Tried and true, this conversion labret post by Neometal is made with implant grade titanium in a large variety of sizes to fit any of your 18g threadless ends.
These labret posts come in 18g, 16g, & 14g, but will ONLY fit 18g threadless ends.
It's a great investment for someone with a collection of 18g threadless ends, and other 16g and 14g piercings.
A variety of 18g Neometal ends to fit these labrets can be found here.
Threadless jewelry is a push pin technology. The removeable end (dark grey) has a slightly curved pin. This pin straightens out when inserted into the shaft (light grey). The resulting spring tension force holds the two pieces together securely.