4.0 on a 4.2 Options

[Rescue Diver]

Everyone says there is significant HP gains to be had by putting a 4.0 head on a AMC 258 i6 / 4.2l . What years will fit, and what intake and exhaust manifolds will work?

I have a pre 1981 AMC 258 i6 / 4.2l and I've been told that a 1994 4.0 head is a direct bolt on for my shortblock with just a few issues with the power steering bracket. In keeping with my keep it as simple and cheap as possible theme, I'm still wanting to stay in the stone ages with a carb instead of fuel injection. I know my pre 1981 intake and exhaust manifolds won't bolt up to a 4.0 head, but will an intake from 1982 or newer AMC 258 i6 / 4.2l work on a 4.0 head?

What kind of HP gain can I expect with a 4.0 head swap that's still carbed? My stock 1bbl AMC 258 i6 / 4.2l is rated at 110 hp. I've read that I can expect about 140 hp with the 4.0 head. Can anyone back this up with real experience?

I'd like to install a later model 4.0 head with a 2 bbl carb and intake.

For gas mileage I want to keep my stock 354 gears and I'm running 33" tires so it's currently a little gutless on the highway. I can maintain 65 or 80 mph with my foot on the floor depending on which way the wind is blowing.

 

[Sasquach]

I did the 4.0 head w/cherokee tube type header and MPI system but I did everything in one shot so can not tell you what good just the head will do for you, I can tell you though what I did made a pretty significant improvement and now has plenty of power. The head I used is a #7120 casting and that was supposed to be the head of choice but can't tell you what year it is. The exhaust used on a 4.0 head must come from another 4.0 model but the intake will crossover as far as I know, the Mopar intake works on either a AMC 258 i6 / 4.2l or 4.0 head with the exception of the two locating dowels being slightly up on the 4.0.

The 4.0 head has larger intake and exhaust valves, a more direct and downward flow to the intake port and a smaller combustion area to give more compression. All good stuff!

 

[CJim7]

I did the swap also, but I started out with a AMC 258 i6 / 4.2l that I had previously put a hotter cam, flat top pistons, TFI, etc...

I had a lot of power to begin with, but adding the 4.0 head jumped my power output to stupid levels. Seems all the AMC 258 i6 / 4.2l needs is a head that breathes better then a 90 year old emphizema patient.

But contributing immensly to the power output I gained was the Clifford intake I used along with the 500cfm 4bbl Edelbrock. I used a late 90's 4.0 head and a Cherokee tube header. This went on an '87 AMC 258 i6 / 4.2l . The 7120 head Sasquach mentioned is the free-est flowing head they used in the early 4.0's available early to mid 90's (my '94 YJ had a 7120 head).

I personally think the 4.0 head is the single greatest upgrade you can do to the AMC 258 i6 / 4.2l for power gains. Clifford Performance isn't lying....with the proper upgrades, the AMC 258 i6 / 4.2l can make better then stock V8 power, and that's what I accomplished with mine.

 

[fzsk4p]

I did the swap also, but I started out with a AMC 258 i6 / 4.2l that I had previously put a hotter cam, flat top pistons, TFI, etc...

I had a lot of power to begin with, but adding the 4.0 head jumped my power output to stupid levels. Seems all the AMC 258 i6 / 4.2l needs is a head that breathes better then a 90 year old emphizema patient.

But contributing immensly to the power output I gained was the Clifford intake I used along with the 500cfm 4bbl Edelbrock. I used a late 90's 4.0 head and a Cherokee tube header. This went on an '87 AMC 258 i6 / 4.2l . The 7120 head Sasquach mentioned is the free-est flowing head they used in the early 4.0's available early to mid 90's (my '94 YJ had a 7120 head).

I personally think the 4.0 head is the single greatest upgrade you can do to the AMC 258 i6 / 4.2l for power gains. Clifford Performance isn't lying....with the proper upgrades, the AMC 258 i6 / 4.2l can make better then stock V8 power, and that's what I accomplished with mine.

Tech Specs

Jeep 4.0 Head Swap

Jim, did this on a buddy`s CJ7 a couple years ago. We also used the Edlebrock but with a Offy intake. That puppy moved out without a doubt.

 

[Rescue Diver]

Tech Specs

Jeep 4.0 Head Swap

Jim, did this on a buddy`s CJ7 a couple years ago. We also used the Edlebrock but with a Offy intake. That puppy moved out without a doubt.

Thanks!
The Tech Specs page had some great info! I'd already read the other link. It's obvious that the 7120 head casting is responsible for all the the big hp gain people report. I plan to use a stock mid 80's AMC 258 i6 / 4.2l intake and port match it. I would be shocked to find out that any of the aftermarket 2 barrel intake manifolds would add more than a couple horse power if any at all over the stock one with a little port work, so I don't plan on spending the extra money on one of those.

 

[Rescue Diver]

One quick question. I plan to go to the local pic-n-pull for the head. Is the 7120 casting number visible on the outside of the head, or do you have to pull the valve cover to see it like on Chevy heads? I hope it's on the outside. That would make it much easier to locate the right one.

 

[fzsk4p]

One quick question. I plan to go to the local pic-n-pull for the head. Is the 7120 casting number visible on the outside of the head, or do you have to pull the valve cover to see it like on Chevy heads? I hope it's on the outside. That would make it much easier to locate the right one.

Look at the top of the #4 exhaust port. I`ll see if I can get a picture on mine.
EDIT. Camera battery dead........Charging..............

