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Frequently Asked Questions
Can I use Loctite 242 instead of a gasket maker?
In this loctite 242 vs permatex ultra black comparison, the answer is no: threadlocker is a thin anaerobic liquid designed for the tight confines of bolt threads. It cannot bridge gaps between flange surfaces or seal against oil the way an RTV gasket maker does. Use gasket maker for surfaces, threadlocker for threads.
Can I use Permatex Ultra Black on bolt threads?
No. RTV silicone is a flexible paste, not a thread adhesive. It will not lock a fastener against vibration the way an anaerobic threadlocker does. For bolts, use a threadlocker like Loctite 242.
How long does Loctite 242 take to cure?
It sets in about 10 minutes and reaches full cure in 24 hours. Allow the full 24 hours before subjecting the assembly to stress or vibration.
How long does Permatex Ultra Black take to cure?
It becomes tack-free in about 2 hours and fully cures in 24 hours. Cure times vary with temperature, humidity, and gap size.
Is Loctite Blue 242 removable?
Yes. Blue 242 is medium strength, designed to be removed with standard hand tools. That is what distinguishes it from high-strength red threadlockers, which need heat for removal.
What should Ultra Black not be used on?
Per the manufacturer, do not use it on head gaskets or on parts in contact with gasoline. It is designed for oil pans, transmission pans, valve covers, timing covers, and differential covers.
Here is the honest truth up front: Loctite Blue 242 and Permatex Ultra Black are not really competitors. Loctite 242 is a threadlocker, a blue liquid you apply to bolt threads to stop them vibrating loose. Permatex Ultra Black is an RTV silicone gasket maker, a black paste you apply to mating surfaces to seal oil pans, valve covers, and transmission pans. They do different jobs, and most DIY garages eventually need both. This guide compares what each one does, its specs, and when to reach for which, so you buy the right tube the first time.
The short answer: buy Loctite 242 when bolts keep loosening from vibration. Buy Permatex Ultra Black when you need to seal a gasket surface against oil. They are not interchangeable, and using one for the other’s job will fail.
Table of Contents
Quick Picks: Loctite 242 vs Permatex Ultra Black
| Pick | Product | Best For | Check Price |
|---|---|---|---|
| For Bolts | Loctite Blue 242 Threadlocker, 6ml | Locking 1/4 to 3/4 in fasteners against vibration, removable | Check Price on Amazon |
| For Sealing | Permatex Ultra Black RTV Silicone Gasket Maker, 3.35 oz | Sealing oil pans, valve covers, transmission pans against leaks | Check Price on Amazon |
Loctite 242 vs Permatex Ultra Black: Side-by-Side Specs
All figures below come from the manufacturers’ technical data sheets, checked the same day this article was written.
| Spec | Loctite Blue 242 | Permatex Ultra Black |
|---|---|---|
| Product type | Threadlocker (anaerobic adhesive) | RTV silicone gasket maker |
| What it does | Locks threaded fasteners against vibration loosening | Forms flexible gaskets that seal against oil and fluids |
| Strength | Medium — removable with hand tools | Flexible silicone rubber gasket |
| Fastener/surface size | 1/4 to 3/4 in (6–19 mm) bolts | Gasket surfaces, fills gaps up to 0.25 in |
| Set time | ~10 minutes | Tack-free in ~2 hours |
| Full cure | 24 hours | 24 hours |
| Temperature range | -65°F to 300°F | -65°F to 500°F |
| Size | 6 ml tube | 3.35 oz tube |
| Typical uses | Valve covers, water pumps, alternators, oil pan bolts, mowers, bicycles | Oil pans, transmission pans, valve covers, timing covers, differential covers |
| Not for | Plastic parts, pure oxygen systems | Head gaskets, parts in contact with gasoline |
The table makes the split obvious: one is for threads, the other is for surfaces. Everything below follows from that.
What Loctite 242 Does
Loctite Blue 242 is a medium-strength anaerobic threadlocker. Anaerobic means it cures in the absence of air, which is exactly the condition inside a tightened bolt thread. You apply the blue liquid to the threads, assemble the fastener, and it locks the threads against loosening from shock and vibration while also sealing them against rust and corrosion.
