81 cj5 engine swap 2.5 to a 4.2 ???? - JeepForum.com
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Unread 04-18-2010, 11:32 AM   #1
defeo
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1981 CJ5 
 
Join Date: Apr 2010
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81 cj5 engine swap 2.5 to a 4.2 ????

I have a 81 cj5 with the gm 2.5 litre 151 engine.when I got it it ran terrible so I replaced almost everything on the engine externally hoping I could to get it to run right. never got it to quit missing so I pulled the head did not find anything wrong,pulled the engine and found a lifter that was beat to death and the camshaft lobe on the intake on the #4 piston was rubbed down to nothing. I have been debating on rebuilding this engine but in the process I have come across a 4.2 litre out of a 80 cj5 with the bell housing that fits my SR4 tranny. Will my tranny hold up with this engine will I have to change the motor mounts. Everything apears to look like it will work but with my past experiences on such things there are always hang ups. So any tips or knowledge that could be passed on would be great. Thanks


Last edited by defeo; 04-18-2010 at 02:01 PM..
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Unread 04-18-2010, 01:15 PM   #2
rodjr
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the motor mounts are different, u will need a radiator. i bieleve the fuel lines on the wrong side, clutch linkage may b different, wiring on your 151 is all set up for gm so there will b changes. i have a 1980 with a 2.5 (151) and i can tell u a small block chev or 4.3 v6 is probably easier, your bellhousing is allready sbc pattern. the sbc deffinately wouldnt b more work than the 4.2, at least all the wiring/fuel lines plug right in. the sr4 sucks i wouldnt trust it behind either one. also the 1981 2.5 isnt amc its a gm/pontiac.
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Unread 04-18-2010, 02:47 PM   #3
defeo
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Thanks, I actually looked into the small block but that meant that I had to swap the transmission as I have read that the small block was to much power for the SR4 and that the 4.2 would not be to hard on it. The fuel pump on the 4.2 is on the same side as the 2.5 and the 4.2 I am looking at comes with the bell housing that will fit my SR4. The wiring well there is not much there with either engine. I was really interested in the 4.3 but that engine is fuel injected and I would have to go into a whole other project finding the right intake manifold and carb. Still debating though... I am really wondering if the stock 4.2 engine mounts will line up with my frame mounts bolted to the sr4 transmission. Thank you for the correction on the engine brand GM not AMC. For some reason I thought that Amc was owned by GM at one point in time but not sure that is a different discussion. LOL
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Unread 04-18-2010, 03:53 PM   #4
rodjr
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the 4.2 mounts will bolt in but the trans crossmember will need moved, this will change shifter location a little, no big deal all factory bolt holes are already there. if u can find an early 4.3 (think 1986) like from a astro van they were factory carb motors. ya the sr4 will work with a 4.2, they came with em, but alot of folks feel there marginal even for that. i just did a full rebuild on my 151, with hypereutectic 9.25:1 pistons, valve job, gasket match, blended bowls, schnieder racing 256 dur cam, stock carb and exhaust. its pretty damn snorty with the new 4.10 gears and still gets 18-19mpg all the time, just a thought. i can send ya some specs and even tune info to build a good engine combo that i know works well, and can b done for reasonable $. i have some sbc swap parts inc 153 tooth flywheel, and t176 trans if u decide to go that route ill cut ya a deal. i was gonna put in a 283 im building, but i really like going 260+ miles on a tank of gas.

Last edited by rodjr; 04-18-2010 at 04:11 PM..
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Unread 04-18-2010, 06:11 PM   #5
defeo
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Thanks the information you just gave me was good stuff. So I will have to move the transmission crossmember back ?? I looked at mine and it is already all the way to the rear of the vehicle on the factory holes. there aren't any bolt holes behind it on the frame. There are three different sets of holes in the bracket itself and the transmission is already mounted to the farthest set of holes to the rear. I am pretty set on the 4.2 I am liking the 4.3 Idea though I am researching into it. The four cylinder I considered rebuilding but I have the whole front end off of the jeep and wanted to upgrade. My last jeep had the 2.5 as well it was a very reliable engine. I am just looking for a bit more power. the sbc route is a little more modifying than I am looking to do and gas is stupid expensive right now.
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Unread 04-18-2010, 06:59 PM   #6
rodjr
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ok, yours is a 5 mines a 7 must b some different mounting going on. my skid is all the way forward, and the trans (sr4) is bolted in the rear set of holes. odd, seems the front half of a 5 would be the same, but i guess not. im sure the 4.2 will b pretty straight forward swap, but i have to wonder if the mileage will b substantially better than a 4.3? and 4.3 parts im sure are cheaper, and there still being produced.

Last edited by rodjr; 04-18-2010 at 07:37 PM..
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Unread 04-18-2010, 07:31 PM   #7
Pacfanweb
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Quote:
Originally Posted by defeo View Post
Thanks, I actually looked into the small block but that meant that I had to swap the transmission as I have read that the small block was to much power for the SR4 and that the 4.2 would not be to hard on it.
You read wrong, the SR4 is dead meat behind the 258. Worst transmission put in a CJ, EVER.
Jeep actually swapped them out for original owners back when they were new, even when the vehicles were out of warranty.

You're going to need a tranny anyway, so reconsider that small block.
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Unread 04-18-2010, 08:13 PM   #8
defeo
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Well I did not know that about the sr4 if it was a good thing it would not be in this jeep the way things are going. LOL well this forum has been helpful it seems everyone is a fan of the small block conversion.
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Unread 01-17-2011, 02:06 PM   #9
MMPSM
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Quote:
Originally Posted by rodjr View Post
the motor mounts are different, u will need a radiator. i bieleve the fuel lines on the wrong side, clutch linkage may b different, wiring on your 151 is all set up for gm so there will b changes. i have a 1980 with a 2.5 (151) and i can tell u a small block chev or 4.3 v6 is probably easier, your bellhousing is allready sbc pattern. the sbc deffinately wouldnt b more work than the 4.2, at least all the wiring/fuel lines plug right in. the sr4 sucks i wouldnt trust it behind either one. also the 1981 2.5 isnt amc its a gm/pontiac.
Are the 2.5 Frame Mounts different, or, if they are the same, are they mounted higher on the frame than the 4.2 needs to be? I know that the vulcanized rubber parts are different for the 2.5 and the 4.2.

My swap of 2.5 to 4.2 results in the engine being higher in the bay than I think it should be using the 2.5 steel frame mounts.

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