Only Expert Mechanics do this

Why are mechanics drilling into the bolt holes of an engine?! ⚙️🔧
When rebuilding a car engine, the threaded holes fill up with rust and old coolant. If you install a bolt into a dirty hole, you get a false torque reading and blow the head gasket! Worse, liquid trapped at the bottom will cause "hydraulic lock" and physically crack the engine block in half! Mechanics use a specialized "thread chaser" or "tap" to safely carve out this grime, leaving perfectly clean threads behind!