What is wrong with 4.0 heads

[doneagle]

         HI     Ladies and Gentlemen  .I have recently had to replace a 4 0 head that was cracked in the water jacket and I called the salvage yards and they soled  them as fast as they get them in so I had to buy a recon one at the parts store for $ 472.00 plus every thing else needed to put it back together ....
the parts man said he sells about 3 a week on average .....does any one know why or what is wrong with that head

This is not the first time that I've heard about that head.  It was a 1999 and I was told that was a good one  and I said no that was a bad one .....hope some one can shed a light on the problem .........don     

[IowaEagle]

Are they cracking in the same place, according to what you have learned?

[JayRamb]

Not to anyone off, but typical Chrysler making the part.

[captspillane]

They commonly crack in one of the castings, I think around 97-99. There were several casting numbers over the years. I know the 1992-93 head doesn't crack and is the most desireable just because those are the two I own that I've confirmed in the past.

Cracks are not that common, even among the bad years. I bet most of his sales are 4.2L owners wanting to upgrade. The better head design accounts for a substantial power and efficiency increase and it goes on a 4.2L with minimal work. It also eliminates the terrible valve cover leaks that annoy most 4.2L owners.

[captspillane]

Here is a link and I copied the important posts to here from the link so that you don't need to go there.

http://www.jeepstrokers.com/forum/viewtopic.php?f=34&t=241

Jeep heads are identified by a casting number on top of the head above the #4 exhaust port, on the manifold side.

The cylinder heads

The stock I6 cylinder head is cast iron and weighs about 60lb. The valve head diameter is 1.91" intake/1.50" exhaust. Casting numbers are as follows:

Year............Casting No
1987-90......2686
1991-95......7120
1996-98......0630
1999-06......0331

Renix heads (can't remember casting number) have the lower ports and presumably less flow (don't recall actual flow numbers though)
7120 (91 to 95 I believe) this head somehow got the reputation as the "best" head... I can only guess because it's popular for the 4.2 head swap because some misinformed early swappers claimed the later 0630 head didn't have a water temp sender boss (sometimes is does sometimes is doesn't).
0630, 96-99 (98 on a ZJ I believe) this head is supposed to be identical to the 7120, I haven't flowed my 7120 yet but I believe it will flow the same. This head supposedly doesn't have a water temp sender location, but it does on 4 out of the 5 that I've seen... there's a small bolt in it depending on what model Jeep its in.
0331 2000+ (99+ on Grand Cherokees) this head is prone to cracking in its first few years of casting. It has Ports that are higher on the head and the intakes flow better than the intakes of a 0630 (and I suspect the 7120 as well but I will confirm that at some point).
The exhausts on the 0331 should be better for torque (perhaps worse for high RPM horse power, perhaps no difference), they are smaller, shapped much better, and the floor has been raised a lot while the "roof" was raised slightly making for a higher velocity port, more exhaust "scavenging" and thus better low end torque. How much is anyones guess as no ones done a dyno shootout of Jeep heads (unless HESCO has but they would never make the sheets public).

I feel the need to point out that the last part about the 0331 will be controversial, because it flies in the face of every assumption about 4.0 heads on the net. It's a crack prone head so it really doesn't matter most people aren't going to be brave enough to take a chance for a few extra foot pounds, if it means possibly having to do it all over again.

The exhaust port floor on previous heads was extremely lazy, the area that Jeep filled up on the 0331 (making the port smaller) was this lazy area, and actually makes the velocities higher in the port, while flowing slightly less volume. A good pocket porting and valve job would (In my opinion) make the 0331 flow significantly better than any previous 4.0 exhaust. I believe there's a WHOLE LOT left on the table because this port is smaller and has more meat to work with in terms of shaping.

[doneagle]

        HI   Chptspillane you are on top of this ....I just gave you a thumbs up .... another Good Job  ....Thank you         don

[doneagle]

               HI   Doug  as far as I know they are cracking in one place . the exhaust  gases were coming into the water jacket and pushing the water out .....and you could not see the crake from the top of the head ..had to take the head off to see the crake .............don

[IowaEagle]

OK, thanks for the update.  Its good to have this information.

[carnuck]

'99 up (if they have the multi coil pack bolted to the head) has the biggest issue.  There was a "silent" recall on them. IE: People who bitched loud and long and public generally got the replacement under warranty.

[doneagle]

            HI Carnuck   thanks for the info it is appreciated ........don