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.
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Reach for the Lubrication
Spray lubricant, such as WD-40, is a non-viscous liquid 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 spray to penetrate the corroded surfaces, you can often remove the screw with an Allen wrench.
How to Use 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 through Mineral Deposits
Corrosion and mineral deposits on a screw can cause it to become fused to the valve stem. A stone-like 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, because the 1/8-inch Allen key you use for such a small screw is fragile, and if a small piece breaks off inside the screw, you'll be out of options for removing the handle without a lot of extra effort. Once the corrosive barrier is broken, apply a bit more penetrating oil and you're good to go.
Applying Heat May Help
Applying heat by holding a heat gun or hair dryer 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.
Use heat before you use spray lubricant, not after. Most spray lubricants are flammable, and while a small flame from a hex screw probably won't hurt you, it will definitely damage the finish on the faucet. If applying heat doesn't work by itself, wait until after the screw has cooled before applying it and attempting to turn the screw with your Allen wrench.