4.0 Head Swap on 4.2 Thoughts? - JeepForum.com
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post #1 of 16 Old 09-25-2017, 08:26 AM Thread Starter
TX-YJ
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4.0 Head Swap on 4.2 Thoughts?

Good Morning Everyone!

I need to replace the head on my 4.2 '90 YJ and have previously read about replacing the head with a 4.0 head and at the same time replacing the intake and switching it from carbed to FI. I am still pretty new to a lot of this and am currently running a 4.2 with a 2150 carb. Most of the links in the FAQs are dead, and was curious if anyone knew of a good write-up still alive out there somewhere. Also, what are your thoughts on the 4.0 Head Swap with FI vs. 4.2 with a 2150 carb? Would the difference be worth the work?

Thx in advance!

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post #2 of 16 Old 09-25-2017, 08:43 AM
jserna
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Yes the differences are well worth the work. I swapped in FI in my 87 about 15 years ago. The single best mod I made. The power difference is pretty significate.
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post #3 of 16 Old 09-25-2017, 10:21 AM
mike134
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You get a big peak horsepower boost going with the newer technology head.

The FI will broaden your HP curve and make you feel like you have a ton more horsepower when you are off peak.

No disadvantages, except that some EPA jurisdictions might prohibit it if you are a resident in one (Los Angeles for example)
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post #4 of 16 Old 09-25-2017, 10:55 AM Thread Starter
TX-YJ
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Awesome, planning on tackling this project in the coming weeks. Thanks for the responses. Would you opt for a clearwater head or try to find a junkyard 7120? Is there any specific model year that is best to try and pull a 7120 and related parts from?
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post #5 of 16 Old 09-25-2017, 11:01 AM
mike134
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Is the head already modified for your 4.2L if you buy it?

If not, you may as well get one from the junk yard, because you might mess up the welding of the coolant ports.
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post #6 of 16 Old 09-25-2017, 12:57 PM
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Quote:
Originally Posted by mike134 View Post
Is the head already modified for your 4.2L if you buy it?

If not, you may as well get one from the junk yard, because you might mess up the welding of the coolant ports.
I've never done this swap, but I thought you just had to epoxy the coolant ports, not weld them? I remember something about stuffing packing peanuts in there and epoxying over the peanuts, and the peanuts melt away later because they are biodegradable......
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post #7 of 16 Old 09-25-2017, 02:25 PM
mike134
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People have been successful with "JB weld" epoxy. A real metal weld is better though.

The risk with the epoxy is if a chunk pops off and ends up floating around the coolant system, its bound to cause a major issue.

I won't discourage it or anything, but make sure you have REALLY good adherence of metal to epoxy.
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post #8 of 16 Old 09-25-2017, 06:03 PM
shaggyjim
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It might be better to have freeze plugs installed instead of epoxy or welding.

Jim...

"That's more fun than watching a hippie get mauled by a bear!"
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post #9 of 16 Old 09-25-2017, 06:12 PM
nyyj1989
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I did this swap almost 17 years ago. I did not use JB Weld but something similar.
It was from a marine store and it's two part epoxy called Marine-Tex.
make sure you get "packing peanuts" that dissolve.
I have had zero issues.

I did the head swap and FI at the same time.

I am very happy with it. The engine is running strong all these years later.

Hope that helps.
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post #10 of 16 Old 09-26-2017, 12:00 AM
lclark2074
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I did mine about 8 years ago and i would not go back. jbweld works good for me too. Its only freeze plug pressure on the jbweld so it works good. Go to your local pic and pull and get the wiring harness, intake, fuel rail,exhaust factory header,charcoal canister and computer.

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post #11 of 16 Old 09-26-2017, 05:31 AM Thread Starter
TX-YJ
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Thanks again for the feedback guys. Iclark, is there a specific cherokee year that I should try to find to pull the parts from? I thought i read somehwhere that the 1996 models had stronger parts than the earlier years.. any truth to that?
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post #12 of 16 Old 09-29-2017, 10:59 PM
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Quote:
Originally Posted by TX-YJ View Post
Thanks again for the feedback guys. Iclark, is there a specific cherokee year that I should try to find to pull the parts from? I thought i read somehwhere that the 1996 models had stronger parts than the earlier years.. any truth to that?
Get one from a 91-95 with the 7120 cast head. Or a 96-99 with 0630 avoid the 00-01 with 0331 heads that crack.

When i did a cam swap and replaced all the seals i was going to swap on a 7120 head (its on my toolbench) but i didnt do it to save money since i could never get a definitive answer on pushrods. Then i screwed up and left a rag in the oilpan. So my 258 got trashed. Now im almost done with my 4.0l swap

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post #13 of 16 Old 09-30-2017, 12:09 AM
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like the previous poster seas 1991-1995 will work. Pull all the hardware off. But the intake off 2001 will flow better than an obd1
you may like this thread for help

JeepForum.com > Models > Jeep Wrangler Forums > YJ Wrangler Technical Forum > 4.2L Fuel Injection and Build

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post #14 of 16 Old 09-30-2017, 09:01 AM
Beach248
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Quick question, will a 4.2 intake manifold work on the 4.0 head? I already have a Howell FI on my 4.2, so I'd rather just use that, since i'm in California, and have my Howell CARB sticker.

EDIT: For anyone wanting to do this, or run a carb, I found out that the 4.2 intake will work on the 4.0

88' YJ, 4.2, Howell EFI, CRT HEI, 8.8 4:10, 95' D30, AX15 ex. slave, Dodge NP231 wide chain
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post #15 of 16 Old 10-01-2017, 12:38 PM
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Quote:
Originally Posted by Beach248 View Post
Quick question, will a 4.2 intake manifold work on the 4.0 head? I already have a Howell FI on my 4.2, so I'd rather just use that, since i'm in California, and have my Howell CARB sticker.

EDIT: For anyone wanting to do this, or run a carb, I found out that the 4.s intake will work on the 4.0
I've been hesitant to do this for the same reasons.

Hope you do a write up when you get to it.
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