The key spec is removability. Blue 242 is medium strength, which means the fastener can still be removed later with standard hand tools. That is the whole point of the blue grade: it holds against vibration but yields to a wrench when you need to service the part. The technical data sheet rates it for 1/4 to 3/4 inch fasteners and a service range of -65°F to 300°F, with full cure in 24 hours.
Typical automotive uses from the data sheet: valve cover bolts, water pump bolts, alternator fasteners, and oil pan bolts. Anywhere a bolt lives in a vibrating assembly and you will want to remove it again someday.
What Permatex Ultra Black Does
Permatex Ultra Black is an RTV (room-temperature vulcanizing) silicone gasket maker. You apply a bead of the black paste to a clean mating surface, assemble the parts, and it cures on exposure to air moisture into a tough, flexible silicone rubber gasket. It replaces pre-cut gaskets entirely or dresses them for extra sealing.
The key specs are oil resistance and temperature range. Ultra Black is formulated for maximum oil resistance with a patented adhesion system, sensor-safe and non-corrosive, rated from -65°F to 500°F. It fills gaps up to a quarter inch and resists the thermal cycling that cracks lesser sealants. Full cure takes 24 hours, with a tack-free skin in about 2 hours.
Typical uses: oil pans, transmission pans, valve covers, timing covers, and differential covers. Anywhere oil wants to escape between two metal surfaces.
Why They Are Not Interchangeable
This is the section that saves you from an expensive mistake. Threadlocker on a gasket surface does nothing useful: it is a thin liquid designed for the tight confines of threads, not for bridging gaps between flanges. RTV silicone on bolt threads is equally wrong: it is a flexible paste, not a thread adhesive, and it will not hold a fastener against vibration the way an anaerobic threadlocker does.
The confusion is understandable because both come in small tubes, both live in the chemical aisle, and both get used around engines. But the rule is simple: threads get threadlocker, surfaces get gasket maker. If you are staring at a bolt, reach for the Loctite. If you are staring at a flange, reach for the Permatex.
Application Tips for Both
For Loctite 242: clean the threads first, apply the liquid to the bolt threads (a little goes a long way), assemble, and allow the full 24-hour cure before subjecting the joint to stress or vibration. It sets in about 10 minutes, but full strength needs the full day. Do not use it on plastic parts, where it can cause stress cracking.
For Permatex Ultra Black: remove all old gasket material from the mating surfaces, apply a continuous even bead (about 1/16 to 1/4 inch), assemble the parts, and let it cure 24 hours. The product skins over in about 2 hours. Do not use it on head gaskets or on parts in contact with gasoline, per the manufacturer.
Both products reward patience. The number one DIY mistake with either is rushing the cure and then blaming the product for a leak or a loose bolt.
Price-Tier Reality
Both are inexpensive as automotive chemicals go: small tubes that last through many jobs. Neither purchase needs much deliberation on price. The cost that matters is the cost of using the wrong one, a leaking oil pan or a bolt that backs out, which dwarfs the price of either tube.
If your garage does not have either yet, the honest advice is to eventually own both. They cover the two most common chemical jobs in DIY car maintenance, and neither substitutes for the other.
Who Should Buy Loctite 242 vs Permatex Ultra Black
Buy Loctite Blue 242 if: bolts on your vehicle or equipment keep loosening from vibration; you service parts regularly and need the fastener removable with hand tools; you work on small engines, mowers, or bicycles as well as cars.
Buy Permatex Ultra Black if: you are resealing an oil pan, transmission pan, valve cover, timing cover, or differential cover; you need maximum oil resistance and high-temperature tolerance; you want to replace a cut gasket with a formed-in-place seal.
Buy both if: you do your own maintenance. These are the two chemical staples of a DIY garage, and they do not overlap.
The Verdict
The Loctite 242 vs Permatex Ultra Black question has a simple answer once you see what each product is. Loctite 242 locks threads against vibration and comes apart with hand tools. Permatex Ultra Black seals gasket surfaces against oil and heat. Buy the one that matches your job, threads or surfaces, and if your garage sees regular DIY work, keep both on the shelf.
Loctite Blue 242 Threadlocker, 6ml: medium strength, removable, 1/4 to 3/4 in fasteners, 24-hour cure.
Permatex Ultra Black RTV Silicone Gasket Maker, 3.35 oz: maximum oil resistance, -65°F to 500°F, sensor-safe.
