They must be available.....

Problem with our kind of engines is that the piston and sleeve have matching materials regarding the expansion. Materials can change due different suppliers so there is a chance that the piston of one production serie does not match the sleeve of other. Beside that due tolerances the dimensions aren't equal, they can differ up to some microns so you must have several pistons try to match your sleeve in size. To prevent that they just deliver a compleete matching P/S set.