DOJ

A double open jaw, or DOJ, is a wrench with two open mouths facing opposite directions on a single handle. Unlike a conventional wrench, which has a fixed jaw on one end and an adjustable jaw on the other, a DOJ presents open jaws at both ends, each independently adjustable. This design allows mechanics to work in tight spaces where a standard adjustable wrench cannot fit, or to quickly swap between two different bolt sizes without retrieving a second tool.

The mechanism consists of a single shaft with two separate worm screws, one at each end, each controlling an independent jaw assembly. Tightening one worm screw opens or closes the jaw on that end; the other jaw operates independently. The jaws typically open to 30 or 35 millimeters, covering most common fastener sizes in aircraft maintenance, from M6 through M12 bolts. The tool is usually made from drop-forged steel with hardened jaws to withstand the torque loads on structural fasteners.

In aircraft maintenance, DOJs are valued in engine compartments and tight panel-access areas where space is severely constrained. A mechanic working on engine bleed air fittings or hydraulic line connections often cannot rotate a single wrench freely, and a DOJ's compact profile reduces the swing radius needed. Because both jaws are adjustable, the tool also accommodates slightly worn or damaged bolt heads better than a fixed-size wrench would.

Limitations and wear patterns

The dual-mechanism design introduces a trade-off: each jaw is inherently less rigid than the jaw on a single-ended wrench because the worm screw is shorter and the jaw structure more slender. Under heavy torque, the jaws can slip on severely rounded fasteners. Additionally, the worm screws wear relatively quickly in high-use environments, and once worn, the jaws no longer clamp reliably. Regular inspection for jaw rocking and worm wear is essential before relying on a DOJ for critical fastener work.

The DOJ name reflects its geometry straightforwardly: double, meaning two jaws; open, meaning they are not fixed in width; jaw, the gripping surfaces. It is distinct from a double-ended wrench, which has a fixed size on each end, or from an adjustable wrench, which has only one adjustable jaw. In hangar practice, DOJs are usually stored in a specific slot in the toolbox because their unusual shape does not nest like conventional wrenches do.