Non HO 4.0 to 4.2 Swap

I did alot of searching but couldn't find the right answers. Everything I found was for the 4.0 HO's.
I have a '89 4.0 XJ with compression of 50 psi, with oil added to the cyl. 150 psi. My nephew has a good running '89 4.2 out of a YJ that I could get for free.
If so what has to be changed.
If someone could point me in the right direction I would appreciate it.
My XJ is offroad only if that matters.

TIA

Guy

Well, I believe the 4.0 head can go on the 4.2 block. You could just swap blocks maybe? That way you could keep the fuel injection.... You would have to use the 4.0 flywheel, and dist. Perhaps searching the stroker posts will give you an idea of what is interchangeable and anything that requires modification.

50psi dry and 150 psi wet? Sounds like a re-ring to me!

"Wet" compression tests (usually done with about one tablespoon of clean engine oil) are usually done to point up whether low compression is due to a leak at the head gasket (the wet test will show no change) or that there is enough damage to the rings/bore (wet compression will be higher than dry.)

With that great a difference between wet and dry numbers, I'm inclined to think you're a candidate for both a re-ring and a re-bore - since you'll have to have the engine out for that, I'm going to go ahead and suggest "stroking" it as well (installing a 258 crank and rods,) since the actual work to install the stroker crank is minimal. You will need to have a shop change the connecting rods for you (they're pressed out, and inserted into heated "small ends,") but the crankshaft will otherwise drop right in with no special work.

Pull the head, cycle the pistons down, and inspect the bores. Scoring can usually be cleared up with a +.010" overbore, but it usually costs the same to get +.030" - and "thirty over" pistons and rings are also easier to find.

5-90

Bottleworks, Idefinitly want to keep the F.I.. Will my intake or cyl. head fit the 4.2?
5-90, I sucked up alot of mud water but not enough to hydro-lock. I think thats what killed my compression.
Right now I just want to get it back on the trails. Building a new motor is a winter project for most of us northerners. Our ride time is short enough. With all the searching I've been doing, I'll guaranty it'll be stroked by spring.

Guy

Muddy water? Yeah - wet sand. That would do nasty stuff - and don't bother going +.010 - go right to +.030".

Still, get yerself a 258 crank and rods if you can, and just do a stroker while you're about it.

And, if you spend a lot of time playing in mud and water, why for you don't have a snorkel? You really should, you know - the XJ air intake is right behind the left headlamp, and should be higher if you can manage it...

5-90

i'd like to know if you do the crank/rod swap if there's anything else needing to be changed ? and would the 4.0 head fit on the 258, nobody's really answered that part yet.
just curious as my bearing's in my '88 are going rapidly and i'm looking to swap out/rebuild mine very shortly and CHEAP. my broke ass doesn't need this now,lol.

anyone know what a rebuild goes for. might go talk to the machine shop and see what they offer as far as the rebuild or stroker set-up but don't wanna go in blind. if only i had a nice CNC mill and a smithy