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Old 12-17-2013, 09:58 PM   #1
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4.2l to 4.0l head swap questions?

My head gasket is blown again on my 4.2l and I think I'd rather swap the head for some more HP. I have 3 4.0l cylinder heads in my garage, 88 xj (renix era), 93 zj (HO), and a 01 xj (obd2). Which cylinder heads will and won't work.

I know chrysler changed stuff to the head in 91 so would the 88 head even work as it is in immaculate shape and would be my first choice?

The 93 head is another choice but I'll have to have it decked and change all the valves, seals, and 2 rockers.

The 01 head has very small exhaust ports on it so I don't know if that is remotely good if I stay carbed which is very likely. Any help would be appreciated as I've never done a head swap on these things

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1989 Base YJ, 4.2l bored .03 over, Borla exhaust, offy intake, Motorcraft 2150 carb (soon a howell TBI kit), HEI ignition, nv3550, nv241or with 4:1 low ,D44 with ox locker & Ford 8.8 with lsd both with 4.10 gearing, Skyjacker 4" Double wrap lift, and Boom Shackles.
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Old 12-17-2013, 11:35 PM   #2
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I found some cast numbers on 2 of the heads. The 01 xj has 0331, the 93 zj has 7120

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Old 12-18-2013, 10:10 PM   #3
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Will the 88 4.2 intake bolt right up to the 88 4.0 head, and does the 88 4.0 head flow any better than the stock head?
I've read that with some [ DIY] porting and clean out the ruff castings the stock head will flow very good.
Deck it and reinstall?
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Old 12-18-2013, 10:44 PM   #4
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Quote:
Originally Posted by Nubby55 View Post
Will the 88 4.2 intake bolt right up to the 88 4.0 head, and does the 88 4.0 head flow any better than the stock head?
I've read that with some [ DIY] porting and clean out the ruff castings the stock head will flow very good.
Deck it and reinstall?
I have all the renix efi stuff so I wanted to go that route including the renix intake. I'm missing certain sensors and such of the mpfi stuff but the renault is all complete and in great shape. I have my stock head ported already with borla exhaust headers and it still isn't doing it for me. We'll see though as this is no light undertaking, and the jeep is my current and only running vehicle at the moment.
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1989 Base YJ, 4.2l bored .03 over, Borla exhaust, offy intake, Motorcraft 2150 carb (soon a howell TBI kit), HEI ignition, nv3550, nv241or with 4:1 low ,D44 with ox locker & Ford 8.8 with lsd both with 4.10 gearing, Skyjacker 4" Double wrap lift, and Boom Shackles.
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Old 12-18-2013, 11:23 PM   #5
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I have not run my motor yet, so I cannot give you any insight there. What I can tell you is that the 7120 head is considered the most desireable of the 4.0 heads. I have a core, and a fully rebuilt one that is on my stroker. I just went through the ordeal of mounting the carb'd YJ intake to the 4.0 head today, and it is not a big deal.

In between the intake runners where the intake meets the head, on the underside, is a hole for the locating studs that the 4.2 heads use. These will need to be ground flat, as the 4.0 locating studs sit about 3/8" lower in relation to the intake ports on the head. See visual aids below (from my engine). I am considering getting some large flat washers to compensate for the lack of support from the bolts after the change in intake location.





Next thing you need to know is that the 4.2 exhaust manifold will not fit the 4.0 head. Also, as I have just found out today, the APN header will not fit without some modification (needed some stratigic dents) to accompany the carb'd 4.2 intake.

Have fun, I have had nothing but hiccups along the way with my build, but 99% of the stroker info I have found is EFI related so I have had to blaze a few trails myself.
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Old 12-18-2013, 11:35 PM   #6
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Quote:
Originally Posted by Wombat Ranger View Post
I have not run my motor yet, so I cannot give you any insight there. What I can tell you is that the 7120 head is considered the most desireable of the 4.0 heads. I have a core, and a fully rebuilt one that is on my stroker. I just went through the ordeal of mounting the carb'd YJ intake to the 4.0 head today, and it is not a big deal.

In between the intake runners where the intake meets the head, on the underside, is a hole for the locating studs that the 4.2 heads use. These will need to be ground flat, as the 4.0 locating studs sit about 3/8" lower in relation to the intake ports on the head. See visual aids below (from my engine). I am considering getting some large flat washers to compensate for the lack of support from the bolts after the change in intake location.

Next thing you need to know is that the 4.2 exhaust manifold will not fit the 4.0 head. Also, as I have just found out today, the APN header will not fit without some modification (needed some stratigic dents) to accompany the carb'd 4.2 intake.

