You can check the pistons for wear(diameter,taper and ring side clearance)to see if they are beyond the wear limits...but I've never seen a high-mileage diesel piston that didn't have a significant amount of wear on the ring grooves. Personally, I would replace pistons and liners. Its easier to do it once the right way instead of doing it twice. Remember,whatever you save on parts the first time through won't count much when it has to be taken apart again.
Regards,
Randy D.

------------------
W.R.Durrance
ASE Master Tech
Mercedes Tech 19 Years