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4.0 block compatability

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Have an ‘83 SX4 we are building and have already done the 4.0 head swap. Using FiTech FI and an hei distributor. The driver side axle mount has already been modified to clear the 4.0 factory header and exhaust. Steering bracket has also been modified to work worth 4.0 head. For driveline, we have an nv3550 mated to the original t4 4 speed bell housing and clutch.

Turns out the original 4.2 bottom end has more miles on it than we thought with a lot of ring blow by once we got it running. So contemplating actually going with a 4.0 block with our 258 crank to create the popular jeep crowd stroker if we are redoing the bottom end anyway, since I believe all the headaches associated with the 4.0 side of it have already been worked out in our combo.

From what I’ve found, the only additional mod might be to the drivers side of the 4.0 block which has some added webbing where our modded front axle bracket would mount. To be sure, I have a few questions:

1. Since we would use the 258 crank and I assume balancer, is it safe to also assume we could use the 258 V belt crank pulley and then all the 258 accessories? Are the 4.0 mounting holes on the front of the motor all the same to allow for this (power steering, alternator and ac brackets)?

2. Again, since the 258 crank will be used, I assume we could also reuse the 258 manual flywheel as well as the 258 starter (will the 258 starter bolt up ok)?

3. I assume the existing 258 Eagle motor mounts will bolt to the 4.0 block with no mods needed?


I ran into issues with the v belt stuff on the power steering bracket. I forget why, but I just modified it and it worked. starter matches flywheel. motor mounts should be fine.

I've not done this personally so am just going off what I've read.  But when using a 4.0 block, just make sure it still has the spots to mount the front drive train (left rear of the engine and on the right side near the motor mounts).  If I remember correctly they stopped including those on the block somewhere along the line.  I think I also read that at some point those mounting spots were still there but you may need to tap the holes.

Good luck, sounds like a fun project!

I'm in the process of doing this swap myself and have already ground down the rib on my 4.0. Attached are some pictures of what it looks like and its location after being ground down ignore the pink sparkly paint that's on the ground down section it's just some cheap fingernail polish I used for clearance confirmation.

Thanks for all the great info gang. Much appreciated. I also stumbled into a detailed 4.2 to 4.0 swap write up that may be useful to others as well:

One more thing I didn’t think about, we already had a new 4.0 head on the 258 which we are going to reuse. It’s the one from Clearwater that had the extra triangular water passages already blocked off, which was great for the 258 so we didn’t have to mess with plugging them. They were welded shut before they milled the head. If we are now going to the 4.0 block for the stroker, do I need to open those water passages back up in the 4.0 head? And if so, can I just drill a smaller hole for each? Thanks.


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