A frequent source of leaks and assembly failure for hobbyists, DIYers, and industrial users is a fundamental mismatch between the thread sealant used and the thread standard it’s applied to.
Understanding Thread Standards and Sealing Methods
Specifically, using a sealant intended for tapered threads on straight threads, or vice versa, often results in a poor seal or complete joint failure. The confusion stems from not understanding how different thread types actually seal. The two most common standards are tapered threads such as NPT/NPTF (National Pipe Thread/Fuel), and parallel or straight threads such as BSPP (British Standard Parallel Pipe) or metric threads.
| Thread Type | Sealing Method | Sealant Requirement | Risk of Mismatch |
|---|---|---|---|
| Tapered (NPT/NPTF) | Metal interference — threads wedge together, sealant fills voids | Requires a sealant, tape or liquid, to fill helix gaps left by machining | A sealant designed only for straight threads won’t provide enough bulk to fill the wedge gaps |
| Straight (BSPP, metric) | Gasket/O-ring — sealing is accomplished by a separate component seating on a face | Requires a thread locker or hydraulic sealant to prevent spiral leakage and keep threads from loosening | A thick sealant intended for NPT on a straight thread with a gasket can interfere with the gasket’s compression and cause leaks |
The key takeaway is that tapered threads rely on the sealant itself for the leak-proof function, while straight threads rely on a gasket or O-ring, with the sealant simply locking the threads and preventing spiral leakage.
One Sealant Chemistry, Two Different Jobs
For most high-performance, clean sealing needs — especially within hydraulic and pneumatic systems where thread-type confusion is common — a medium-viscosity, chemical-resistant liquid anaerobic sealant provides good versatility across both standards, though it performs a different function on each:
- Tapered threads (NPT): A consistent, medium viscosity is well suited to completely filling the gaps in the thread helix, providing the reliable, high-pressure seal that NPT/NPTF joints require.
- Straight threads (BSPP/metric): When used on straight threads in addition to a mechanical seal like an O-ring, the sealant cures to prevent spiral leakage — slow seepage along the thread path — and locks the joint against vibration loosening. As a liquid, it applies thinly enough not to over-bulk and interfere with the seating of the primary gasket seal.
Because it’s a liquid, the sealant eliminates tape shredding and the associated contamination risk regardless of thread type, which matters in sensitive hydraulic circuits. Its medium viscosity fills deep voids in tapered threads while staying thin enough to avoid hydraulic lock or interference with gasket seating on straight threads, and its chemical resistance keeps it compatible with a wide array of hydraulic fluids, oils, and gases used across both thread standards. If you’re not certain whether your fitting is tapered or straight, Email Us with a photo or the fitting’s part number and we can help identify the thread standard before you seal it.
Getting It Right the First Time
To ensure successful sealing regardless of thread type:
- Identify your thread type first — check the fitting’s markings or measure the thread taper to confirm NPT versus BSPP or metric before selecting a sealing method.
- Clean your threads thoroughly — surface contamination affects both mechanical seating and anaerobic cure quality equally on either thread type.
- Respect the gasket on straight threads — never over-apply sealant on a straight/parallel joint, since excess material can prevent the O-ring or gasket face from seating correctly.
Frequently Asked Questions
Q: How can I visually tell a tapered thread from a straight thread?
A: Tapered threads narrow slightly from the base to the tip when viewed along the fitting’s axis, while straight threads maintain a constant diameter throughout. When in doubt, a thread gauge is the most reliable way to confirm.
Q: What happens if I use only tape on a straight thread with a gasket?
A: Tape adds bulk without contributing to the gasket’s sealing function, and it can actually prevent the gasket face from seating flush, causing a leak that has nothing to do with the threads themselves.
Q: Do metric and BSPP threads always use the same sealing approach?
A: Generally yes, since both are parallel thread standards that rely on a separate gasket or O-ring for the primary seal, with the sealant serving a locking and anti-spiral-leak role rather than the primary seal itself.
Getting the thread-type and sealant pairing right the first time avoids the repeat failures and rework that come from treating every joint the same way. For related reading on adhesive selection by application, see which UV glue delivers higher bond strength and UV glue vs epoxy for transparent bonding. Contact Our Team if you need help matching a sealant to your specific thread standard.
Visit www.incurelab.com for more information.