It's the Alabama Cylinder 4.0 head which can be put on the 4.2L. I've read some articles with conflicting advice on whether to use a junkyard head, Clearwater, or Hesco; in addition to specific model years where there are concerns about the heads cracking.
Hesco sells a 4.0 with the water jackets sealed, gives you a 4.0 valve cover, and the gasket for $1100. Pricey, yes, but I didnt want to seal water jacket ports. And I got a pretty sweet looking aluminum valve cover with it (included).
So, depends on if you want the bolt-on product delivered to your door (like me), or you want to do the work yourself.
I did the 4.0 head swap from a junkyard in my 88 YJ and noticed a really good power gain in the low end. The only downside to going junk yard is that you will have to machine the head and fill in the ports. When you factor in the cost of the head and the machine shop work, you are almost better off to get one that's already done.
..... what are the stock valve sizes of said 4.0 7120 head?
While I can't exactly answer the question to that particular head, the "Mopar Performance Parts Jeep Engines" build manual list the 4.0 intake valve head diameter as being 1.782-1.792" and the exhaust as 1.401-1.411"
In speaking with Ron McGinnis @ Alabama Cylinder Head, I'll offer this. He tells me that they cast (iron), mill and installs all new parts while using the #0331 head pattern. That intake valve head diameter is 1.90" and the exhaust head is 1.49".
Alabama Cylinder Head is affiliated (in house) with "Patriot Performance" If the head is walked across the hall then that modification process includes a five angle hardened seat insert installed, its bowl blended and the runners are "cleaned", Patriot Gold springs are installed.
Ron can be reached @2564426773
I want to point out that the modification to the water jacket holes is not performed.
Hesco's site is a little confusing....you have to hunt for it. Even having bought the thing, it just now took me about five minutes to find it.
What you saw immediately was the price for the aluminum head ($1,995.00).
Way to expensive for me.
Here's the information from the website on the standard head. Hesco modifies the head to accommodate the 4.2L intake, but they say they will weld the mounting studs on wherever you want them and that it can be reversed later to take the 4.0 intake. I was really happy with this- showed up at my door and I put it on. Done.
Cylinder Head Conversion Kit #HES4240CHC
Description: Converts the 4.2L to the 4.0L cylinder head using the superior 1991-up high port head casting. Designed to let your fuel starved 4.2L perform to its fullest potential. Provides a snappy throttle response with increased torque and horsepower. Complete bolt-on kit includes modified cylinder head with head gasket and valve cover. Will require a 4.0L exhaust manifold or header.
Part Number: HES4240CHC
Application 1: 4.2L Six Cylinder Jeep engines, to be converted to 4.0 head design.
1) Hesco does whatever you want them to do, so to me that is a great advantage. They set it up for either the 4.0 intake or the 4.2, and they told me that if I changed my mind, I could have the mounting studs moved in the future, so I liked the sound of that.
2) I'd say that having the head delivered with the water jackets filled, a new valve cover, valve cover gasket (the metal and rubber type- not cork), and really expensive head gasket (it's the metal kind, with several layers...the factory gasket is more like one layer with some kind of felt on it....the Hesco gasket is clearly a high-pro version). So if you add up all the trouble of getting the modifications done, and finding and buying the upgade parts, I'd say yeah, it is worth it. I got the thing in the mail and installed it. Bing bang boom. Add it all up and compare- you are still paying more, but for me it was paying to remove a headache. You do need to buy new head bolts for a 4.0, something else I mistakenly forgot in my excitement.
3) I used a weber carb, so not sure from you saying that you have a 4.0 intake if your intent is to do a full MPI swap, but I'll give you my experience whether it helps or not (might help someone): I maintained the factory pulse fuel pump, and I wish I hadn't. I had to use a fuel regulator, and the fuel lines go everywhere because of mounting locations, and I still have the hard fuel line that runs over the valve cover (which I hate), and the pressure gauge pulses, so my perfectionist self wishes that I had coughed up a couple bucks for an electric fuel pump for the weber, which I know is available.
Maybe someone else can answer this one for the MPI if that is your intent.