Have fun, I have had nothing but hiccups along the way with my build, but 99% of the stroker info I have found is EFI related so I have had to blaze a few trails myself.
I know how to install, I have just never heard of anyone using a renix head and was curious to know if it'll work. I want to go efi really bad now with a stock 4.0 efi system including the 4.0 intake, and I'm going to go down that route one way or another either renault efi or mopar mpfi. One of the cylinder heads I have is the 7120, but it will need A LOT of work and I don't really have the time. However if I can find a good obd1 4.0 computer and wiring harness with sensors, I'll have the 7120 head decked which will cost a lot because it's not level. I'll have to replace the valves as they're either slightly bent or burned, and a few rockers, and also I'll need new push rods for the 7120 head. I'm just trying to weigh all my options as this is going to have to be done withing 4 days max as my jeep is my daily driver
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1989 Base YJ, 4.2l bored .03 over, Borla exhaust, offy intake, Motorcraft 2150 carb (soon a howell TBI kit), HEI ignition, nv3550, nv241or with 4:1 low ,D44 with ox locker & Ford 8.8 with lsd both with 4.10 gearing, Skyjacker 4" Double wrap lift, and Boom Shackles.
Build Thread: http://www.wranglerforum.com/f330/fi...ld-385106.html
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Old 12-19-2013, 12:02 AM   #7
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How much is the 7120 head build going to run you friend?


Local machine shop here will take a cracked casting and return a complete rebuilt new head for $500. If that is significantly cheaper than your local rates we can talk shipping.


Another option since your renix head is good, I have read that the renix intake can be run carb'd.
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Old 12-19-2013, 12:10 AM   #8
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How much is the 7120 head build going to run you friend?


Local machine shop here will take a cracked casting and return a complete rebuilt new head for $500. If that is significantly cheaper than your local rates we can talk shipping.


Another option since your renix head is good, I have read that the renix intake can be run carb'd.
All the parts should be under $150, and having it decked should be around $200. It's not bad, I'd just only want to do that if I had all the mpfi parts. I have everything for the 88 renix head and all efi for it, but there are a few problems with either. The renix and mpfi will need a crank position sensor which my bellhousing doesn't have the location because I retained my stock ax-15 bellhousing when doing my tranny swap. The renix system also uses a knock sensor for timing control, but I think I can weld a bung for that on the block. I have an inline fuel pump already so I'm good there. I'd love to find a 91-95 computer and wiring harness with all sensors though so I can use the 7120 head and mpfi
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1989 Base YJ, 4.2l bored .03 over, Borla exhaust, offy intake, Motorcraft 2150 carb (soon a howell TBI kit), HEI ignition, nv3550, nv241or with 4:1 low ,D44 with ox locker & Ford 8.8 with lsd both with 4.10 gearing, Skyjacker 4" Double wrap lift, and Boom Shackles.
Build Thread: http://www.wranglerforum.com/f330/fi...ld-385106.html
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Old 12-19-2013, 12:20 AM   #9
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Complete curiosity here, I am not a forum crawler who likes to challenge everyone's every decision.


You have struck me as very knowledgable on the carb options on these rigs, why are you so set on EFI? Carb's are simpler, cheaper, and simpler. Why are you wanting to complicate things when you already have a 2100? All I read is that they are just as good as EFI on the angles.
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Old 12-19-2013, 12:35 AM   #10
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Complete curiosity here, I am not a forum crawler who likes to challenge everyone's every decision.


You have struck me as very knowledgable on the carb options on these rigs, why are you so set on EFI? Carb's are simpler, cheaper, and simpler. Why are you wanting to complicate things when you already have a 2100? All I read is that they are just as good as EFI on the angles.
I'm a carb whiz, yet I would rather just be able to start my car in the morning and just go without warming it up. I also do all my offroading up in my local mountains which are about 6000-7000 ft elevation change and my vehicle runs like crap until I adjust it for the altitude then readjust it when I go back down the hill. Who knows though, I'll probably just give up half way through and stick to the carb after swapping the head. That's usually how things go down with me haha. The motorcraft 21xx is by far the best 2 barrel around on an offroad vehicle though. As long as you can get fuel to it, it will run. You'd also be surprised by how many private messages i get on this forum about carburetor questions and swaps haha, so if you ever need help, let me know
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1989 Base YJ, 4.2l bored .03 over, Borla exhaust, offy intake, Motorcraft 2150 carb (soon a howell TBI kit), HEI ignition, nv3550, nv241or with 4:1 low ,D44 with ox locker & Ford 8.8 with lsd both with 4.10 gearing, Skyjacker 4" Double wrap lift, and Boom Shackles.
Build Thread: http://www.wranglerforum.com/f330/fi...ld-385106.html
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Old 12-19-2013, 12:46 AM   #11
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You will be hearing from me the minute I need help with a carb! I have little experience there.

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'88 YJ 4.6L stroker, HEI, MC2150, AX15 swap
4" rub/ex, 33" BFG A/T's, SYE, rear locker
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