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I have a 90 YJ with a blown 258 and I have a opportunity to buy a 258 out of a 83 CJ. Finding a block out of a 87-90 is hard it seems...

What, if any, difference is there between the blocks? I will reuse the correct intake, exhaust, etc., but will the blocks simply swap over?

Thanks for any help.
 

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that's what she said
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it is the same engine. i have an 83 cj7 258 4.2L and a 90 yj 258 4.2L. engine parts are swapped from one to the other frequently. the block and head are the same.

Source: http://en.wikipedia.org/wiki/AMC_Straight-6_engine#258
258


Engine bay of a 1980 AMC Spirit AMX with numerous emissions controls


The 258 cu in (4.2 L) in-line six was produced from 1971-1990. It featured an undersquare 3.75 in (95 mm) bore and 3.895 in (98.9 mm) stroke; it was otherwise similar to the 199 and 232. Later 258 models (starting with the 1980 model year for California AMC Concords and Spirits, 1981 for California Jeeps, California Eagles, and 49-state Concord and Spirits, as well as in 1982 for 49-state Eagles and all other applications) are equipped with AMC Computerized Engine Control (CEC) system. This engine is "reliable, inexpensive, and has horse power and torque peaks at low RPM."[5]
Applications:

 
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