Moen, like many manufacturers of plumbing supplies, offers faucets whose handles attach to the faucet's valve stem via a hex screw, a screw whose head has a six-sided hole that fits a hex wrench or Allen wrench. Over time, exposure to water can cause corrosion to build up on the screw, fusing the screw to the valve stem and making it difficult to remove. You can usually loosen a corroded hex screw by using penetrating oil and a bit of force to lubricate and break the mineral deposits on the screw. Once the screw is lubricated, you can usually remove it with an Allen wrench.
Penetrating oil is a non-viscous lubricant often supplied in a pressurized can. Many manufacturers include a nozzle or tube that attaches to the can of penetrating oil so the oil may be precisely applied. Once the nozzle or tube is attached to the spray button on the can, you can apply the oil to the hex screw with minimal over-spray. After you wait a minute or so for the oil to penetrate the corroded surfaces, you can often remove the screw with an Allen wrench.
An Allen wrench is a hexagon-shaped steel rod bent into an L configuration. To apply the greatest torque while using an Allen wrench, insert the short end of the wrench into the hex screw head. To keep the wrench from slipping, press with one thumb the bend in the Allen wrench toward the screw and use your other hand to turn the wrench counterclockwise to loosen the screw.
Cracking the Corrosion
Corrosion and mineral deposits on a screw can cause it to become fused to the valve stem. A stonelike mineral barrier can prevent the penetrating oil from reaching the inner threads of the hex screw. To break the barrier, you may have to insert the wrench into the head of the screw and tap the bend in the wrench with a hammer. Do this carefully to prevent bending or breaking the valve stem. Once the corrosive barrier is broken, more penetrating oil may be applied.
Applying heat by holding a heat gun close to the hex screw will cause the metal in the screw and valve stem to expand. With sufficient expansion of the metal the mineral deposits can break, allowing the hex screw to be removed. After the screw has been heated and allowed to cool, apply more penetrating oil before attempting to turn it with an Allen wrench.