So I have been reading about the 4.0 piston slap and why they blow pistons. To my understanding Chrysler wanted to reduce friction in the engine so they undersized the pistons and that created pistons slap. I have also been told that the reason the 4.0 likes to blow pistons is because of a casting error in the pistons. The piston slap combined with the casting error causes them to break the skirts off then they soon blow up from there. I was told that this mainly applies to engines 98 and after.


I have a 97 engine all torn down that I am considering overhauling to prevent it from blowing a piston as my 98 and 2000 have both blown pistons and I want avoid this happening again. The 97 has 170,000 miles on it and I really don't have all that much cash so if possible would like to avoid boring it for new pistons but I will if it's likely it will blow.

 

Maybe it's because peeps use 50k mile oil change intervals and only open the hood after "something" explodes?
Usually things like pistons falling apart can be attributed to some type of severe abuse.

 

Never heard any of that and I was Service Manager at a Jeep dealer.

Chrysler needed to REDUCE the piston to cylinder clearance to make customers happy.

It's not likely to blow.

 

Inline 6's tend to flex a bit. That leads to bore distortion, which can be hard on pistons.
Ford's 300-6 had similar problems, the OEM cast pistons were the weak link there. I've not yet heard of a 4.0 with aftermarket pistons cracking a skirt, so infer what you like from that. We'll see what happens when those rebuilds hit 250,000 miles. Perhaps it makes no difference.

 

Second that. If the 4.0 is running right you wont have problems, sounds like looking too deep. Mind you most have cracked exhaust manifolds that will make noise, some have slapping timing chains, and if your fuel pump has 1ook on it, you will have a lean misfire at idle when starting up. Now, that, could cause a problem, you would want to fix that asap.

 

'Flex a bit?' Seriously? The inline engine is one of the most if not the most sturdy block ever designed, it's used in all the big equipment like earth movers, tanks, 18 wheelers....caterpillar stuff.,,

 

I think you will hear more the other way about how many miles they run when taken care of just a little, most will run 300,000 mi. and not loose a piston.

 

You got that right. The Ford 300 is a tank, one of the most reliable engines ever made. It's been installed in everything from tractors to trucks, generators and trenching machines, stump grinders and forklifts.

Because it's just about unbreakable, like our 4.0s. Flexing? Get real.


OP, wherever you read that load of horse manure about piston slap and 4.0s blowing pistons, don't waste any more of your time there.

I have 266k on mine, and it's got factory perfect compression numbers, and mine is in no way unusual.

 

You're pretty unusual though!!! LOL.

 

I resemble that remark!

 

At 315,000 when the head starting to let go on my 2001 XJ, I replaced the head after I found no evidence of blow by & had excellent compression. As long as you keep oil & coolant in the engine, and prevent the two from mixing , the engines last a long, long time without much more than spark plugs & gaskets when needed.

Sure some folks have piston skirts breaking off and other issues, but there wouldn't be as many XJ's on the road today if many/most of them had problems... even though they haven't been made in 15 years, where I live they are on every street corner.

 

Sure do!!!

 

Exactly.

 

Giggle if you like.
The idea the block had some longitudinal flex and bore distortion isn't mine. It came from Chrysler's own documents during one of the several lawsuits over 4.0 piston slap. Snippet of the testimony below.
And that is exactly what Chrysler tried to fix in 1996. By stiffening the block with additional ribbing and a bearing brace.
How did they stiffen a block that had "no flex"? Good question.


Absolutely it is. Nobody suggested it wasn't. Geez, you'd think I offended someone's religion.
Go back and read what I actually wrote: OEM cast pistons were the weak link.
Fact is, they were. Did it happen to the vast majority of engines? Hell no. But if you're going to have a hard failure in that engine, it's likely to be a piston.
We saw it more in the automotive engines than the stationary/industrial. Cast aluminum pistons with steel expansion gussets + thermal cycling.
Certain 351W engines had the same piston problems, guess I might as well **** those guys off too.


You can't read more than a page or two here without someone mentioning piston slap and/or chunks of skirt in the oil pan as a possible source of noise. Some of us have had it happen to our own Jeeps despite conservative use and proper maintenance. It's a legit concern when you hit higher miles...and we're talking engines that often have over a quarter-million miles on them.

Perhaps pistons are not "blown" as the OP mentions, but I really hope we're not arguing semantics. A failed piston is a failed piston. Nor is anyone questioning the longevity of the 4.0 or suggesting that they all have issues. But some do.
Recall the OP here has three jeeps, two with failed pistons. He's beyond an argument over "If" and wants to know "why".

 

I had this problem on my 98 TJ with only 150,000 miles on it. The number 4 piston broke from the oil ring down on one side. It did not score the block and it still had the factory honing marks on it. And it did not burn any oil wile it ran with the broken skirt. I was very good about changing the oil at 3000 miles and overall general maintenance. I think that the problem lies in the piston casting its self. Not the block. It is a problem but not enough of a problem to start recalling engines and replacing pistons. Now I have a 98 XJ with 276,000 miles on it and no signs of piston slap or any thing. Like said before it is hit or miss weather you are going to have a piston problem or not.
Now Tedparis if you are worried about possibly blowing a piston you could always buy a new OEM set, but I know that you stated that for financial reasons this is not possible. If it is a concern of yours see what it would be to send out the pistons and get them tested for cracks and any other stress issues that could cause them to break. Just remember that if you decide not to do anything and a year or more goes by after rebuilding the engine and one of the pistons goes you will have to pull the head again to remove it.