This part acts as a cushion when the piston/driver rects bainto the cylinder after g fired. Because this part get so much abuse it has a tendency to craand fall apart. If the part is damaged or falls out you should rep it.This part is compatible with the following machines: Porter Cable FN250B 16 Finish NailerThis part provides a seal for the Head ValveCondition: NewManufacturer Part Number 904685Colour:As shownMaterial:plasticPackage Contents:1 * Piston StopOnly the above package content, other products are not included.Note: Light shooting and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.