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ozusa 09-16-2019 10:29 PM

4.0 swap for 4.2L vs serpentine pulleys issue
Can anyone offer advice on what 4.0 engine fits serpentine groove pulleys?
Understand the 4.2 has certain length crank snout and others are different.
Want to rebuild a 4.0 that can use serpentine belts pulleys with a compressor.
All research and experience doing similar work appreciated.


Axhammer 09-17-2019 03:02 AM

Yes, the 4.2 was a long snout crank, and the 4.0 used a shorter snout, and some 4.2 between 1987 and 1990. There is a spacer available to get around this issue. I will have the same problem since Iím putting a long snout crank in a 4.0 block (in my CJ-7) from what I understand the spacer is an easy way around it.

dirtdudeaz 09-18-2019 08:27 AM

Are you building a stroker (4.2 crank in the 4.0 block)? What is the casting number on the crank?

ozusa 09-18-2019 04:07 PM

Spacer to use serpentine pulleys sounds good. Will plan that route.

Yes plan to stroke or get a titan block ready to match with the 4.9 #7120 head I have and will add Holley EFI. Having disassembled my daily driver yet. It is an 83 CJ7 with original 258 in it. Is there something I should watch for with the crank casting #?


Axhammer 09-18-2019 06:24 PM


The 258 (4.2) crankshafts we use as the basis of most of our stroker builds can be found in two basic forms, twelve or four counterweights. The heavier 66 lb., 12 counterweight, with itís 64mm nose, was in use from 1971 to 1981 and the lighter, 46 lbs., four counterweight crankshafts, in use from 1982 through 1990. This four-counterweight crankshaft can be found in early form with a 64mm nose or in the later 54 mm nose. Both 258 crankshafts use a 3.895" stroke. Many feel the 12-counterweight design is the stronger of the two.

All 4.0L crankshafts utilize the same main journal and rod journal sizes as the AMC crankshafts. All 4.0L
Crankshafts are nodular iron, use only 8 counterweights and have the 54mm nose. The stroke in the 4.0L engine is 3.44".
Year_____Crank Casting No._Part No.
1971_____3199738________4488665 For Borg Warner automatic, small pilot
1972-74__3214723________8120508 12 counterweights, 66lb, 64mm nose
1975-79__3214723________8125450 12 counterweights, 66lb, 64mm nose
1980_____3214723________8132619 12 counterweights, 66lb, 64mm nose
1981-86__3235477________3235444 4 counterweights, 46lb, 64mm nose
1987-90__3727___________53005535 4 counterweights, 46lb, 54mm nose

No, there is nothing wrong with any version other than maybe the 1971 automatic crankshaft, but Iím sure a good machine shop could open it up if you needed it. Iím using a 3235477 light weight, long snout crank in my build. It polished up within specs for standard bearings, so Iím good to go concerning my crankshaft.

dirtdudeaz 09-19-2019 07:59 AM

As axhammer beat me to it; I was asking to let you know if you even need a spacer. The 4.0 cranks use a 54mm snout, so if you have a 3727# casting, you don't need a spacer or to machine the crank 1CM.

You could also make a spacer if you had access to a lathe. Take an old balancer and cut yourself a 1CM piece from the mounting portion. It would save you $30 for a thick washer with a keyway cut in.

ozusa 09-28-2019 09:42 AM

Makes sense with a spacer and sounds easy.
Thanks for the insight, am looking for all the serpentine belt pulley, bracket and components now.

The time now is 08:14 PM.

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