97 legacy outback - piston slap

I bought a 97 outback with a fried engine. Bought a used 2.5 engine with 70,000 miles on it. Replaced the engine, buttoned up everything and fired it up, heard a noise that to me was a slapping piston or something that went away within a few minutes of warm up. Sound is on both sides of the engine.

I've never owned a 2.5 engine, just my current 2.2 so I went searching. Apparently there is an issue with piston slap until it warms up.


<li type="disc">Diagnosis: The pistons are engineered for operation at temperature. In an effort to save weight in the engine, the piston skirts were shaved down. When the pistons are cool, however, they do not have sufficient contact with the cylinder to keep from 'slapping' or rattling against the cylinder sides. At temperature, due to thermal expansion, the pistons match up properly with the cylinder and do not rattle. <li type="disc">Reported solutions: In some cases, SOA has rebuilt the engine short block, while in others SOA prescribed "countermeasure pistons" with longer skirts that do not slap. Most dealers will initially state the sound is "normal." <li type="disc">Detrimental effects: SOA has said that the noise causes no engine problems or abnormal engine wear; Subaru engine expert CCR Engines of Colorado has said that the engine may experience a 10% reduction in life."


So I am thinking I am good to go with this engine transplant but I thought I would run it by the forum. Engine runs well on the road.

Yes you are fine. My 2.2 does it too. It's kind of universal for older subies of this age to do it.

It was just a bit weird hearing an engine sound like a diesel engine at start-up.

Originally Posted by Tr00b

Yes you are fine. My 2.2 does it too. It's kind of universal for older subies of this age to do it.

Start to worry when she rattles at 2000 rpm and up when warm that is main bearing slop.

+1 on Troob's comment ... very common on these engines. I've had it on my Subie for 15 years and the engine is just fine after 100K miles.