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Unread 01-18-2014, 05:15 PM   #1
pinnick-46
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4.2 Head swap

Hey Guys, I read somewhere, and im not sure whether or not this is true, and cant find the post again, that a pre 87 head off a 4.2 would be a good swap for my 90 4.2 due to the head having better flow, i was wondering if anyone has done this before, and knows this is true
thanks guys
eric

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Unread 01-18-2014, 05:36 PM   #2
GrEmLiN
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Why do you want to swap the engine head? It's not the easiest thing nor is it the hardest, but it's a committed effort, so I'm only asking what end result you're going for.


I'm not 100% sure about that claim of better flowing heads, especially limited to just the early YJ's.

However, what I'm 100% about is that with some careful, patient but entirely feasible home brew modification, the hot ticket for the 4.2 head swap is a 4.0. It depends on the year, too. Late YJ 4.0 heads are the best, with early TJs a runner up. 99+ 4.0's changed again. I think the valve sizes were different? I forget.

A 4.0 head from, say, a 95 YJ on an early YJ 4.2L, the coolant passages are different (actually most are the same, but there's more of them on the 4.0 than on the 4.2 block, so the extras in the head must be plugged, JB Weld works for this) but the better flow and performance, and above all the ready ability to accept the Mopar MPFI from the 4.0's. If you got a 4.0 head, intake, exhaust (required, the manifolds are different), MPFI setup, computer, fuel tank/pump/lines/rail/harness you could add a TON of power, torque, reliability and performance/economy to your 4.2. It's an investment though. And if you can locate a donor for all that stuff then the only way I wouldn't do a full 4.0 swap is if the internals were bad, otherwise a 4.0 swap is also a very viable option.
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Unread 01-18-2014, 05:42 PM   #3
pinnick-46
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my reason is ive bought a cam intake header and hei, and i was gonna have it apart..., didnt learn about the 4.0 swap till after id already bought the header, if swapping to a different 4.2 doesnt do anything i wont worry about it
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Unread 01-18-2014, 05:52 PM   #4
GrEmLiN
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Oh. Well if you're that far committed, the stock 4.2 head flows pretty damn decent. More than enough to negate the hassle and expense of a swap. Those bolt ons will make a nice difference.

Not for nothing, which carburetor will you run with that setup? A Holley 350 or some kind of TBI would be the hot ticket with the HEI and header.

My 87 runs 4.2, stock internals, Holly 500 carb (way big) and Borla exhaust. Stock manifolds though. Oh, I have a home brew HEI. Using stock distributor as a pickup I use an HEI module from a Chevy V8 and a Fox body Mustang E-Coil. Big difference over stock, but it lacks the timing retard feature for hot weather starting that the Duraspark box has. I retained both ignition systems so I can switch if one fails.
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Unread 01-18-2014, 05:56 PM   #5
pinnick-46
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ive got a howell TBI setup, which i actually just had to replace the wiring harness because of, the guy before did some butchery putting it in, and i went and bought a crt HEI, and a borla header, and an offy intake, and a crower cam, should run decent when im done
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Unread 01-18-2014, 06:15 PM   #6
roadyrob
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never heard of the earlier heads being better but there are a few things you can do to yours. theres a casting ridge in the ports, remove it with a die grinder. you can also port match it to your intake, theres a video on youtube to show you how. you can also mill about 20 thousands off to up the compression.
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Unread 01-18-2014, 08:30 PM   #7
fishadventure
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My understanding is that the old carbureted 258 heads moved air better and offered better torque at low RPMs as compared to the "late" 4.2 heads and definitely better at low RPMs than the 4.0 heads. 4.0 heads are much better at higher RPMs when used with fuel injection.
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Unread 01-18-2014, 09:37 PM   #8
jeepguy85
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Quote:
Originally Posted by fishadventure
My understanding is that the old carbureted 258 heads moved air better and offered better torque at low RPMs as compared to the "late" 4.2 heads and definitely better at low RPMs than the 4.0 heads. 4.0 heads are much better at higher RPMs when used with fuel injection.
What years are we talking with old 4.2 heads? That engine was used for a good while.
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Unread 01-18-2014, 09:48 PM   #9
GrEmLiN
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Can you get casting numbers of the heads in question? Or splits of the years? I might be able to cross reference something amongst my AMC information related to this topic. (A good source of information on the AMC 6 & 8 would be BullTear performance. Look em up.

