A coil spanner is an engineering tool with various uses.
In 2372, Lon Suder murdered fellow Voyager crewman Frank Darwin by striking him in the back of the head with a two-kilogram coil spanner. (VOY: "Meld")
Chief O'Brien asked Worf to give him the coil spanner when he repaired a console on the OPs. (DS9: "Bar Association")
On a salvage mission to Empok Nor in 2373, Boq'ta asked Amaro to retrieve a coil spanner so he could loosen a bolt. Amaro returned with a flux coupler instead, to find Boq'ta killed by a Cardassian soldier. (DS9: "Empok Nor")
On Enterprise (NX-01), Lorian used a coil spanner to make adjustments to a plasma injector. (ENT: "E²")
In 2375, Ezri Dax used a coil spanner to tune a phase amplifier on AR-558. (DS9: "The Siege of AR-558")