We see many engines damaged every year by "locking devices" BTW: The thread pitch is national extra fine and is a special aircraft pitch, which is normally cut on the lathe! We simply hold the engine upside down [glow plug installed] fill the cylinder through the exhaust port with after run oil (Do Not Use Silicones) and let it "hydraulic lock". This evenly distributes the load over the entire surface of the piston. When done simply pull the plug and drain the excess oil!!! No harm done and cost pennies