What Loctite is and when to use it

Loctite is a brand of threadlocker and adhesive products that prevent bolts, screws, and other fasteners from loosening due to vibration or temperature changes. The most common version is Loctite 243, a medium-strength threadlocker that comes in a small bottle with a thin applicator tip. You apply it directly to the threads of a bolt or screw before assembly.

Use Loctite when you're fastening something that will experience vibration — an engine component, a piece of machinery, anything on a vehicle, or equipment that runs regularly. You should also use it on fasteners in damp environments where rust might cause them to seize. For permanent bonds between metal parts with no threads involved, Loctite makes separate adhesive products like Loctite Super Glue or Loctite Metal Epoxy, which work differently and are covered separately.

Loctite is not necessary for fasteners that will be removed frequently, fasteners in dry indoor environments with no vibration, or anything held in place by a lock washer or nylon-insert lock nut. If you're unsure whether a job needs it, the fastener probably doesn't.

Key Takeaways

  • Apply Loctite 243 to the threads of a bolt or screw by squeezing a small drop onto the tip, then thread the fastener in normally — do not soak the threads.
  • Let the product cure for at least one hour before the joint experiences vibration or load, and allow 24 hours for full strength.
  • Loctite 243 is removable with hand tools if you need to disassemble later; stronger grades like 271 require heat or power tools.
  • Clean old Loctite residue from threads with a wire brush or cloth before applying fresh product to a reused fastener.
  • Store Loctite with the cap tightly sealed and keep it at room temperature, as the product hardens if left open or exposed to cold.

Preparing the fastener and threads

Before you apply Loctite, the threads must be clean and dry. If you're reusing a bolt or screw that had Loctite on it before, scrape or brush away the old residue with a wire brush or a cloth. Dried Loctite looks like a thin plastic coating on the threads — remove as much as you can reach. If the fastener is visibly dirty or oily, wipe it with a clean, dry cloth or paper towel.

Do not apply Loctite to wet or damp threads. If the hole or bolt has moisture on it, let it air-dry or wipe it dry first. Loctite cures by reacting with metal, and water interferes with that reaction. For the same reason, do not apply it to fasteners made of plastic or nylon — Loctite is designed for metal threads only.

Applying Loctite to the bolt or screw

Squeeze the bottle gently to release a single small drop of Loctite onto the tip of the bolt or screw. The drop should be about the size of a match head — roughly 2 to 3 millimeters. You do not need to coat the entire length of the threads. One drop is enough for a standard bolt; use slightly more for a longer fastener with more threads, but still only a small amount.

If the bottle has a brush applicator instead of a pointed tip, dab the brush once onto the threads near the tip of the bolt. Do not paint Loctite along the entire length of the threads — a light coat in one or two spots is sufficient. The product will spread as you thread the bolt in.

Immediately thread the bolt or screw into the hole by hand or with a wrench, turning it until it is snug. Do not over-tighten. Loctite works best when the fastener is tight enough to stay in place but not so tight that you strip the threads or crack the part. As you thread the fastener in, the Loctite will distribute itself around the threads.

Curing time and when the joint is ready to use

Loctite 243 begins to cure immediately after you thread the fastener in, but it does not reach full strength right away. The product will be set enough to hold the fastener in place within one hour, so you can move or transport the item after that time. However, the joint should not experience vibration, load, or stress during that first hour.

For full strength, allow 24 hours before the fastener experiences heavy vibration or load. If you're fastening something that will run or vibrate soon after assembly — like an engine bolt or a machine component — wait the full 24 hours if possible. If you must use the item sooner, be aware that the fastener may loosen slightly until the Loctite fully cures.

Temperature affects curing speed. Loctite cures faster in warm environments (above 70°F) and slower in cold environments. In very cold conditions, curing can take significantly longer, so plan accordingly if you're working in a garage or outdoor space in winter.

