4.0 on a 4.2 Options

[Rescue Diver]

Yeah, I'm right there with you. I plan on tearing into the donor 4.0 this weekend. I'll need to get the head work done.

It will probably take me a few weeks to get that done. There is a machine shop local, but the guy I talked to said they wouldn't do any work on it if the water jackets were filled with JB weld. They would have to drill and tap for freeze plugs, and they are not siue if they have the right bit and tap size. I am going to have to call around for a few other shops.

Just let me know how your project goes. Looking forward to it.

Thanks, Dan

It only took my prefered machine shop a couple days to do mine, but I waited until I got it back from them to do the water jackets with JB weld. After reading countless threads on many different forums, I have yet to hear from anyone that had a leak when they filled them that way. Even the stock 4.0 head gasket covers those ports, so it's not certain that we would even have a leak doing nothing at all. I read that the problem can arise from some AMC 258 i6 / 4.2l blocks being slightly narrower on the manifold side than on a 4.0 block, so the head hangs slightly over the edge, leaving these ports too close to the side of the block. I took care to clean the ports well, and the JB weld is 3/4" thick blocking the ports. They won't leak. I'll know more about how well it lines up with the block when I finally get to bolt it on this weekend. I've heard they changed the block in 1980 and newer AMC 258 i6 / 4.2l 's so maybe my 1978 block casting is wider? 1980 is also when they changed to 7/16" head bolts from 1/2". They switched back to 1/2" for the 4.0, so I can just use my stock 1/2" head bolts without spacers. Can't wait to hear it run!

 

[Rescue Diver]

The JB weld thing never made much sence to me as it gets soft at about AMC 150 deg. I see the need to ensure the hole does not extend past the edge of the block but I have never thought JB weld would have been my first choice.
Having done a bit of thnking on the matter I came to the conclusion that the orlando head was quite cost effective having been milled, valves ground and the holes welded closed.
How much was the cost of having the holes filled with freez plugs??

And yet, not one person who did it the JB weld way has ever posted on any forum that that I can find anywhere that they had a leak.

I don't call $400 plus shipping on an 80lb item cost effective when I have only $44 in the head and $130 in machine work. I can't complain.

 

[IOPort51]

that fact does make me wonder if the JB weld is needed at all, Has anybody just allowed the head gasket to seal the hole?

Your machine shop is a bit cheeper than mine. I figured that the welding cost me about $100 if I didn't count a day at the grave yard pulling a ,hopefully, good 4.0 head. When I did this to the J-10 I pulled two heads and both failed die pen tests.

I think the shipping is free but I would have to confirm that. Returning the core is a bit of a PITA.

And yet, not one person who did it the JB weld way has ever posted on any forum that that I can find anywhere that they had a leak.

I don't call $400 plus shipping on an 80lb item cost effective when I have only $44 in the head and $130 in machine work. I can't complain.

 

[Rescue Diver]

that fact does make me wonder if the JB weld is needed at all, Has anybody just allowed the head gasket to seal the hole?

Your machine shop is a bit cheeper than mine. I figured that the welding cost me about $100 if I didn't count a day at the grave yard pulling a ,hopefully, good 4.0 head. When I did this to the J-10 I pulled two heads and both failed die pen tests.

I think the shipping is free but I would have to confirm that. Returning the core is a bit of a PITA.

You pulled two bad 4.0 heads? What year and casing number were they??? The only ones I've heard of having problems are the 0331 castings from mid 1999 to 2002. I haven't heard from anyone that had a bad or cracked 7120 or 0630 head from 1991 to early 1999. The shop said my 7120 was in great shape and only needed the valves reground, new seals and resurfaced.

I read on one forum that not having a leak when mounting a 4.0 head on a AMC 258 i6 / 4.2l without filling the extra water holes is hit or miss depending on what year block you have, but they didn't specify which blocks worked.

Still, I have a lot of faith in the JB Weld method. Like I said before, lots of people have done it that way and reported back that it worked great. None have said otherwise that I've been able to find. I did it right, so I'm not worried at all.

 

[IOPort51]

I am still wondering how much the freeze plug install costs.
They were both 7120's and the one I put on the J-10 is also. It has been repaired where it cracked between the valves. I thought seriously about sending it back but decided to give it a try, so far so good.

 

[Cheepjeep85]

I always thought jb weld held up to alot more heat than that. Although I don't have a package to look at handy.

Sent from my DROID2

 

[Rescue Diver]

I always thought jb weld held up to alot more heat than that. Although I don't have a package to look at handy.

Sent from my DROID2

Funny you should mention that right now. I just read a thread on another forum earlier today where the guy said JB Weld will hold up to 800 degrees. I'm going to have to look for the info from an official source.

