4.0L Head Swap on a Jeep 258 4.2L Engine
What you need to install an HO 4.0L cylinder head on your older Jeep 4.2L inline-six engine
I'm having the 4.2L 258 in my 1985 Jeep CJ-7 rebuilt by a local machine shop. The shop just called and said the head is no good and that I need to get another one that is a rebuildable core. I'm having trouble locating a 258 head, but 4.0L heads are everywhere. Could I use the 4.0L head on a 258 block? Would my intake and exhaust manifold bolt up?
We're a little surprised that you're having trouble finding a good 258 head, as the engines were used for a long time in quite a few Jeep models. If you haven't been able to find one locally, you might try some online resources, such as car-part.com or even eBay.
If you continue to strike out on a 258 head, you have a few different options. The short answer to your question is, yes, a 4.0L head will physically bolt to a 258 block. Your intake and exhaust manifolds will work on the later head with some minor modifications, namely grinding the intake for the dowel pins present in the 4.0L head. You can use the 4.0L gasket as a template to see where the modifications need to be made. There might also be some minor accessory bracket things that need to be addressed, but the whole thing is largely a bolt-in affair. If we were in your shoes, however, it would be hard not to justify retrofitting the entire 4.0L fuel injection system to your CJ-7. The 4.0L system is fairly simple and extremely reliable. To do this, you would need to harvest the intake manifold, injectors, fuel rail, throttle body, engine wiring harness, computer, distributor, and crank sensor from a 1991-1999 YJ, XJ, or TJ. If you use a 1996 and newer donor, you'll also benefit from OBD-II diagnostics. Paring down the engine harness to only what is needed to make the fuel injection work will take some effort, but it's fairly straightforward with the aid of a factory service manual. The 4.0L EFI offers vastly superior drivability and performance over the computer-controlled Carter BBD system that's on your CJ, and it's worth doing if you're going to a 4.0L head anyway. If you're leery about using a junkyard head, Edelbrock () offers a brand-new aluminum 4.0L head that's also a viable option.
Aside from the wiring, the only modifications required to run the 4.0L EFI is a crank sensor. Stock 4.0L crank sensors mount to the bellhousing and count the notches machined into the flywheel, which your 258 flywheel doesn't have. You'll want to invest in a Hesco () conversion kit that moves the crank sensor to the harmonic balancer.
Other than the carburetor, the downside to retaining the stock carbureted intake manifold is that the ports on the intake are round, while the 4.0L head intake ports are roughly rectangular. You can smooth the transition between the two with some porting work, but with a stock engine, we're not sure you're going to notice much of a difference anyway.