Cylindrical Constraint
A pin or cylindrical constraint controls the axial movement of a cylindrical surface, while allowing angular movement, and keeping radial movement fixed. It is particularly useful when a surface must move in one or more directions, but must be fixed in the remaining directions—for example, a piston that slides within a cylinder, but remains fixed to the inner cylinder wall and does not rotate.
1. Click > 
Pin. The Pin Constraint dialog box opens.
Pin. The Pin Constraint dialog box opens.2. Specify a Name for the constraint or accept the default name. Optionally, click the color swatch adjacent to the Name box to change the color of the icon, the distribution, and the text displayed for the constraint.
3. Select the cylindrical surfaces to constrain. You can select the geometry before or after you open the Cylindrical Constraint dialog box. The References collector displays the selected surfaces.
4. Select one of the following options from the axial displacement 
list :
list :◦ Free—Allows axial displacement.
◦ Fixed—Constrains axial displacement.
5. Click OK to create the constraint and save it.