Threadlockers are the unsung heroes of mechanical assembly, filling the microscopic gaps between mating threads to prevent vibration loosening, corrosion, and galling. However, "just put some Loctite on it" is dangerous advice if you don't specify the color. Understanding the difference between Purple, Blue, Red, and Green threadlockers isn't just about preference; it's about the difference between a secure assembly and a snapped bolt during maintenance.
Here is a comprehensive breakdown of the four main threadlocker colors, what they mean, and exactly when you should reach for each one.
The Marketing Myth: Color vs. Product Number
Pro Tip: Read the Number
While colors are a great shorthand for strength, they are essentially marketing categories. The product number (e.g., 242, 243, 271) is the actual specification. Formulations change over time—for instance, the old standard blue was 242, but today's general-purpose benchmark is 243, which offers better oil tolerance. Always match the number to your specific technical requirement.
1. Purple Threadlocker (Low Strength)
Purple is often the most underrated and underutilized threadlocker on the shelf. It is specifically designed for delicate assemblies where you need vibration resistance but absolutely require easy removal later without damaging the hardware.
Purple (e.g., Loctite 222)
- Strength: Low
- Fastener Size: Small (Under 1/4"6mm)
- Removability: Easy with standard hand tools
When to use Purple:
- Tiny machine screws (e.g., laptop screws, eyeglasses)
- Carburetor adjustment screws or delicate instruments
- Fasteners threading into soft materials (aluminum, brass, titanium) where higher strength threadlockers might strip the internal threads upon removal.
2. Blue Threadlocker (Medium Strength)
If you only have one bottle in your toolbox, it should be Blue. It's the "Goldilocks" of threadlockers: strong enough to hold securely under significant vibration, but weak enough that a standard mechanic's tool can break the bond for maintenance.
Blue (e.g., Loctite 242, 243)
- Strength: Medium
- Fastener Size: Medium (1/4" to 3/4"6mm to 20mm)
- Removability: Standard hand tools (spanners/sockets) with normal force
When to use Blue:
- General automotive maintenance (brake caliper bolts, water pumps)
- Securing flywheel and pressure plate bolts
- Bicycle components and power tools
- Machine shop fixtures and general assembly where future teardown or adjustment is expected.
3. Red Threadlocker (High Strength / Permanent)
Red threadlocker is the heavy muscle. It is designed for assemblies that you intend to never take apart again. It offers the highest resistance to shock, vibration, and torque.
Warning: Red Means Heat
The biggest mistake novices make is treating Red like a "better Blue." Do not use Red on small fasteners or delicate parts. To remove a fastener secured with Red Loctite, you typically must apply localized heat (upwards of 500°F260°C) with a torch for several minutes and wrench it out while it is still hot. Applying Red where Purple or Blue is needed usually results in snapped bolts or stripped threads.
Red (e.g., Loctite 262, 263, 271)
- Strength: High / Permanent
- Fastener Size: Large (1/4" to 1-1/2"6mm to 36mm and up)
- Removability: Difficult. Requires localized high heat and heavy tools.
When to use Red:
- Heavy machinery and large structural bolts
- Automotive suspension components, engine mounts, or heavy-duty driveline bolts
- Applications subject to extreme, continuous vibration where loosening could be catastrophic.
4. Green Threadlocker (Wicking / Penetrating)
Green threadlocker is the wildcard of the group. While Purple, Blue, and Red are applied to the threads before assembly, Green is applied after the fastener is already torqued down. It has a very low viscosity (like water), allowing it to wick into the microscopic gaps of pre-assembled threads via capillary action.
Green (e.g., Loctite 290)
- Strength: Medium to High
- Application: Post-assembly (Wicking)
- Removability: May require heat depending on thread engagement
When to use Green:
- Securing fasteners that are already assembled and torqued (where disassembly is impractical)
- Pre-calibrated adjustment screws or instrumentation
- Electrical connectors or hard-to-reach assemblies.
Quick Reference Comparison
| Color | Strength Level | Application Timing | Removability | Best For |
|---|---|---|---|---|
| Purple (222) | Low | Pre-assembly | Hand Tools (Easy) | Small screws, soft metals, instruments |
| Blue (242/243) | Medium | Pre-assembly | Hand Tools (Standard) | General automotive, machine maintenance |
| Red (262/271) | High / Permanent | Pre-assembly | Heat + Hand Tools | Heavy equipment, structural, high-vibration |
| Green (290) | Medium/High | Post-assembly (Wicking) | Hand Tools or Heat | Pre-assembled joints, set screws |
A Note on Plastics
Standard anaerobic threadlockers (Purple, Blue, Red, Green) are designed for metal-to-metal contact. Do not use them on plastics. They can cause stress cracking or degradation in common plastics like ABS or PVC. If you are fastening plastics, you need specialized cyanoacrylate adhesives (like Loctite 425) designed specifically for that purpose.