I've thought about taking the 258 out of my 76 AMC Gremlin and somehow integrating it with my Jeep stuff but the 70s vintage 258 has a few differences from my Jeep's 258, like the thermostat housing is different, I'm sure the timing covers and water pumps might be different. The heads could be a different casting but it's possible that the valve seats in the newer head might be more robust due to better metallurgy etc. The 4.0 head on the 4.2 block gets its reputation not just for higher-RPM flow and power but I believe offers a higher compression ratio... can anyone confirm this? I'm not sure if the 4.0 combustion chambers are smaller for the smaller motor and FI, or larger for better quenching.

Maybe someone can chime in with more info.

If you're going to lengths to change the cam and manifolds, the head flow theory goes out the window. The aftermarket cam and manifolds will require different air flow rates than factory and thus what may work for one combination may hold back another. Run whatcha brung, can't go wrong with those go fast goodies you picked out.

Also, for cheap thrills and lots of torque, the AMC V8 is a great option. Hands down the easiest V8 swap into a Jeep. Mounts are almost the same, electrical and fuel and cooling and bolt patterns are all compatible.
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Unread 01-18-2014, 10:01 PM   #10
jeepguy85
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Quote:
Originally Posted by GrEmLiN
Can you get casting numbers of the heads in question? Or splits of the years? I might be able to cross reference something amongst my AMC information related to this topic. (A good source of information on the AMC 6 & 8 would be BullTear performance. Look em up. I've thought about taking the 258 out of my 76 AMC Gremlin and somehow integrating it with my Jeep stuff but the 70s vintage 258 has a few differences from my Jeep's 258, like the thermostat housing is different, I'm sure the timing covers and water pumps might be different. The heads could be a different casting but it's possible that the valve seats in the newer head might be more robust due to better metallurgy etc. The 4.0 head on the 4.2 block gets its reputation not just for higher-RPM flow and power but I believe offers a higher compression ratio... can anyone confirm this? I'm not sure if the 4.0 combustion chambers are smaller for the smaller motor and FI, or larger for better quenching. Maybe someone can chime in with more info. If you're going to lengths to change the cam and manifolds, the head flow theory goes out the window. The aftermarket cam and manifolds will require different air flow rates than factory and thus what may work for one combination may hold back another. Run whatcha brung, can't go wrong with those go fast goodies you picked out. Also, for cheap thrills and lots of torque, the AMC V8 is a great option. Hands down the easiest V8 swap into a Jeep. Mounts are almost the same, electrical and fuel and cooling and bolt patterns are all compatible.
Swap to a 304? If I were doing a V8 swap, 304 would be the last on my list. IMHO, the 304 was a huge POS. I would take a 258 over it any day. If I were to go through the time and trouble to swap in a V8, LS1 gets my vote.
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Unread 01-18-2014, 10:12 PM   #11
GrEmLiN
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304 is hard to find... And about as efficient as a 258.
All AMC V8's are the same block and bolt pattern... there's no small or big block AMC V8, the 304/343/360/390/401 block are all the same externally.

So a 360 / 5.9L from a Jeep Grand Wagoneer or J-truck, motorhome, etc are easier to come by, since they were used up through the 1991 model year. Easy to get parts for, very reliable, and one of the best V8s ever made. AMC hit the jackpot with their 3rd gen V8. They didn't suffer from many of the problems the SBC does, or even a few of the SBF's ailments. And it's still in the spirit of keeping things Jeep.

