tommy bar

A tommy bar is a short cylindrical rod, typically 4 to 8 inches long and 0.25 to 0.5 inches in diameter, inserted through a hole in the head of a wrench to increase leverage. The bar acts as an extension of the wrench handle, allowing you to apply greater torque to stubborn fasteners or adjustments without swinging a longer wrench in confined spaces. It is used in machining, toolroom work, and equipment maintenance wherever nut runners, box-end wrenches, and adjustment wrenches are fitted with a cross-hole or eye.

The bar must be rigid steel or iron; it carries bending stress and can snap if undersized or made from brittle material. A loose fit in the wrench hole allows the bar to rotate and slip rather than transmit force; a tight fit risks stripping or cracking the wrench head. The bar is inserted perpendicular to the wrench handle and gripped at one end, so its length and position on the handle govern the mechanical advantage. A 6-inch bar used 2 inches from the wrench pivot gives better leverage than the same bar used 6 inches out.

The term comes from its original form and use: a simple iron bar, sometimes called a tommy rod or tommy iron. Tommy likely derives from the generic male name or from British slang for a soldier's kit, reflecting its common, everyday character rather than a proprietary design. The bar has changed little in over a century; it remains one of the most basic and durable aids in the toolroom.

Common failures occur when the tommy bar is too short to move the fastener, or when force is applied in the plane of the wrench head (sideways) rather than perpendicular to it, which bends the bar or cracks the hole. Applying rotational force rather than pulling or pushing in a straight line also introduces unwanted stress. A skilled operator places the bar level, applies steady pressure, and knows when to switch to a powered wrench or breaker bar rather than trying to force a seizing fastener with hand tools.

The tommy bar remains standard kit in toolrooms and machine shops where wrenches with cross-holes are issued. Modern variants include collapsible bars for saving space and adjustable bars with multiple grip holes, but the basic 0.25-inch steel rod remains the most reliable. It is too simple to obsolete and too effective to abandon.