sorry for the misunderstanding. Lets try again. No the piston doesn't wear out. The high rate of modulation or cycling actually wears the valve body out at the 6 o'clock position, ie the bottom of the bore. The hole in the valve body that excepts the piston becomes egg shaped in the bottom of the bore allowing fluid to bypass the piston on top or 12 o'clock position. The repair has to be made one of two ways. Replace the valve body housing with a new one to the tune of $150 min. All the pieces in the old valve body should drop right in the new valve body no soft wear will be disturbed. The other fix is reaming the bore with the tool I mentioned from Sonnax. When purchasing the reaming tool a new over size piston is provided as part of the kit I think or may be sold separately. The over size piston diameter is matched to the diameter of the reamer. The 2 choices are replace the valve body or repair the old one. If you want to find out the condition of your existing valve body pull the piston out of the bore in the hole marked as 50.100. Then stick your finger in the bore. If the vale body is shot you will feel a ridge or step inside the bore. Example; have you ever taken a motor with high miles apart? At the top of the cylinder (combustion chamber) there is usually a ridge worn in the top of the cylinder where the compression ring stops. Any good mechanic doing a engine over haul has a tool called a ridge cutter for cutting the ridge out of the engine block cylinder. This is the same principle that happens in the valve body. That's what this reamer tool from Sonnax does. It's kind like punching out a cylinder bore to install over size pistons or cutting the ridge off the top of a cylinder to save the engine block. Hope this clarifies it. Got a little long winded. Sorry. PS you may have trouble trying to get the piston out of the valve body. Use compressed air from the back side and blow it out. It goes without saying when pulling these vale bodies apart make sure all the springs and things go back in the correct order. Good luck. PSS; This tread also applies to 3 series bimmers and possibly 7 series. The key to this is a ZF 5HP 19 transmission. It doesn't matter what car it's in, it's still a ZF 5HP 19 tranny.