My 2001 Subaru Outback Wagon developed loud piston slap before hitting 30,000 miiles. 2.5L automatic.

The car was always well maintained with proper oil changes but the light tick sound that started kept getting louder over time.

Our local Subaru Dealer inspected it and said they are "unable to verify condition", so they wouldn't fix it. They could clearly hear it knock, but always recorded it in the paperwork as "the customer states" this or that. They would never confirm it themselves or be responsible. Later as the knock got worse the dealer insisted that "Engine knock is normal for a Subaru", which was a pretty stupid thing for a dealer to say.

The noise was worst when the engine was cold, and 70% better as it warmed up due to piston thermal expansion filling the gap between the undesized, out-of-round pistons and the cylinder bore. If they didn't use such an extremely short piston skirt (almost non-existent) vs traditional old style pistons,,, then it may have gotten by.

The noise kept getting worse over the next 5,000 miles until it knocked all the time, really bad cold and kinda bad warmed up. It was clearly worse as I dropped the car into gear too, vs being in Neutral. As the noise got worse I took it to the dealer again this time it was loud enough they could not deny it. They kept the car for 2 days, had no loaner available, and later they said they fixed it. All done. When I got it back it still knocked the same and I found out they replaced the timing belt tensioner instead of really fixing it. Apparently that can mimic the same noise in some cases. And then Chilson Subaru would not work on it anymore. They refused.

So I tried another Subaru dealer and they agreed it sounded pretty bad and they would take care of it. 2 days later I got it back again .. still knocking. Turns out they also put in a timing belt tensioner. Duh. I just had that done. And it still knocked of course.

Then neither dealer would look at it any further. Engine knock is Normal they insisted. As it got worse I tried again 6 times over the next year and they refused to fix it. One day I happened to catch the US Central Region Subaru Rep on-site at the dealer. He heard it knocking WHILE it was warmed up. I demonstrated that it didn't clear up when fully warmed up which had been their previous excuse and I had it knocking on Video too.

They said there is no amount of knocking that is too much if it's cold. It can echo off buildings 3 blocks away and that is still fine. But I showed that in my case the sound was ok in neutral and got worse from simply putting the car into drive. Then I pulled the spark plug wire off cylinder #4 (and used a dummy spark plug) and showed them that with no load on that cylinder the noise was completely gone. It was cured. Gee.. they could not explain why it had one special piston that was noisy and 3 "normal" ones so this proved that piston slap was a defect. The corporate rep said I was right and ordered the dealer to fix it. and they did. sort of.

The dealer tries to weasel out of anything they can, so their 3rd repair was putting in 1 piston under warranty and it sounded fine for 50 miles and then it got bad again since they didn't replace the block. I could see that the removed piston was visibly scratched up again. They would not let me keep the bad piston, they sent it back to the factory. Then they repaired it for the 4th time time and that time they fixed it right with a new short block and new pistons. The problem was finally solved. The motor was quiet from then on, and then I sold it.

Subaru knew about the issue for years and didn't fix it until 2005. Several sources also say the turbo models didn't have that issue because they had different piston rings and pistons, and the H6 motor was also fine but it's pretty hard to find one since it was not an option, literally. You could not buy a H6 motor. not unless you paid another $6000 to get the LL Bean or VDC version that cost abour $31,000. So almost all have a 4 cyl since they would not sell anyone a 6 cyl for years.