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4.0 maybe a rebuild?


NAXJA Forum User
Peabody, MA
so i haven't had luck with my jeep lol

i bought it with a blown motor. that also was not the original. its an 88 and had a 91-95 engine in it. but cylinder 5 was destroyed. and 3-4 had a blown head gasket. swapped engines with my 90 4.0 and then the flexplate cracked and i assumed it was a knock.

grabbed a $500 junkyard motor, it was spotless inside. no sludge at all. however that knocking came back and again this time i assumed it was the flexplate it was used. nope... I'm guessing a piston skirt broke and the the piston shattered.

ok so now that i have the worst luck. im thinking of putting the 300k engine back in. but its got some blowby could i just get away with honing and new rings or should i just grab a rebuild one? and will i net any improvement using the 7120 or 0630 head with all the renix stuff?

You won't notice any difference with the HO heads, but you can use them.

I made one engine out of three Renix motors for a family member. they ran the crap out of it. The blocks are hard. Pick the best block and go for it.
i think what i will do is pop the engine i have back in it. and at a later date buy a rebuilt one.i suppose i will just stick with the renix head if im not going to net anything.
Iron blocks will take a hone and new steel rings very well most of the time. I would pull the pan on your current motor and verify broken piston skirt first though. Ive seen where a lot of people developed rod knock from bad bearings and had good luck with just popping new bearings in with a little crocus cloth work on the crank.
I had rod and main bearings done on my '92 back around 175K miles due to a lower-end knock. I'm at 256K now and counting, no subsequent issues with them.
I had a crank go bad. I put in a VG used one. Also, some will replace broken pistons with used ones. I was involved with a 3 to 1 4.0L build. I supplied the block. It came from one of the 2000 cylinder head being cracked. One of the cam lobes was wiped out.
The oil pan had been pulled and what's left of the piston is on the floor in my garage. The shirt is in one piece and not all beat up. The wrist pin punched a hole in the cylinder wall so coolant filled the oil. I'm going to do some work while the engine is out and I may just try and pick up a rebuilt one.
That is a bummer to find a broken piston and a punched hole in the cylinder wall! I've had my share of broken cast pistons and broken cast iron rings over the years. That taught me to stop using cast pistons and only use forged pistons. Some cast piston designs use steel reinforcement plates cast into the aluminium. Unfortunately over time, the different thermal expansion rates between steel and aluminium causes catastrophic piston thermal-cycling fatique failure to occur. Also to eliminate future grief, use either ductile iron piston compression rings or steel piston compression rings if you can find them. Good luck on getting another engine into your XJ.

Best regards,
Yeah, you're not going to be able to "buff out" a hole in the cylinder wall.

Going "rebuilt" (I'm guessing something like a Jasper) is probably a good idea - at least then you get some sort of warranty on it for at least a while.