Jeep Strokers • Cylinder Head FAQ

REBUILT 4.0 AMC / CHEROKEE / JEEP LAREDO CYLINDER HEAD - eBay (item 270671979734 end time Jan-25-11 11:17:35 PST)

Not Those Jeeps Again! The Facts About 4.0L Cylinder Heads: Engine Builder


Tried to get the oil fill cap as reference.

 

[CJim7]

Dont be dissapointed if you dont find a 7120 head...any of the 4.0 heads is a huge upgrade over the AMC 258 i6 / 4.2l head.

I didnt use a 7120...but rather a 3xxx, and power output due to breathability was more then abundant.

 

[Rescue Diver]

Dont be dissapointed if you dont find a 7120 head...any of the 4.0 heads is a huge upgrade over the AMC 258 i6 / 4.2l head.

I didnt use a 7120...but rather a 3xxx, and power output due to breathability was more then abundant.

I really don't expect to have a problem finding one. They're pretty much all 7120 from 1991 to 1995 from what I've read. Those years are a dime a dozen at the junk yards.

 

[CJim7]

I really don't expect to have a problem finding one. They're pretty much all 7120 from 1991 to 1995 from what I've read. Those years are a dime a dozen at the junk yards.

Well shoot...get me one too then

Kidding. But when I was searching for parts to do my swap, I couldnt find a 7120 to save my butt
Jeep parts were scarce where I was living at the time, thankfully I had friends in low places (California) that hand delivered a head for me, water jackets brazed and all. Wasn't a 7120, but worked ok.

 

[Rescue Diver]

Well shoot...get me one too then

Kidding. But when I was searching for parts to do my swap, I couldnt find a 7120 to save my butt
Jeep parts were scarce where I was living at the time, thankfully I had friends in low places (California) that hand delivered a head for me, water jackets brazed and all. Wasn't a 7120, but worked ok.

Guess I'm lucky. We have like 3 u-wrench-it type places in KC and they have lots of 90's cherokees. My Jeep is running fine with the freshly redone stock head. If for any reason I can't find one right away, I'll just keep checking until one shows up. I'm pretty sure it won't be difficult to get one though.

 

[Cheepjeep85]

You can save yourself some time and machine work if you buy a new one from alabama cylinder heads

Sent from my DROID2

 

[Rescue Diver]

You can save yourself some time and machine work if you buy a new one from alabama cylinder heads

Sent from my DROID2

What do they charge for them? I can get a basic valve job and resurfacing done pretty cheap at the machine shop I usually use, and the head won't cost much from a u-wrench-it. I plan to just fill the extra coolant ports myself rather than having the shop weld them in or machine them for freeze plugs. Some people frown upon filling them with JB Weld, but I've read a ton of posts and articles on the 4.0 swap and have yet to read a single report of one leaking when someone used JB Weld with bio degradable corn starch packing peanuts to hold it in place while it cures.

 

[Cheepjeep85]

I don't have catalog right now I can find out later, quadratec, 4wd hardware and others sell them. If I remember right sasquatch bought one when he done his

Sent from my DROID2

 

[Rescue Diver]

Another quick question. I might have a line on an 0630 casting from the later 1990's. They're supposed to be about as good as the 7120 but some say they don't have the hole for the temp sender. I can live with that. My concern is I found a site that says the two center exhaust ports are somewhat combined and are different than the 7120. The picture of the #0630 showed that the two center exhaust ports have one single open area where the manifold flange would bolt up.

Can anyone tell me if the exhaust manifold for a 1991-1995 #7120 head will still work on the 1996-1999 #0630 head even with the different exhaust ports?

 

[Sasquach]

some good info here;
Jeep Strokers • 7120 vs 0331 head

 

[Rescue Diver]

some good info here;
Jeep Strokers • 7120 vs 0331 head

Thanks. I'm still not finding if the exhaust manifold is different for a #0630 casting than one for a #7120. It doesn't really matter for now because the DB guy with the #0630 removed his ad from Craigslist without having the courtesy to email me back that it was sold so I wouldn't be waiting by the phone and checking my email all evening.

I'd still like to find the answer to that question in case I run across another #0630 head. I'm still shooting for a #7120 because it definitely has the hole for the temp sender but would use a #0630 if the manifold isn't something that's harder to find new.

 

[fzsk4p]

Thanks. I'm still not finding if the exhaust manifold is different for a #0630 casting than one for a #7120. It doesn't really matter for now because the DB guy with the #0630 removed his ad from Craigslist without having the courtesy to email me back that it was sold so I wouldn't be waiting by the phone and checking my email all evening.

I'd still like to find the answer to that question in case I run across another #0630 head. I'm still shooting for a #7120 because it definitely has the hole for the temp sender but would use a #0630 if the manifold isn't something that's harder to find new.

Understand your question but have no concrete answer for you.

Bennie at Hesco would know for sure.
Give him a call.

Hesco Inc. - High Performance Engine Service

 

[Sasquach]

I did what Cheapjeep mentioned and bought a ready to go conversion head were the jackets are welded and machine work done ready to bolt on, cost me $400 approx. It was from Clearwater cylinder heads.
Your concern with the xxxx head having some strange exhaust port layout is news to me and would like a pic if you have it.

 

[Rescue Diver]

I did what Cheapjeep mentioned and bought a ready to go conversion head were the jackets are welded and machine work done ready to bolt on, cost me $400 approx. It was from Clearwater cylinder heads.
Your concern with the xxxx head having some strange exhaust port layout is news to me and would like a pic if you have it.

Here's the link to the page I found the info for the #0630 head casting and the picture of the exhaust port from that page.

Not Those Jeeps Again! The Facts About 4.0L Cylinder Heads: Engine Builder