The motor is SHOT.

[bonedaddy]

So I have no choice but to replace the motor. Can anyone help clarify exactly what year blocks are a direct replacement for my 83 SX 4 258? I've been told 75-85, but the eagle ran till 88, I'm confused.

[carnuck]

That's wrong info. '80 to '90 (last year of 4.2L in YJs) have the boss on the block for mounting the front axle. Previous years didn't. Pre '00 4.0L will also bolt in with the 4.2L accessories, oil pan and timing cover bolted to it (I planned to use 4.0L accessories, but I can't afford the $350 serpentine pulley for the York AC compressor) Make sure the holes are already drilled/threaded in the front axle bosses (they went away after '00 I was told) or do that before installing (it's a pain AFTER installing!)
   There might be other minor changes (like the coolant temp sensor and "massaging" the intake to fit the head) but it's pretty much a bolt in otherwise.

[TLC87Eagle]

If you plan to keep your original head, I believe you have to find a block in the 1981-1986 range, as they are the only ones that have the 7/16" size head bolts. But if you change the whole engine, 1980-1990 258s will work, providing you get the matching cylinder head with it. As far as the 4.0L swaps are involved, I'm not too familiar with all the possible interchangeabilities, but there are a good deal of parts that are compatible with both engines.   

[carnuck]

You can ream the holes for the headbolts in the 4.2L head to slightly bigger than 1/2" to use your head on a 4.0L bottom end.

They are available from Jeep YJs from '80 till '90, FSJs '80 till '88 (make sure it has the casting for the  front axle/pinion hanger on the driver's side!) as well as most 4.0Ls in Jeeps till '00, but '87 to 08/90 is the easiest.

The BEST option is a 4.2L from an 87/88 Eagle or Wrangler '87 till '90 (it'll be carbed) since they will have the aluminum non-leaky valve cover stock.

Putting one on my Eagle was the best upgrade (besides the 727) that I have done so far.

If you find a parts Grand Wagoneer or J10 ('83 to '88) with 4.2L and 727/NP229 (or NP228) you can put the whole drivetrain in with only a few changes (best done before installing)

Oil pan (motor) and accessories (if they are different), shifter linkage on the trans from your old one, front yoke from the NP129 to the FSJ tcase (you'll need to add a low range shifter) and the tail stock from the NP129 to the NP229/228.

NP208 is different and requires a 5" longer driveshaft in the rear and mods to the OEM tailcone to attach a speedo cable.

[bonedaddy]

Thank you so much for this amazing info, this really helps me move this along, and adds a ton of options. I'll let you guys know when I find one, cross your fingers. Thanks so much!!

[TLC87Eagle]

Sure thing, you will finds loads of great info and helpful people on here. I know if it wasn't for the Nest, my Eagle probably wouldn't be in as great of shape, or may not even be on the road. But thanks to a member that sold a 2.35 axle carrier, my Eagle will be on the road for years to come.

I actually have my original 86 casting engine sitting in my garage. I retired it 2 years ago because of high miles and it was making a lot of blow-by. Good candidate for a rebuild though, but it seems you are too far away for that. I'm sure there are AMC straight 6s much closer than me. Plus a small part of me wants to keep the numbers matching engine, just in case our Eagles start to pick up in the collector car world. 

[bonedaddy]

 Ok, so it took forever but I'm gonna pull the trigger. Just want to clarify if you don't mind helping. I can get a 4.2 out our an 89 Wrangler at a good deal, and I'm just making sure of what I have to re-use from the old motor. It has nice Pace Setter headers, which I think have to go, and I have to switch the oil pan, correct? If you could please throw me a simple list, I'll go for it. I'm not concerned with wires etc, I'm gonna run this like a rat rod as simple as I  can.

[BenM]

Oil pan and oil pickup need to be Eagle. If you reuse the intake/exhaust manifolds from the Eagle you're set.

You may need to use some brackets from the old motor depending on options and what was modified to clear the headers. Most of the driver's side accessories will come off anyway when you remove the manifolds.

Axles and mounts should bolt up just fine. There was very little change after 82 on the 4.2 engine or its emission system.

Check your flex plate for cracks around the bolt holes before you put the "new" engine in.

[carnuck]

motor mounts and axle hangers have to be Eagle too.

[bonedaddy]

Thanks!! What is the reason I can't leave the wrangler intake/exhaust manifold? Just for curiosity sake. I don get why different parts fit the same block? I understand the whole oil pan thing.

[eaglefreek]

The intake should be the same as the Eagle. The exhaust will probably interfere with the Eagle's front axle.

[BenM]

The intake should be the same as the Eagle. The exhaust will probably interfere with the Eagle's front axle.

AMC used a few different brackets for the throttle cable as well.

Mostly, it's easier to just disconnect the manifold set from the wires, exhaust, and block and then put them back in as a unit. No worries about ports, sensors, etc. There's a dozen little doodads on the intake that could be slightly different.

You could always test-fit the headers to see.