Great Tips from tougeagle

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By AMC Eagle Nest member tougeagle

I will be honest in telling you that all of the information I gathered for doing the swap came either from the Nest, Jeepstrokers.com or jeepforum.com.

The best head casting to get for the 4.2L is 7120, this head came in jeeps 1991-1995. 1996-1997 Had the same head but without the coolant temp sensor port, which means finding an alternative way to wire in your coolant gauge. There are a few big differences between the 4.0 head and the 4.2 head, and some pointers that should be considered carefully:

1. The 4.0 head will raise compression slightly thanks to the smaller combustion chambers, however, I cannot provide exact numbers.

2. The 4.0 engine used 1/2" diameter head bolts. Some 4.2's used 7/16" headbolts, which leaves a little bit of wiggle room. I don't recall the exact year AMC made the switch to 1/2" head bolts, but I believe it was 86. If you use 7/16" head bolts, I'd highly recommend buying two head bolt spacers from Hesco, or using copper pipe as bushings, to make sure the head gasket is lined up. I chose not to use the spacers because I didn't find either of them until after I'd replaced my head gasket for the third time, meaning...

3. NEVER re-use your headbolts. I am under the understanding that they are torque-to-yield bolts, which means they can only be used once. Even if this is not quite the case, I wouldn't recommend it, and I had to find out the hard way.

4. From reading up on personal anecdotes about performance changes with the 4.0 head swap, I can tell you I'd HIGHLY recommend installing a 4.0 cam as well, 91-up. The 91-95 cam is a single pattern with slightly higher duration than the later cam. However, the later cam is dual-pattern, with more duration on exhaust, which allows the engine to breathe a little easier because though slightly less air/fuel mix is permitted per cycle, proportionately more exhaust is relieved in the same amount of time. I personally went with the earlier cam, and am loving the results. I recommend this because the only way to remove the lifters is to remove the head first, and from there getting to the cam is not very hard since the lifters can simply be lifted out of the block with an extending pen magnet, for as long as you have a crank damper puller/installer. Make sure to use the lifters, pushrods and rocker arms from the 4.0 engine if you swap the cam.

5. I would not recommend simply slapping the head on after pulling it from a vehicle. You'll experience the full improvement potential the head has to offer if you spend a little extra to have the head cleaned up and machined. It will also rule out any leaks your engine may be experiencing or experience at the time of the swap. New valve stem seals are around 20 bucks. The local machine shop magnafluxed, glass beaded, dipped, replaced seals, machined flat and 3 angle valve grind job for my head was 200 bucks even.

6. If you pulled your head from a high mileage vehicle, replace the valve springs at this point. You won't pay addition cost for labor at the shop, they'll have to remove the springs to replace the seals anyway.

7. You will NOT be able to use the 4.2L exhaust manifold, which means you will have to either use a Renix (87-90) exhaust manifold and custom fab an EGR tube, and/or not live in an area that requires smog testing, as you will have to disable your EGR. You will also have to fabricate a custom differential bracket and upper exhaust piping to work with the 4.0 manifold.

8. You can use your 4.2L intake manifold, though the dowels will not go through the holes, but under them. Also, I would recommend a carburetor upgrade at this point, as the Carter is going to vastly restrict the amount of air/fuel mixture going into the cylinders, hampering positive improvement characteristics. You could also easily use the opportunity to upgrade to a junkyard jeep EFI setup and double the benefits of your head swap (not kidding).

9. Your 4.2 thermostat housing will not work, you will need a 4.0 thermostat housing, though you can still re-use your 4.2 thermostat. 10. The work is somewhat time-consuming, but by the end you'll know how to replace a head gasket like the back of your hand Wink

Since most of the actual swap information I've gathered came from this, I'll lead you right to the source.. [1]