258 4.0 Head Swap Results

[Out of my mind CJ5]

It'll be a while before i have the time to focus on the 75 that much but that sure would be pretty cool

 

[Rescue Diver]

I managed to locate one of the articles I read wayyyyy back regarding the 4.0 head swap. The article from jeep.off-road.com is fine. The comments left about the article are another story. One of the responders asked the author what he did to solve the oiling problem since "the AMC 258 i6 / 4.2l has solid push rods!" This info is definitely wrong. It's garbage like this that leaves countless people scratching their heads.

4.2L Head Conversion: Off-Road.com

So far I haven't run across the info I read regarding what year AMC went to the modern hollow pushrod oiling system, but I'll keep looking. One thing I definitely know is that all AMC 258 i6 / 4.2l 's had the hollow push rods.

 

[Rescue Diver]

the AMC 232 i6 is from a 75, do you all think that the head might be too big for the little AMC 232 i6 , i know its 3.8L but still just curious

I found a thread on another site where everyone seems to be in agreement that the AMC 232 i6 and AMC 258 i6 / 4.2l take the same head. If they're correct, the 4.0 head swap should be done exactly the same on a AMC 232 i6 as on a AMC 258 i6 / 4.2l , and you should expect similar performance gains!

AMC 232 head on 258? - JeepForum.com

I also re-read Tim Weaver's 4.0 head swap page(Great Info by the way) and he says that the AMC 232 i6 and AMC 258 i6 / 4.2l share the exact same block, which confirms everything else I've read.

http://www.monsterslayer.com/pages/jeep/40Head/40Head.htm

If you do the swap, be sure and tell all of us what your results are!

 

[ThisGuyUKnow]

the tim weaver article is the one I used to get my step by step for my swap.

 

[jeepindavis]

1978 Stock Push Rods(Correct Length for my swap)

 

[jeepindavis]

I replaced/updated the ignition system and got a major improvement

what did you replace the ignition system with? What components were replaced?

 

[Rescue Diver]

I replaced/updated the ignition system and got a major improvement

what did you replace the ignition system with? What components were replaced?

Who are you asking? I'm running a cheapy HEI that I bought on ebay new last Summer, way before I did the head swap. I have roughly 7k miles on it so far and no problems with it.

 

[TheDirtyCJ]

So you're doing the 4.0 head swap, but you're sticking with the same intake and carb, and you're just changing from one header to another, so any performance gain you have will be due stricktly to the head? And you plan to dyno before and after????

You rock! It's about time someone did that!

I'm eagerly awaiting the results!

Done and Done! It took a little longer than I thought. But the results are well worth it #7120 head-I had some time on my hands so I polished the ports then did the swap. We ran it on the dyno 3 times I got this long piece of paper for b4's and then peak afters. Here are my results- b4 was 142@ 3200 rpm peak(with headers and 4bbl truck avenger carb on offey intake) afters peak 178@ 3200 rpm. thats 36 HP gain! I have the torque and and a whole bunch of other written down, but I'm not gonna type it all right now. The real test was going up the hill to red mountain by my house it always bogged down-no faster than 55-60 on that road. remarkable difference now easily 75+ in the same spot. and thats on 37" tires with 4.56 gear ratio. Thats all I have for now will have step by step pics later!

 

[IOPort51]

and the only thing you did was change the head??

Done and Done! It took a little longer than I thought. But the results are well worth it #7120 head-I had some time on my hands so I polished the ports then did the swap. We ran it on the dyno 3 times I got this long piece of paper for b4's and then peak afters. Here are my results- b4 was 142@ 3200 rpm peak(with headers and 4bbl truck avenger carb on offey intake) afters peak 178@ 3200 rpm. thats 36 HP gain! I have the torque and and a whole bunch of other written down, but I'm not gonna type it all right now. The real test was going up the hill to red mountain by my house it always bogged down-no faster than 55-60 on that road. remarkable difference now easily 75+ in the same spot. and thats on 37" tires with 4.56 gear ratio. Thats all I have for now will have step by step pics later!

 

[Rescue Diver]

Done and Done! It took a little longer than I thought. But the results are well worth it #7120 head-I had some time on my hands so I polished the ports then did the swap. We ran it on the dyno 3 times I got this long piece of paper for b4's and then peak afters. Here are my results- b4 was 142@ 3200 rpm peak(with headers and 4bbl truck avenger carb on offey intake) afters peak 178@ 3200 rpm. thats 36 HP gain! I have the torque and and a whole bunch of other written down, but I'm not gonna type it all right now. The real test was going up the hill to red mountain by my house it always bogged down-no faster than 55-60 on that road. remarkable difference now easily 75+ in the same spot. and thats on 37" tires with 4.56 gear ratio. Thats all I have for now will have step by step pics later!

