Piston erosion on intake side
 

I recently pulled the engine out to do a rebuild and found all 4 pistons like this
https://cimg7.ibsrv.net/gimg/www.mia...39b8deb6f4.jpg
On comparison the exhaust side:
https://cimg4.ibsrv.net/gimg/www.mia...1ead6d610c.jpg

Engine is a stock BP6D 10:1 with 190.000km, 90k of them turboed 14 years ago. It's been running 14-15 psi on a 2871 with slow boost ramp to be glentle with rods.

It looks worse in the picture than it is

What can be causing this? imperceptible knock? only damge to the intake side?
Head is perfect, no signs, no nothing
I'm planning on rebuild with forged rods + stock pistons, so i need to address the cause in order to not run in to the same problem again.

wow, fascinating. I mean, 14 years at 1 bar with stock pistons... worst you could do is get another 14 years at the same power level. I suspect there aren't too many out there who have anywhere near that amount of use. No noticeable wear on the far softer and more porous head would seem to me to be a significant clue. Injectors leaking?

Piston is running too hot. You should use a different piston if you are going to forged rods. If you want to stay with a cast piston you might want to consider a BP4W. Although a forged piston will be more equal to a sturdier aftermarket rod.