4.2 block with a 4.0 head

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4.2 block with a 4.0 head

Postby Reednuts2 » November 22nd, 2013, 11:41 am

I've seen the write ups about huge gains by swapping a 4.0 head to an old 4.2... I've got a low mileage 4.2 out of a 79 FSJ cherokee just sitting around doing nothing. I drive a lifted 97 xj sport (DD) and have stock (very clean) 97 country with a blown rod (would like to make my dd w/the good 4.0 motor)... My question is can I put a cam in the 4.2 block, swap belt drives/head/all external components and what not or will I be just making one big giant turd?... any help would be great and if the best solution is to build a stroker I'm fine with that too lol just wondering why this head/(block swap in my case) wouldn't work in reverse?
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Re: 4.2 block with a 4.0 head

Postby DaemonForce » November 22nd, 2013, 2:26 pm

If it weren't for the low mileage part I would say turd. I would go for it. It's kind of a pain but as long as you have the engine out and on the stand then cam it while it's still possible. As far as belts go, my Eagle has an air conditioning pump that I won't touch so I'll go against the norm and V-belt my stroker. That's literally the only thing keeping me on V-belts. Tear the head apart, dip it in caustic and the next day key in a puffball brush for the drill and some small cup-brushes for the dremel, clean up the head and paint it.
Be gone vile rust particles! :smack:

Also, watch out for dust. The swap itself isn't hard but it's definitely a process thing. Especially on an older block where the head bolts are probably smaller than the Renix or HO heads expect. Digging out the valve compressor tool and adjusting it for these tiny springs is maddening. I have some old Snap-On device that resembles a fugly C clamp and it's annoying. I go back and forth between Ford and American heads so the surface pieces are never convenient or correct when I need it. I'm lazy. The stock AM springs will bind when they've been seated for a while and it's hard to separate the valves from the retainers without making the keepers fly. Close the door and wear goggles. :/ I usually spin my valves against sandpaper with a noisy Milwaukee drill to clean out the carbon. Earplugs are great.

Reseat the valves and lap them the old way with the a cute little suction cup tool you get at NAPA for $5. It takes a while but compared to shipping the whole thing off for a reman, I prefer this. Two oil based grits give a better finish than the single tube of water based Permatex. The rest is just teardown, clean and swap. Mind the torque specs.
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Re: 4.2 block with a 4.0 head

Postby Reednuts2 » November 22nd, 2013, 2:36 pm

Well a remanufactured 4.0 head is 350 or so of money well spent and I'll just use the QUik steel in the water jackets like I've read about, the v belts wouldn't be bad as it would leave the alternator about 12-18" higher, don't have/need ac, what kind of cam and power could I make?
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Re: 4.2 block with a 4.0 head

Postby DaemonForce » November 22nd, 2013, 3:13 pm

That's really up to you. I don't have a stroker or cammed engine so that's an experiment to me as well. I've been looking at Crane and Crower cams now that I'm focused on a stroker. COMPcams interested me when I still had a functioning 258 but I'm over it. Right now I'm trying to figure out the bind height on my stock springs. I'm pretty sure I can't reuse them but if I can figure out how to get by with it then I'll try. I have weird smaller than stock diameter springs that seat at 1.700" and the cams I'm looking at have a much larger than stock lift that makes me more concerned about predicting the engine behavior.

As far as head swapping a 258 goes, I never got around to it but I'm pretty sure 80% of the power gain is in going to electronic fuel injection. It's not even about the high output head. Most of these swaps are done for Eagles, Hornets, Ramblers and some CJs since the Chrysler Jeep stuff is far less complicated than dropping a 4.0 into a classic AMC. A lot of these swaps are done the lazy way by using a Renix electrical harness and some minor splicing to avoid rewiring much else. I forgot about the water jacket issue until now but I'm not really sure why it's even a thing. The paths look about the same to me. I tore apart a 258 and 4.0 block side by side last month and I still can't tell the difference without looking closely. The bore, fuel pump port and oil filter mount are the main differences. The freeze plugs are different and the pipe plugs typically have an outer square on the 258 instead of the extremely troublesome 5/16" squares that tend to seize and rust shut in the 4L.

I'm not sure what your Cherokee is like in the engine bay but if you have a Dana 30 up front like I do then I'm guessing the crossmember is also thick and cramped. If you're planning on building a stroker then I'm willing to bet you're going to be struggling with stupid like I did if you try to match it up with the 4L pan. Only the 4.2L pan clears my crossmember and the last part of my bill is the most annoying trying to find a 4L high volume pump with a pickup tube angled for the 4.2L. :frustrated:
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Re: 4.2 block with a 4.0 head

Postby Reednuts2 » November 22nd, 2013, 3:40 pm

It seems to be all pain no gain... I guess I'm just gonna have to build a stroker then :D , I'm okay with it. Thanks for the help
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Re: 4.2 block with a 4.0 head

Postby Cheromaniac » November 23rd, 2013, 2:43 am

Reednuts2 wrote:I've seen the write ups about huge gains by swapping a 4.0 head to an old 4.2... I've got a low mileage 4.2 out of a 79 FSJ cherokee just sitting around doing nothing. I drive a lifted 97 xj sport (DD) and have stock (very clean) 97 country with a blown rod (would like to make my dd w/the good 4.0 motor)... My question is can I put a cam in the 4.2 block, swap belt drives/head/all external components and what not or will I be just making one big giant turd?... any help would be great and if the best solution is to build a stroker I'm fine with that too lol just wondering why this head/(block swap in my case) wouldn't work in reverse?


You could use the 4.2L short block as the heart of your new engine, swap in the 4.0L cam with new lifters/pushrods, and bolt on the existing '97 4.0L head assembly with intake/exhaust manifolds and all your existing '97 EFI stuff. You could also use a new 4.0L harmonic balancer and run a serpentine belt.
The end result won't be as powerful as a stroker but your "new" 4.2 will have more low/medium rpm torque than your old 4.0 and will represent a mild upgrade.
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