That is awesome! 36 HP is pretty close to the 40 that people have been saying, and totally believable with the increased compression, bigger valves and better flow that you get with the 4.0 head. I believe my gain was even greater than yours since you were already running a header while I was running a stock very restrictive 1979 exhaust manifold and switched to the 1995 cherokee (header style) manifold while doing my head swap.

Thank you VERY much for getting the before and after dyno numbers and finally proving what all of us that have done this swap already knew!

 

[jeepindavis]

#7120 head-I had some time on my hands so I polished the ports then did the swap.

so is this the same head used on all XJ's? I have one on my garage floor that I'm considering installing in my CJ but was curious of the prep you did on the head. Did you have it machined? Revalved? You said that you polished the ports. What does that mean exactly.


Many, many thanks for the dyno results.

 

[Rescue Diver]

so is this the same head used on all XJ's? I have one on my garage floor that I'm considering installing in my CJ but was curious of the prep you did on the head. Did you have it machined? Revalved? You said that you polished the ports. What does that mean exactly.


Many, many thanks for the dyno results.

The #7120 head casting was used on 1991 to 1995 XJ's. This one is considered the best for a AMC 258 i6 / 4.2l swap. From 1996 until early 1999 they used the #0630 head. It's also an excellent head but some do not have the hole at the back for the temp sender. Avoid the Mid 1999 to late 2002 #0331 heads because they're prone to crack between the number 3 and 4 cylinders.

You can tell which one you have by the number on the top of the drivers side of the head in the center next to the valve cover. If it's not one of the numbers I listed, it's probably a pre 1991 head without the raised intake ports. Those heads don't give much improvement over the stock AMC 258 i6 / 4.2l head.

If you do make the swap, pay a good machine shop to do a valve job, seals and resurface. The shop I use only charged me $140.

You'll also have to plug the extra 12 water jackets between the combustion chambers and the driver side edge of the head. You can pay the machine shop to weld the holes in before they resurface it or drill them round and install freeze plugs, or you can fill them yourself with JB weld and disolving packing peanuts as a lot of us have done. Mine doesn't leak with several thousand miles on it. I haven't read anywhere else about anyone reporting leaks doing it the JB weld way either.

 

[TheDirtyCJ]

The #7120 head casting was used on 1991 to 1995 XJ's. This one is considered the best for a AMC 258 i6 / 4.2l swap. From 1996 until early 1999 they used the #0630 head. It's also an excellent head but some do not have the hole at the back for the temp sender. Avoid the Mid 1999 to late 2002 #0331 heads because they're prone to crack between the number 3 and 4 cylinders.

You can tell which one you have by the number on the top of the drivers side of the head in the center next to the valve cover. If it's not one of the numbers I listed, it's probably a pre 1991 head without the raised intake ports. Those heads don't give much improvement over the stock AMC 258 i6 / 4.2l head.

If you do make the swap, pay a good machine shop to do a valve job, seals and resurface. The shop I use only charged me $140.

You'll also have to plug the extra 12 water jackets between the combustion chambers and the driver side edge of the head. You can pay the machine shop to weld the holes in before they resurface it or drill them round and install freeze plugs, or you can fill them yourself with JB weld and disolving packing peanuts as a lot of us have done. Mine doesn't leak with several thousand miles on it. I haven't read anywhere else about anyone reporting leaks doing it the JB weld way either.

What Rescue Driver said is right on the money! All I can add is polishing the ports is just taking your time and go in to the manifold side and use a good working Dremel tool to smooth out the bore going from a lower grit to higher. DO NOT gouge the inside area just a smooth sanding till it looks like a mirror. It allows for a smooth air flow in and out of the head. Check out powerblocktv.com they did a whole epidode on this......

 

[IOPort51]

Dirty;
I will raise a glass to you on a regular basis you being the only one, I am aware of, to actually document this modification. Is there a chance that you will be able to scan the documentation into this site sometime in the future? Jeep guys will speak your name in reverent whispers from this day forward.
Have you already upgraded the ignition system? That would be another terrific dyno session. I have always been of the opinion that I got more positive results from the TFI/distributer upgrade than I did from the head swap.
On the subject of polishing the ports, when I have had that much free time on my hands, I have opened the ports on the head and manifold to match the gasket opening. I like a carbide burr on an air die grinder and would hope anyone doing this is using goggles and a respirator of some sort. Powdered iron and/or aluminum are for sure not something you need to take internally.

