rompnXJ

So a couple weeks ago I was out wheeling and broke a motor mount, which caused the mechanical fan to slash the radiator open, which led to a nasty overheat and consequently melted the #6 piston.



So I replaced the #6 piston & pin, replaced all the rod bearings, and replaced the rings on all pistons with the gaps set 180* apart from each other. The cylinder walls didn't have any apparent damage so I naively buttoned everything back up without honing the cylinders.

Anyways, it ran great for about a week, (Took a couple days for the rings to fully seat), good power, no oil consumption, but then I started noticing a low rpm stumble, which I tracked down to low compression in cyl #6. The engine still runs great with the exception of the low rpm stumble.

As it sits now, cylinder 6 has 81psi dry, 124psi wet.
Cylinders 1,2,3,4, & 5 are ~150 dry , all are within 5 psi of each other.

I'm in the process of tearing it back down, and while I would love to drop the block off at a machine shop with a blank check; funds are tight.

My current plan of attack is to use a 240-grit ball hone on cylinder 6 & re-ring piston 6, with the exposed rod journal taped off. I figure if it doesn't work, I'll only be out some labor..

Anybody have any thoughts on this??

Hayden98XJ

If your budget is pretty tight I would consider having a machine shop bore the number 6 cylinder out the least possible (.001 or .002) and then run an oversized piston in just that cylinder. It may not be ideal to run just one cylinder oversized but it sounds like theirs no cylinder wall damage in the other cylinders.

rompnXJ

Thanks for the reply, I hadn't thought of that. Would running a single oversize piston cause a noticeable imbalance?

Anybody know the average/typical price of having a single cylinder bored vs boring all 6?
Just curious... but if I do end up having to pull the block from the vehicle, I'll probably end up going .020" over on each cylinder.

Hayden98XJ

I have only heard of a company that has done this that sells rebuilt engines. I'm not sure how good it is for the engine though... I agree doing all 6 would be a better idea if you do end up pulling it.

rompnXJ

Update!

Ended up running a 180grit flexhone through it, took me about an hour to tape off every little hole, crack, & crevice lol. Here's what I ended up with...



Looked OK to me, but wtf do I know

put her all back together, and after a 2 hour drive the rings fully seated and the thing ran great ...for 3 days. then compression in #6 dropped back down....SH**. Guessing the bore is tapered from the overheat.

So my new plan of attack, is to pull a motor from the junkyard and do a full rebuild; hone, rings, bearings, gaskets, that whole bit. But I have a question...

Those of you that have had 4.0's apart know that the 96+ 4.0's have flimsy thin crappy pistons that are very prone to dropping their skirts in the oilpan. Is there any reason I couldn't take a motor from say, a WJ, and put 87-95 pistons in it?

Fred/N0AZZ

What did you really expect using a band aid when it needed major surgery??

You need a complete engine overhaul and yes that includes the head as well. If you don't do it at the same time it will give up in short order being the weak link.