Choosing the right Loctite strength for your job

Loctite comes in several strength grades, and the most common is Loctite 243 (blue), which is medium-strength and removable with hand tools. This is the right choice for most jobs: engine bolts, machinery fasteners, automotive work, and anything you might need to disassemble later. It holds firmly but can be unbolted with a wrench if needed.

Loctite 271 (red) is high-strength and permanent — it requires heat or power tools to remove. Use this only if you're certain the fastener will never need to come apart, such as on a critical structural bolt that will never be serviced. For most people, 243 is the better choice because it gives you options later.

Loctite 222 (purple) is low-strength and is used for small fasteners like set screws or fasteners that need to be adjusted frequently. It's removable by hand and cures faster than 243, but it's not strong enough for vibration-prone applications.

If you're unsure which grade to use, start with 243. It handles the vast majority of fastening jobs and is forgiving if you change your mind later.

Removing a fastener that has Loctite on it

If you applied Loctite 243 and need to remove the fastener later, use a wrench or socket that fits snugly and turn it counterclockwise with steady pressure. The fastener will come loose — it may take slightly more force than an unthreaded bolt, but hand tools are sufficient. Once the bolt is out, you can clean the old Loctite off with a wire brush or by wiping it with a cloth.

If the fastener is stuck and won't budge, apply penetrating oil (like WD-40) around the bolt and let it soak for 15 to 30 minutes. The oil helps break the Loctite bond. Then try again with the wrench. If it still won't move, apply heat with a heat gun or hair dryer to warm the bolt — heat softens Loctite and makes it easier to remove.

If you used Loctite 271 (red, high-strength), hand tools alone may not work. You'll need a heat gun to warm the fastener significantly, or in some cases a power tool. This is why 271 is reserved for permanent installations.

Storage and shelf life

Keep the Loctite bottle tightly capped when not in use. If the cap is left off, the product will begin to cure inside the bottle and will eventually harden into a solid. Once hardened, it cannot be used. A properly sealed bottle will last for years at room temperature.

Store Loctite in a cool, dry place — ideally between 50°F and 85°F. Extreme cold can thicken the product and slow curing; extreme heat can cause it to cure prematurely. If the bottle has been stored in a cold garage or car, let it warm to room temperature before opening.

If the applicator tip becomes clogged with dried Loctite, soak the tip in acetone (found in some nail polish removers or paint thinners) for a few minutes, then wipe it clean with a cloth. Do not try to force the clogged tip open, as this can damage the bottle.

Frequently Asked Questions

Can I apply Loctite to threads that are already assembled?

No. Loctite must be applied to the fastener before it is threaded in. If the bolt is already in place, you would need to remove it first, clean the threads, apply Loctite, and reinstall it. For a fastener that is already tight and not loosening, there is no reason to do this.

What if I use too much Loctite?

Excess Loctite will squeeze out around the bolt as you thread it in. Wipe away the excess with a cloth or paper towel before it cures. Using too much does not make the joint stronger — it just wastes product and creates a mess. A single small drop is sufficient for most fasteners.

Does Loctite work on plastic or nylon fasteners?

No. Loctite is designed for metal threads and will not cure properly on plastic or nylon. For plastic fasteners, use a nylon-insert lock nut or a lock washer instead. Loctite makes separate adhesive products for bonding plastic parts, but these are not threadlockers.

Can I apply Loctite to a fastener that is already under load?

No. The fastener must be removed, cleaned, and reassembled with Loctite. You cannot apply Loctite to a bolt that is already tightened in place. If a fastener is loosening under load, remove it, apply Loctite, and reinstall it while the joint is not under stress.

How do I know if Loctite has fully cured?

Loctite 243 will feel solid to the touch after one hour, but full strength takes 24 hours. You can tell it is fully cured when the fastener no longer turns easily by hand — it will require a wrench to move. If you need to check before 24 hours, try gently tightening the fastener with a wrench; if it does not move at all, curing is complete.