2001 Cracked Piston

I've got a 2001 with a cracked #5 piston, the skirt came off in pieces. haven't taken the head off yet but from the bottom it looks like the cylinder might be ok. Does anyone know what caused this? I have searched on here and seen other references to cracked pistons but no one has put a theory out there.

JP1234,

Typical OEM pistons are cast aluminum, which are low strength. To strengthen them, steel inserts are cast into them around the piston pin. Unfortunately, cast aluminum and steel have different coefficients of thermal expansions. So as the piston heats up, the cast aluminum and steel expand at different rates. Result? High thermal stresses occur between the cast aluminum and steel inserts and the metals eventually separate. Over time, the cycling induced high thermal stress causes fatigue cracks to develop in the piston and failure occurs.

Solution? Replace cast piston with forged high strength aluminum pistons which have no metal inserts cast into them. Forged pistons also run about 80F cooler and do not develop any internal thermal stresses. Forged pistons are competitively priced, readily available and offer long term trouble-free operation. After you crack a couple of cast pistons, and go through the expensive rebuild process (i.e. engine disassembly, cylinder re-surfacing or re-machining, reassembly, etc.) you become a believer. Many racers use forged aluminum pistons for a very good reason.

Best regards,

CJR

no need for a forged piston in an otherwise stock engine. Plus there are no off the shelf forged pistons for the 4.0L. That means custom, which are usually $400-$600. Just get a set of hypereutectic cast for under $200.

As for why there could be any number of reasons. Dropped prior to install, cylinder bore distortion, excessive piston to wall clearance, defective piston, etc. Have any pics?

His piston also didn't have a steel insert.

jp1234 said:

I've got a 2001 with a cracked #5 piston, the skirt came off in pieces. haven't taken the head off yet but from the bottom it looks like the cylinder might be ok. Does anyone know what caused this? I have searched on here and seen other references to cracked pistons but no one has put a theory out there.

Click to expand...

How many miles on it? As noted by the other posters, there are several types of pistons. The most common that I have heard of in the Jeep 4.0L are cast pistons. These are the weakest pistons but generally hold up pretty well in basic engines without power adders. The next step up are hypereutectic (sp??) pistons which is an aluminum alloy with additional silicon added for strength. These are typically used in higher performance engines but are not as strong as the cream of the crop: Forged pistons.

The reason I asked about mileage is that the rings will eventually wear the cylinder walls down creating a loose fit. On high mileage engines it is possible for the increased gap to allow the pistons to slap against the side and this can cause the piston skirts to break.

Here is a presentation describing the different types of problems. Note slide 11 (cylinder bore distortion).....

http://www.mlcmotorfactors.co.uk/Trouble%20Tracers/PistonTT.pdf



HTH

It has 113,000 miles, I understand the general limits of oem pistons, I'm looking for a reason that #5 cracks. If you search on here you will see a few references to this piston. I don't have a picture but it's smaller pieces of the skirt up to the oil ring on both sides.

Pictures would be good if possible. A cracked skirt would cause me to think that the casting had a flaw.

Eventhough the pistons are cast, they are fairly strong for an OEM. Absolutely no need for Forged pistons for a stock motor, or even mild forced induction.

It sounds like its from the pin boss down, which could also be an elongated pin boss. Did you ever drive through a lot of water, like something that could get sucked into the intake.

I can't find if the stock pistons are cast or hypers.

I have forged pistons in my stroker, but I don't need the forged part I just wanted to use the longer rod. They do offer some mental assurance I suppose.

I just bought a 2001 with what I thought was a rod knock. Turned out the #2 piston skirt was broken. 107k miles, owner reported good oil pressure and said it ran cool. Cylinders had no ridge, and original hatch marks are still visible in bore. A quick measurement gave me only about 0.001" of cylinder taper.
The other pistons had scored skirts.

While I could get by with a light hone and a new set of pistons, I'm going the stroker route with a set of forged KB944's and a 0.020" overbore.:sunshine:

Have a machinist measure your cylinder bores and go from there. Cast hypers are harder but more brittle than plain cast. They'll take more abuse, but when they break, they break real good! A good choice if you don't abuse it too much.