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post #1 of (permalink) Old 11-26-2001, 11:44 PM
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4.0 Head on 4.2 Block?

I just pulled the head off the 4.0 MPI on my new (used) jeep...head looks ok but piston #5 has a hole in it...no telling what the crank looks like. On my engine stand is a 4.2 with 5K miles and about $1300 in it. My initial plan based in input and some of the ariticles, was to have the 4.0 head re-done and slap it on top of the 4.2 block....but after looking at the water cooling passages on the 4.0, I'm begining to wonder if this is such a good idea.

If I had to speculate...I would say that Jeep added the extra cooling passages for a reason (changes cost money). My fear is that if I block them, then the head is going to run hotter than it should. I know some guys out there have done this....how many miles do they have on the enigine? Any reliability problems?

Any input? Suggestions? Speak now or forever let me wonder if I made the right decision.

John...southern CA
84CJ7, 3"lift, 32"BFG, 4.10's, ARB Rear, Detroit Front, Solid Axle's, Durabak, York Air

They say we learn by our mistakes......I guess thats why we are all so smart.
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post #2 of (permalink) Old 11-26-2001, 11:51 PM
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Re: 4.0 Head on 4.2 Block?

I have about 7,500 milles on my 4.0 head with no problems. The reason for blocking the water jackets in the head is some heads have been known to overlap the 258 block. I have read a lot on it and have never heard of a problem related to the filling in the jacket at the surface.

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post #3 of (permalink) Old 11-27-2001, 12:18 AM
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Re: 4.0 Head on 4.2 Block?

Some people have gotten away with not blocking the passages. The gasket sometimes works fine on it's own to stop the coolant flow.
The head itself still has the same amount of water going through it, it just doesn't circulate back through the other side of the block in as many places.
I haven't had any heating concernes at all. I overheated the engine once but that was still with the old 4.2 head, which caused the crack which caused the swap...
If you have all the MPI parts and the whole 4.0 motor, you are really going to enjoy the way swap will turn out. The 4.2 with the HO 4.0 head AND MPI is supposed to scream![img]/wwwthreads_images/icons/laugh.gif[/img]

'88 Sahara, 4.2 that runs thanx to every Cheap trick on this board. The rest is getting there![img]/wwwthreads_images/icons/blush.gif[/img]
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post #4 of (permalink) Old 11-27-2001, 08:17 AM
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Re: 4.0 Head on 4.2 Block?

I did this and had problems with it leaking, but what I later found out was that I should have done it my self instead of letting a mechanic friend of my dad's do it for me, which ruined the head I bought and I'm hoping that it didn't ruin the block because of the overheating that it caused. I was in college and didn't have the time to spend on modifying the head, etc... so the mechanic ended up half-way filling up the head with a block selant compound, even after I provided him with some liteature about how to pack a styrofoam peanut in there to keep it from happening and asked him to use quick steel. The guy didn't do either and ruined the head.

I ended up saving up and ordering a hesco head that had already had the waterports plugged up that needed to be plugged up. Then slowly I began to put it back together as time allowed. I got it in and the power was outstanding, but it still leaked near the back of the engine. After honing down the block to flatten the deck a little, I have gotten the head back on, this time using the spray copper to help the gasket stick. All I have to do is find time to put it in. I'm not in college anymore, but I got married last summer and haven't had much time after re-arranging my priorities.

I had a bad experience initially, but by my fault for not doing it my slef. I did all of the research and knew how it had to be done, but I let someone else do it and ended up with a problem.

Was the conversion, with the MPI worth it? I think so. For the brief time when I was able to drive it with the new head on it the power gain over the original 258 setup was amazing. And having the YJ sitting in the driveway for 2 years now has got me itching to get it done.... It shouldn't be more than a month before I get it all in and "driveable" again..

My only advice is to do more research than you could ever possibly need so you know what route you want to go. The mpi is good, but others have had success with a carb or the howell system on their jeeps.

Good luck with your conversion

4.2 w/<font color=blue>(4.0 head & MPI)</font color=blue>
After over 2yrs of driveway jeep'n, She's almost put back together.
post #5 of (permalink) Old 11-27-2001, 07:38 PM
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Re: 4.0 Head on 4.2 Block?

DDawg16 wrote:
> If I had to speculate...I would say that Jeep added the extra cooling passages for a reason (changes cost money). My fear is that if I block them, then > the head is going to run hotter than it should.

No, it won't run any hotter. If you look at a head gasket for the 4.0L engine, you'll notice that those passages are blocked off by the head gasket in the stock set-up. I've been running my 4.0L head for several years now with no problems with over heating or reliability. The 4.2L block and 4.0L head are a great combo with no downside of which I'm aware.



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