I can't answer how long it will last. Mine does it. Its embarrassing. I take good care of my cars and it rattles like a paint can.
Jdm motors are about $1200 around here.
Subaru makes two size pistons A and B. the A is larger
On the top of your motor behind the alternator it says what piston they used.
Mine is AA AB matches which cylinder used what piston.
You can install an A piston in a B cylinder but not many choices for the A piston. other than rebore with oversize piston.

 

If it were mine, at 175,000 I would just let it be. If it were a fresh build I would have to do something about it, but it is not. Engines can run for a really long time with piston slap before they will grenade because of it, most die of other causes before they die as a result of piston slap.

There is a .06mm(.00236") oversized piston in both "A" and "B" form that Subaru used as a TSB fix for piston slap. Along with the normal oversize options.

It is probably cheaper/faster to just drop a replacement engine in vs deal with correcting the piston slap in your block. To do it right the cylinders would need to be bored, which means the cases need to be split to remove the rods, at which point you are into full rebuild territory.

 

MY04 XS has 166K and in the last 2 years, the cold piston slap is LOUD. 10 minutes later it's very mute, then after 30 minutes or more of driving, no longer audible.

Being the original owner and planning on keeping it 4 or more years, until our children are out of college, I just plan on normal maintenance.

Next biggie for me will either be the next timing belt change,
Oil pan, it's starting to rust and bubble and I have treated it with POR already,
Or the clutch, although it is still working fine, I've gone further than any other manual I have owned and I have been driving since 1970.

Maybe at the 210K mark, if not sooner, I MAY pull the engine to do all the above, just as preventive maintainance .

So I guess unless it causes a problem, other than a cold diesel sounding engine, I would suggest to live with it. Subaru has stated over and over that it causes no harm to the internals of the engine.

 

Thanks for the responses, I am going to have the valves and lifters checked when the timing belt is replaced and switch to 10w-30 dino oil to see if that helps any. It probably wont but it couldn't hurt. I guess for now I will just live with it. I have been looking at used motors to have rebuilt by the local shop (he used to specialize in Porsche, so I trust him with the flat 4 motor. Even raced a few in the 80's and 90's). I know it will be costly but I don't have the time, tools, or knowledge to rebuild a motor that will last for years. But if I can squeeze a couple more years out of it then that will be great.

 

Switch to 50 weight racing oil.

 

Seeing as how I change my oil every 3 weeks or so that will get way too expensive for me. I used to run Red Line in my Saturn and it was about $50 every time I changed the oil.

I suppose Rotella T is a little too thick... But it is equivalent to SAE 5w-40 I think. Oh well, I will stick with the normal stuff for now.