Loctite 638 vs. 640: what’s the difference?
Loctite 638 is specifically designed for metal-to-metal applications and may not be suitable for use on an O.D. rubber seal surface to metal housing; Loctite 640 has a lower viscosity and is suitable for press/shrink-fitted assemblies.
Loctite 638 vs. 640: Specs & Uses
⏰Cure Speed
Loctite 638: Faster with decreasing bond gap and increasing temperature. Full cure within 24 hrs.
Loctite 640: Faster with decreasing bond gap and increasing temperature. Full cure within 24 hrs.
🆚Compressive Strength
Loctite 638: 29 N/mm2 (fully cured)
Loctite 640: ≥15 N/mm2 (fully cured), ≥22 N/mm2 (heat cured)
🛠️Common Uses
Loctite 638 can be used to secure machine parts, repair metal structures, or assemble metal components; seal gaps, joints, or connections in metal components, providing resistance against leaks, vibrations, or environmental factors.
Loctite 640 can be used to seal hydraulic systems; withstand temperatures up to 400°F, making it suitable for applications where heat resistance is crucial; dealing with aluminum lawn mower engine blocks and crankshaft oil seals.
Technical Info
My Hands-on Test of Loctite 640
✅Rock-Solid Bond That Won’t Budge Under Extreme Conditions
Loctite 640 is commonly used to bond bearings, bushings, and sleeves to shafts and housings. Its ability to cure in the absence of air, even in tight clearances, ensures a rock-solid bond that won’t budge under extreme conditions. Whether you need to secure a crankshaft oil seal in an aluminum lawn mower engine block or prevent bearing buddies from falling off your jet ski trailer, Loctite 640 has you covered.
Adhesion to Inactive Metals
What sets this product apart is its exceptional adhesion to inactive metals like aluminum, stainless steel, and plated surfaces. However, when working with aluminum, it’s essential to use the Loctite SF 7649 primer to ensure proper curing. Once fully cured after 24 hours, Loctite 640 is compatible with oils and other lubricants, making it a reliable choice for sealing applications prone to fluid exposure.
Loose and Jiggly Metal Parts
It’s good at dealing with loose and jiggly metal parts. The adhesive, a translucent green liquid, proved to be both strong and lightweight. Its ability to wick into tiny cracks made it ideal for securing loose metal parts.
Once dried, it became barely noticeable, blending seamlessly with the surroundings. The bonding process started within minutes, and after a day of curing, the previously loose metal part became solid as a rock.
You might also want to learn about the 
best silicone adhesive for metal.
High-Strength Sleeve Retainer Repair
Loctite’s reliability extends beyond small-scale repairs. In a busy auto repair shop, we recently encountered a Chevrolet pickup that had lost its differential carrier bearings, causing damage to the differential housing. Since replacing the housing was costly, we decided to give Loctite 640 a chance.
To our satisfaction, it proved to be an excellent high-strength sleeve retainer repair solution, effectively addressing the problem. So I’ve written an article about 
permatex sleeve retainer vs loctite, feel free to read it!
❌Not Recommended for Repairing Loose Connections
While Loctite 640 excels in certain scenarios, it may not be the best choice for others. For instance, it’s not recommended to repair loose connections between windshield wiper blades and arms, where an epoxy like Gorilla Epoxy or Shoe Goo would be more appropriate.
Gorilla Glue vs Shoe Goo: Fix Leather or Rubber Shoes, Granite, Marble, and Limestone
Similarly, it may not be the ideal solution for sealing leaks around spark plug heli coils in car cylinder heads, as its thin liquid consistency may not provide an adequate seal.
FAQ of Loctite 640
Q: Has anyone successfully used Loctite 640 to repair a loose connection between a windshield wiper blade and a wiper arm?
