that the 4.0 head would overlap the 1983 4.2 block and need to have its water jackets welded. This turned out to be not true at all-it was a perfect fit.  Definitely test fit your 4.0 head first. It could save you some time and money. 

I decided to purchase new rockers and pushrods which turned out to be a good move. The rockers had really been torqued on and I wound up bending the bridges when removing them. As for the pushrods 77-80 4.2 engines use 9.622 length pushrods while 81-88 use 9.700. The 4.0 head uses 9.640 pushrods. I figured my stock 4.2 pushrods might be a little long especially since the head had been milled at the machine shop.  I also purchased a new aftermarket 4.0 header for years 91-99. It comes with expansion joints which are supposed to keep it from cracking. For $100 its worth a shot. 

Since I've never done anything this involved on an engine I wasn't really sure what to expect but I planned to take my time and do it right. Taking apart the engine took about 2-3  hours and was by far the easiest part. Once I had it apart I could see that the cylinders were in good shape. The old head had quite a bit of carbon build up so it will be nice to start fresh and clean. The old exhaust pipe didn't want to budge so I had to crawl under the jeep and cut it off (not fun).

I decided to use a felpro head gasket and alignment washers purchased from Clifford Performance to get the 7/16 bolts to line up properly. Since the gasket looked like it was going to move when I put the head on I would up taking two of the old head bolts and cutting the heads off them. I used them as studs to keep the gasket in place and get the head aligned properly. Once it was on I just removed them and put the new bolts in the head.  

In order to keep the egr valve functioning I had to have the exhaust shop weld the old tube that goes from the intake into the exhaust manifold. I measured it up and had them do this before I installed the manifolds. To make the stock 258 intake fit the 4.0 head I had to notch below where the locating pins would go.
 I also had to grind a very small amount off the webbing on the intake in two spots to get it to fit. The stock intake matches up pretty well with the 4.0 head once you do this. The ports are a little bigger on the head than the intake so its not going to flow quite as well as it could but it should work fine. I know clifford performance puts out an intake just for this swap but at $289 plus another $50 for an adapter its a bit too pricey not to mention the lack of egr port on their intake. To mount the intake and exhaust manifold required me to make several washers out of steel flat. I found that only 3 of the stock cupped washers were large enough to bridge the gap between exhaust and intake. This was by far the most time consuming part of the swap as each washer needed a different shape to fit properly. I'm sure there are washers out there that can be special ordered but I found that trying to locate them was taking longer than just getting out in my garage and grinding.  

The valve cover was another issue as well. The first one I had that came with the 95 head had two small holes that allow an elbow to twist in and lock. On these models the pcv or ccv valve is located in the intake. I found another cover (year unknown) that has larger holes with rubber grommets. I got the stock 258 pcv valve to fit into these grommets by inserting the smaller 258 grommets into the larger ones that came with the valve cover. I believe later year 258's should have the correct fittings for the pcv valve but I got it to work anyway. The 95 valve cover would have been a bit more challenging. 

As for the power steering pump it has several locations to bolt onto the engine. The fact that the 4.0 head sits a little higher makes the holes on the bracket no longer line up with the holes on the intake. It still bolts to the engine in four other locations so I may decide to just leave it be. The power steering bracket cleared the stock 4.0 header so no grinding was necessary.

Onto the exhaust. Since the 4.0 head flows much better than the stock 4.2 it stands to reason that you would want to upgrade to a bigger better flowing exhaust system as well. The local exhaust shop will be bending me a new 2 1/4 exhaust with new cat and high flow muffler. The egr tubes and pulse air tubes will be welded back in to keep it emissions friendly.  

7/02/04 Just got it back from the exhaust shop today.  I wound up going with a vortex muffler which was recommended to me by the shop. It has a nice mellow tone- definitely not too loud and doesn't have that high speed drone I hear people mention in regards to flowmasters. Overall I'm very pleased with the outcome of the head swap.  The engine accelerates quicker and feels much smoother when doing so. I imagine the performance would be even better with the weber 38 carb. The 32/36 is ok for now but I'll probably upgrade in the near future.  Fuel injection would be outstanding on this setup but that will have to wait for another day.

7/21/04 Well it didn't take too long for me to start wanting more power from this setup so I bought a new weber 38 and installed it yesterday. The difference between the 2 carbs with the 4.0 head is incredible! I'm now passing cars with ease, accelerating up hills and just loving to drive my jeep. I never ran a fuel pressure regulator before but I decided to give it a try since my gas mileage was now suffering and the carb was running a bit rich. Since then I've been able to lean out the mixture and the gas mileage has improved. I've heard of some people keeping the carter, putting a mc2100 or smaller weber on after doing the head swap but now I know why they were disappointed with the results. If you're going to do this swap definitely opt for a larger carb that flow at least 400 cfm.