win/win/win.
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Unread 01-18-2014, 10:22 PM   #12
jeepguy85
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Quote:
Originally Posted by GrEmLiN
304 is hard to find... And about as efficient as a 258. All AMC V8's are the same block and bolt pattern... there's no small or big block AMC V8, the 304/343/360/390/401 block are all the same externally. So a 360 / 5.9L from a Jeep Grand Wagoneer or J-truck, motorhome, etc are easier to come by, since they were used up through the 1991 model year. Easy to get parts for, very reliable, and one of the best V8s ever made. AMC hit the jackpot with their 3rd gen V8. They didn't suffer from many of the problems the SBC does, or even a few of the SBF's ailments. And it's still in the spirit of keeping things Jeep. win/win/win.
My DD is a 2012 Chevy Silverado. 5.3 liter, excellent engine. I had an 05 before, same engine. It used a little oil, but it was still a great power plant. I would not hesitate to swap one into my Jeep. Very common and very easy to get parts for. We will have to agree to disagree on that.
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Unread 01-19-2014, 07:15 AM   #13
fishadventure
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Quote:
Originally Posted by GrEmLiN View Post
Can you get casting numbers of the heads in question? Or splits of the years? I might be able to cross reference something amongst my AMC information related to this topic...I've thought about taking the 258 out of my 76 AMC Gremlin and somehow integrating it with my Jeep stuff but the 70s vintage 258 has a few differences from my Jeep's 258, like the thermostat housing is different, I'm sure the timing covers and water pumps might be different. The heads could be a different casting but it's possible that the valve seats in the newer head might be more robust due to better metallurgy etc. The 4.0 head on the 4.2 block gets its reputation not just for higher-RPM flow and power but I believe offers a higher compression ratio... can anyone confirm this? I'm not sure if the 4.0 combustion chambers are smaller for the smaller motor and FI, or larger for better quenching...If you're going to lengths to change the cam and manifolds, the head flow theory goes out the window. The aftermarket cam and manifolds will require different air flow rates than factory and thus what may work for one combination may hold back another...
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Originally Posted by jeepguy85 View Post
What years are we talking with old 4.2 heads? That engine was used for a good while.
I am uncertain of what exact years of the 4.2 head are preferred. I have a 1978 AMC 258- next time I am over at the shop I will clean the goo and get a casting number- Now I am interested in 'knowing' this, too but IIRC anything before 1981 counts. Not sure. Maybe someone will chime in with real data?

Valve seats: this is another question I have had. Are those old heads acceptable for use with unleaded fuel? Leaded fuel used to keep the valve seats from wearing; unleaded causes problems for some older motor's valve seats.

As far as compression: the 4.0 head gives more compression because the combustion chamber volume is smaller in relation to cylinder volume. That doesn't make the 4.0 head special or better other than if you have the octane to use the higher compression, you *can* make a little more horsepressure with it. As far as the 4.0 head being superior: it flows a lot better at mid to high rpms. For my kind of wheeling and road use, this doesn't do much for me, but for mudding, bogging, or sand drags I don't think you can beat the stock 4.0 head even as an easy swap.

Both the 4.0 and the 4.2 stock cam lobe specs are the same at .253 and I believe they both have 1.6:1 ratio rockers, so a cam change WILL make a difference. There are one or two years of 4.0 cams that have longer duration cam profiles (I have been told) but other than that, stock-for-stock, a carbureted motor would probably love a mild cam and header. A small-ish diameter header would likely be an all-around improvement, but would especially benefit lower-rpm power.

Those are the things I know, some opinions based on non-jeep experience, and some questions raised about things I don't know. Loving the discussion
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Unread 01-19-2014, 08:17 AM   #14
roadyrob
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[
. Loving the discussion [/QUOTE]

it is an interesting thread, there are so many ways to take a 4.2. AMC did us tinkerers right when they built it. I've seen posts from people having dynoed 235HP, more than double the stock numbers.

back to the stock head, clifford claims a 60% increase in power by running chevy valves. they also claim you can mill the head up to .060 ( with their deep dish pistons) I took .020 off mine and can still run 87 gas.

http://www.cliffordperformance.net/h...jeepfacts.html
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Unread 01-19-2014, 08:22 AM   #15
jeepguy85
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I love the 258 and always will. I think it is one of the best engines ever put in a Jeep. The 304, they really missed the mark on.
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Originally Posted by Exparrot View Post
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Quote:
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