Sealed Fasteners and Thread Sealants: O-Rings, Anaerobic, PTFE Tape

1. A leaking hydraulic valve block

In 2020 a hydraulic systems integrator delivered a power unit, and a pressure-test port on the valve block leaked during commissioning. The customer wanted it fixed within 24 hours. The plug turned out to be an ordinary M10×1 hex plug with no sealing feature at all.

At 16 MPa the oil seeped along the helical clearance at roughly 200 mL a day. PTFE tape slowed it but it still wept after three days. An O-ring sealed plug solved it.

The lesson is the classic one: designers assume "tight is sealed", but a thread pair is a clearance fit. This guide follows ISO 16092, GB/T 3452.1 and field practice.

2. Why threads leak

Ordinary M, G and R threads are a mechanical fit, not a sealing fit:

• There is clearance between the flanks so the parts can be assembled.
• There is clearance at major and minor diameters.
• Together these form a continuous helical channel along which fluid or gas escapes.

Sealing means blocking that channel somewhere. The four methods block it in different places:

• O-ring: a rubber ring at the nose of the screw is compressed and closes the channel.
• Anaerobic sealant: applied to the thread, it cures once air is excluded and fills the clearance.
• PTFE tape: a thin film wrapped on the thread is crushed into the clearance.
• Bonded washer: a rubber ring under the head seals the bearing face.

3. Four methods compared

4. O-ring sealed screws

4.1 Structure

An annular groove at the nose or under the head carries a standard O-ring. When tightened the ring is compressed 15–25 % and fills the gap between the screw nose and the bottom of the tapped hole.

4.2 O-ring compounds

4.3 Points to watch

• The O-ring is a consumable; replace it at every disassembly.
• Groove depth and width follow GB/T 3452.2 — do not measure your own.
• Excess torque extrudes the ring, so the tightening torque is lower than for a plain screw.

5. Anaerobic thread sealant

5.1 How it works

The liquid stays uncured while exposed to air. Once the joint is tightened the thread clearance excludes air, and metal ions catalyse curing into a solid plastic that fills the helical path.

5.2 Strength grades

5.3 Application

• Coat two or three threads, not the whole length.
• The thread must be clean: oil, water and rust all inhibit cure.
• Fixture time 10–30 minutes, full strength at 24 hours.
• For gaps over 0.15 mm use a gap-filling grade.

6. PTFE tape and bonded washers

6.1 PTFE tape

The cheapest and most familiar option, but only for low pressure (under 2 MPa). Wrap in the direction of tightening, three to five turns.

Drawbacks: fragments contaminate the media, so it is not suitable for hydraulics; it extrudes under pressure; and it must never be used in oxygen service, where PTFE can ignite.

6.2 Bonded washer

A metal washer with a rubber sealing bead vulcanised to it, fitted under the head. It seals the bearing face rather than the thread, and suits drain plugs and port fittings.

7. Media and temperature

8. Five-step decision tree

Step 1 — What pressure?

Under 2 MPa → PTFE tape. 2–10 MPa → medium anaerobic or a bonded washer. Above 10 MPa → O-ring sealed screw.

Step 2 — Will it be serviced?

Regular maintenance → O-ring or medium anaerobic. Permanent → high-strength anaerobic.

Step 3 — What is the medium?

Use the table above: oil takes NBR, water EPDM, fuel FKM.

Step 4 — What temperature?

Above 150 °C, no ordinary anaerobic sealant or rubber ring; use a metal seal.

Step 5 — Cleanliness requirement?

Hydraulic, food or medical duty → no PTFE tape, because of fragments.

9. FAQ

Q1. Can an O-ring sealed screw be reused?

The screw yes, the O-ring no. Compression set means it should be replaced at every disassembly.

Q2. How much sealant?

Two or three threads. More squeezes out into the medium; less leaves the clearance unfilled.

Q3. Why is PTFE tape discouraged in hydraulics?

PTFE fragments travel in the oil and block valve orifices and filters. A stuck valve costs far more than the sealant saved.

Q4. How long does anaerobic sealant take?

Fixture in 10–30 minutes, full strength at 24 hours. If you are pressure testing, wait the full 24.

Q5. Is the torque the same as a plain screw?

No. The O-ring adds friction, so reduce torque by 10–20 % or the ring will be damaged.

Q6. Is screw sealing enough for an IP67 enclosure?

No. Screw sealing is local; the enclosure joint needs a gasket and the whole assembly must be validated to IP67.

Q7. Is thread sealant the same as threadlocker?

No. Sealant is formulated to fill clearance and stop leaks; threadlocker is formulated to add friction and stop loosening. Do not substitute one for the other.

Q8. Can anaerobic sealant be used on plastics?

Depends on the plastic. It can attack PC and ABS, so test first.