Anvil
Affiliate link — we may earn a commissionIn an impact driver or impact wrench, the anvil is the output shaft that receives the hammer strikes and transfers them to the bit or socket. The hammer spins freely until the bit meets resistance, then it strikes the anvil in rapid blows, delivering high torque without the tool twisting your wrist. On impact wrenches, the anvil has a square drive (1/4, 3/8, or 1/2-inch) where the socket attaches. On impact drivers, it's a 1/4-inch hex collet that accepts hex-shanked bits. Anvils are hardened steel, but they do wear over time, especially on high-torque wrenches used for heavy automotive work.
Why It Matters
The anvil is the part that takes all the abuse. A worn or damaged anvil develops slop, which reduces torque transfer and can round out sockets. On a wrench you use daily, inspect the anvil for mushrooming or rounding. Replacing a worn anvil is cheaper than replacing the whole tool.