 

[TheDirtyCJ]

Dirty;
I will raise a glass to you on a regular basis you being the only one, I am aware of, to actually document this modification. Is there a chance that you will be able to scan the documentation into this site sometime in the future? Jeep guys will speak your name in reverent whispers from this day forward.
Have you already upgraded the ignition system? That would be another terrific dyno session. I have always been of the opinion that I got more positive results from the TFI/distributer upgrade than I did from the head swap.
On the subject of polishing the ports, when I have had that much free time on my hands, I have opened the ports on the head and manifold to match the gasket opening. I like a carbide burr on an air die grinder and would hope anyone doing this is using goggles and a respirator of some sort. Powdered iron and/or aluminum are for sure not something you need to take internally.

Oh man, that made me laugh! Clever writing always peaks my interest. The report is a little wider than my scanner and I could possibly do it in sections so I'll get it on here as soon as I can. I upgraded to the TFI Juicebox gold with the stouter 8mm spark plug wires some time ago, so thats allready done. Insofar as powdered iron and aluminum go I always believe to consume in moderation!

 

[Rescue Diver]

I've built a whole lot of engines for the drag strip over the years, and one constant I've found is you really don't gain any noticable power or faster quarter mile times from ignition upgrades unless your old ignition wasn't tuned or working properly. It's true that if you're building a really high RPM motor, a more powerful spark will help burn the charge more completely on the top end, but I haven't seen anyone building an 8000 RPM AMC 258 i6 / 4.2l for a CJ yet.

IO, I supect that your big gain was because you swapped in a properly working distributor for one that had problems.

Of course, you can never go wrong switching from an old points distributor to electronic ignition. But even an old points distributor will give all the spark that a 4000 RPM jeep motor can use as long as it's tuned right and kept dry.

My money is always on the tangible upgrades like higher compression, better flowing ports and valves, a good cam profile for what you're wanting your engine to do, and having the right size carb or fuel injection for your application.

 

[jeepindavis]

That's what I do with the Nordkog CJ speedometer. It has the timer built in. Nice feature.

what are you guys timing? You (3CJ and Dirt Diver) are talking about measuring time vs. HP. I don't understand...sorry for my ignorance.

 

[jeepindavis]

The hot setup these days is to bore a 4.0 block .030 over, and install a AMC 258 i6 / 4.2l crank in it to make a 283 cubic inch stroker jeep motor. I plan to build this as my next motor. The really cool thing about it is, I already have the head and crank for it. All I have to do is get a good 4.0 block, have it bored, and buy a good cam and a new set of pistons!

Rescue Diver - my apologies for calling you Dirt Diver in one of my previous posts (The Unit was one of my favoite TV shows and he was a main character)

Like you I have a 4.0 block, head and exhaust and intake manifolds along with my running AMC 258 i6 / 4.2l '84 CJ. As a bonus I have a non running '84 CJ with a AMC 258 i6 / 4.2l that spun a bearing (only one) so I have a spare to build on if I get that crank repaired.

I am very anxious to hear about your Stroker project. Do you have any idea where you're going to get your cam and pistons from? Do you have any other info about this project? Love to hear about your research...

 

[Rescue Diver]

Rescue Diver - my apologies for calling you Dirt Diver in one of my previous posts (The Unit was one of my favoite TV shows and he was a main character)

Like you I have a 4.0 block, head and exhaust and intake manifolds along with my running AMC 258 i6 / 4.2l '84 CJ. As a bonus I have a non running '84 CJ with a AMC 258 i6 / 4.2l that spun a bearing (only one) so I have a spare to build on if I get that crank repaired.

I am very anxious to hear about your Stroker project. Do you have any idea where you're going to get your cam and pistons from? Do you have any other info about this project? Love to hear about your research...

I'm not running a 4.0 block yet. That stroker motor is a project for me in the future. Right now I'm running a rebuilt but stock AMC 258 i6 / 4.2l bottom end with a 4.0 head.

 

[Kane]

IO, I supect that your big gain was because you swapped in a properly working distributor for one that had problems.

Don't forget that the computer controlled years had distributers with very little mechanical advance. The computer was responsible for allot of the advance so the distributer didn't need it. Once the computer is gone now the total advance is hindered. I have seen this more than once on the old AMC 258 i6 / 4.2l motors. If you switch to a newer distributer its like majically gaining horsepower. The old distributer could be adjusted by filing the factory advance stops so it allows more timing and the same results would shine through.