These metal reinforcement plates solve the problem of locks pulling through hollow core doors. Slide one into the lock bore before installation - it spreads the load and prevents the thin door skin from failing under stress. Each pack contains 5 reinforcers.
They work with cross bores from 1.5" to 2.125" on doors up to 1.75" thick. You bend them to fit (they ship flat) using the included template. Commonly used in apartment retrofits, mobile homes, and anywhere builder-grade hollow doors need upgraded locks without replacing the door.