Simple, effective and virtually indestructible. 
If you’re looking for a cheap but excellent piston head, you can’t go wrong with these! 
Comes with an o-ring. 
Please note the internal spacer uses an 8.9mm square type screwdriver end to tighten however a large flat-head will also work. 
We recommend using Loctite 266 threadlock on the screw to ensure a reliable fit.