A: While I’m uncertain about the specific bonding requirement, it’s important to note that Loctite 640 is not a glue. It is designed to allow for the separation of joined items with the application of heat. If the connection between the wiper blade and arm involves metal components joined in a cup and receiver fashion, there is a possibility it may work. However, for rejoining rubber to a metal arm, it is recommended to use a silicone adhesive instead, for example, best silicone for undermount sink.
Q: I plan to install Bearing Buddies on my jet ski trailer, but I’ve heard complaints about them falling off during travel. Can Loctite 640 prevent this problem?
A: Using Loctite 640 would indeed help prevent the problem. I recommend using SF 7649 primer to cure LOCTITE 640 properly. Keep in mind that Loctite 640 is permanent and high strength. To remove the worn bearing after applying LOCTITE 640, heating it up to 500°F will be necessary. However, for bearing applications, it is advisable to consider using a removable product like LOCTITE 641.
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My Hands-on Test of Loctite 638
✅Pros
Now, let’s get back to the performance. My new tap handle kept switching directions while I was using it. Super annoying. So I tried this and it worked. Just make sure you move it periodically while it’s curing so it doesn’t seize. This Loc-Tite 638 compound is awesome stuff.
Loctite 638 is versatile. While it’s primarily designed for securing metal-to-metal cylindrical assemblies, I’ve also had success using it on rubber-to-metal sealing applications. It provides excellent sealing properties, preventing leaks and ensuring a tight, secure fit.
Gearbox
It secured the bearings in my gearbox, ensuring smooth operation and minimizing any potential wobbling or shifting. I’ve had my fair share of disappointments with other products, but 638 lived up to its reputation. It’s a reliable choice for mechanical applications, and I couldn’t be happier with the results.
Tamiya Model
Front axles popped out, same as some have experienced with the re-release. Loctite 638 to the rescue.
Keep a Pinion Gear On the Motor Shaft
I also used Loctite 638 to keep a pinion gear on the motor shaft of my 31 lbs RC car, and it has been holding up remarkably well. It’s been through some intense off-road adventures, and the bond remains strong.
So, if you’re looking for a solution to keep your gears in place, Loctite might just be the answer.
Setting Time: Loctite 638 is a quick-setting formula, so it’s essential to have everything prepped and ready for assembly before applying it. Once cured, it provides a low-strength hold, making it unsuitable for critical applications like securing a flywheel to a crankshaft on a lawn tractor. For such tasks, I’d recommend a stronger threadlocker like the blue variant.
Loctite Blue Vs. Permatex Blue : Secure Engine Mounts, Suspension Bolts, and Brake Calipers
❌Concerns on High-Temperature Or Heavy-Duty Applications
While Loctite 638 is an excellent choice for securing small screws and cylindrical assemblies, it’s essential to select the right product for your specific application. For high-temperature or heavy-duty applications, I’d suggest exploring options like Loctite’s orange or red threadlockers, which offer superior heat resistance and holding power.
Loctite Blue vs Loctite Red🌟️ A Comprehensive Review
FAQ of Loctite 638
Q: How much should a new bottle weigh? Mine feels kind of empty.
A: Ideally, the bottle should have a weight that indicates it is half empty. This allows for the necessary presence of air to prevent the content from hardening within the bottle.
Q: What country is Loctite 638 manufactured in?
A: The product originates from India.
Q: Can Loctite 638 be used on an O.D. rubber seal surface to metal housing, or is it only suitable for metal-to-metal applications?
A: This particular product is specifically designed for metal-to-metal applications and may not be suitable for use on an O.D. rubber seal surface to metal housing.
I’m Ronald Stevens, a Mechanical Engineer obsessed with adhesives, glue, and all things sticky! My passion began in childhood when I’d always be the one playing with glue during arts and crafts. And I’d spend hours researching and experimenting. Happy reading, and remember: stick with the best, and you’ll never be stuck in a mess!
Feel free to contact me and join our discussion:
Does this 638 formula have a quick setting time?
Yes, I believe it does. After approximately one minute, it becomes difficult to separate the bonded parts.