Make: Volume 78

Begin by cutting a square groove, ⁄" deep by

¼" wide, in your piston. The best way to cut the

groove is with a lathe. A router is nearly as good

if you have a table for it. If you have neither of

these items, then you’ll have to improvise. I built

a simple wooden jig that allowed me to rotate the

piston over a vise holding a Dremel tool with a

cylindrical grinding head attached (Figure

C

). By

carefully rotating the piston I could accurately cut

grooves in the Delrin piston.

Once the groove is cut, place the O-ring in the

groove and check the fit. There must be about

⁄" of extra width in the groove for the necessary

O-ring expansion. In ...