 

[Rescue Diver]

The JB weld thing never made much sence to me as it gets soft at about AMC 150 deg.

Ok, you forced me to look it up because I really didn't think you could possibly be right with such a low temperature claim.

I feel more safe with my choice than ever. This is from the JB Weld web site. Can't get much more official than that!

J-B Weld Company - Frequently Asked Questions

"A: J-B Weld (Part # 8265-S, 8265, and 8280) can withstand a constant temperature of 500 degrees F. The maximum temperature threshold is approximately 600 degrees F for a short term (10 minutes). "

 

[Rescue Diver]

Wadya Know? She's running with the new head!

It started right up and idles great with the 95 4.0 head and exhaust manifold, 80's intake, and freshly rebuilt MC2100 carb. I have an appointment in the morning to get the exhaust mated up to the new manifold and I'll finally be able to see if I really gained the power everyone says this head swap is good for. I'll post up some new pics tomorrow.

 

[ColoradoCJ7]

I am thinking about doing the same thing on my 84 and have a free 4.0 HO head, FI and complete wiring harness. My concern is getting the wiring straight. Any simple diagrams or even conversion harness that can be bought? The 4.0 is from a 92 Cherokee that I will be parting out next weekend. Any input is appreciated. Especially from those that have done it before.

 

[Rescue Diver]

The exhaust is now modified to hook up to the new manifold, and it only cost me $15! It helps that I went to high school with the guy that ownes the muffler shop. He went to a lot of work making that pipe match up and leave enough room for me to later add a clutch slave cylinder when I switch my clutch to hydraulic. I feel bad that he wouldn't take more for all of his hard work.

I still need to set the timing and tune the carb, but I can already tell a huge difference! The engine feels much smoother. It doesn't lug at lower speeds in 4th gear like it did with the original head, and it's very zippy reving. Most important, it holds it's speed better on the highway! It was definitely worth every penny and all the work!

 

[Cheepjeep85]

That's great man. So there is a definate difference in yours, could you tell it right away or did you have to drive it around a while? I wish somebody would put a jeep on the dyno before and after the swap, I'd like to see some numbers.

Sent from my DROID2

 

[Rescue Diver]

That's great man. So there is a definate difference in yours, could you tell it right away or did you have to drive it around a while? I wish somebody would put a jeep on the dyno before and after the swap, I'd like to see some numbers.

Sent from my DROID2

I could tell immediately. It has a totally different personality. The best way to describe it is zippy. I'll be doing some tuning this afternoon. The new MC2100 carb idles great and is really smooth, but it has a bit of a dead spot coming off of idle that is either do to the excelerator pump needing adjusted, or the power valve isn't opening because my vaccum is too high. I have an assortment of power valves left over from my Holley days with the corvette, so I won't even have to buy a different one if I need it.

 

[Cheepjeep85]

That's awesome.


Sent from my DROID2

 

[Petescj]

Best to search and or start another thread.

I am thinking about doing the same thing on my 84 and have a free 4.0 HO head, FI and complete wiring harness. My concern is getting the wiring straight. Any simple diagrams or even conversion harness that can be bought? The 4.0 is from a 92 Cherokee that I will be parting out next weekend. Any input is appreciated. Especially from those that have done it before.

 

[Rescue Diver]

The dead spot is fixed. It turns out it was neither of the things I suspected. It was due to the PCV valve I bought causing a big drop in vaccum. I switched back to the CJ PCV valve and it gained several inches of vaccum and it now runs even better. Since the CJ PCV is the skinny type, I'll have to shop around for one that's the same weight but that'll fit the grommet I'm using.

 

[Rescue Diver]

A few pics of the swap.

 

[Cheepjeep85]

That looks great

Sent from my DROID2

 

[cavie98]

Man, everything look awesome. I hope mine works out the same way. Just gave the head to the machine shop.

I have a question. Do you have to use the mc2100 carb or will any 2bbl carb work? I have the intake lined up and can get a carb but just not sure what it is. I do know it is not the mc2100 or not even a mc. It might be a holley of some sort.

Glad to see yours went smoothly.

Thanks, Dan

 

[Rescue Diver]

Man, everything look awesome. I hope mine works out the same way. Just gave the head to the machine shop.

I have a question. Do you have to use the mc2100 carb or will any 2bbl carb work? I have the intake lined up and can get a carb but just not sure what it is. I do know it is not the mc2100 or not even a mc. It might be a holley of some sort.

Glad to see yours went smoothly.

Thanks, Dan

You can use pretty much any 2 bbl carb on it. Most will require an adapter that you can get just about anywhere. I'd shoot for a carb that's in the neighborhood of 300 cfm. A Holley 2 bbl would be great. Of course you'll have to fabricate a throttel cable bracket and find a way to hook up the return spring and the choke.