Overheated block = bad block??

I bought a 2000 XJ with a 4.0 ltr 6 with what I was told was bad head gasket. Turned out it had a hole in the #1 piston. I was going to have it rebuilt and a friend told me that once a block has gotten hot enough to melt a hole in a piston that it is no good and I should look around for a used engine if I want to rebuild it. Is there any truth to my friends comment?

I replaced a piston that had melted in a mazda 4 cylinder once. It was fine afterwards, I didn't even pull the engine or use new bearings, just swapped a burnt one for non burnt. There are other reasons for burning a piston than overheat, in that case the timing was way screwed up and the detonation melted the piston top.

The only way to know for sure is to pull it and take it to a machine shop and have it looked over.

Agree with 87Manche.

Your 00 may have the nefarious 0331 head casting, which will eventually bite you in the butt when it cracks between cylinders 3 and 4, unless it has already been replaced.

You might be better off to source another engine, an 02+ 4.0 with the improved "TUPY" foundry head.

joe_peters said:

You might be better off to source another engine, an 02+ 4.0 with the improved "TUPY" foundry head.

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Agree wholeheartedly, but wasn't it 2003 when the TUPY heads arrived?

I vote new engine as well. The old one might be more of a hassle than its worth

CumminsJeep said:

I vote new engine as well. The old one might be more of a hassle than its worth

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if you're rebuilding it one block is as good as any other, as long as it passes some pretty basic standards.
The crank and cam journals must be true.
There can't be any cracks in it.

Other than that you're going to be replacing/machining everything else in a proper rebuild, so it doesn't matter.

I see no reason to toss the block away because of a burnt piston. I would try and diagnose what happened to the piston first. Is there splatter? A clean hole or burnt hole? What does the connecting rod bearing look like for that cylinder? Plastiguage the mains bearings and see the tolerances. Look in the pan for other chunks of metal.

The motor might be fine, the bottom end might not need rebuilt. It's entirely possible that piston was damaged by a foreign object, or a broken spark plug. Might be a good candidate to drop a new piston in that cylinder and run it. The 4.0 is a glorified tractor motor, you're not turning a small block at 8 grand here, it doesn't need to be balanced and blueprinted. It may be entirely possible to hone the affected cylinder a touch, drop a new piston with some new rings in it, re-ring the other pistons and run it for another 100K miles.

Really what the OP needs to do is find a good old boy that's local to him, have a sixer of his favorite brew in the garage and have someone come look at it. A good visual inspection of the block will answer all of these questions.

I have fixed 3 XJ's with blown head gaskets and burnt pistons, all had the same thing happen... a compromised cooling system, they kept driving baking the head gasket, usually craking the head between #3 and #4 and once the engine gets hot enough it will burn the edges of one or two pistions. Depending on how hot it got. The 3 I have done didnt didnt have any major scoring to the cylinder walls so I was able to hone them with a dingleberry hone on a drill without taking too much material. You can buy new single pistons and ring sets and while I was at it I would replace the rod bearings to be safe since you will already have the oil pan off. All 3 I fixed are still running great and I've driven mine daily since '05. The only thing you will probably notice is some "piston slap" when you frist start the jeep everyday but once the engine warms up this will usually go away, 4.0's are notorious for this to some extent whether they have been overheated or not anyway.

birchlakeXJ said:

Agree wholeheartedly, but wasn't it 2003 when the TUPY heads arrived?

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Your right, my bad, 03+ for TUPY.