How does a seal actually work, and why does tightening it not help?
O-rings, gaskets and compression olives seal in three different ways. Which one you are looking at decides whether tightening helps, does nothing, or destroys the seal — and it is why so many leaks get worse after someone attacks them with a spanner.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 6 min
- Safety
- caution
Short answer
An O-ring is squeezed in a groove and pressure pushes it harder against the sealing face — nothing is tightened, and a leak means the ring or the surface is damaged. A flat gasket is crushed evenly between two faces and needs even compression, tightened in sequence. A compression olive is permanently deformed by the nut and seals once — over-tightening flattens it past the point of sealing.
The universal response to a leak is to reach for a spanner. For one of the three common sealing mechanisms that is exactly right, once. For the other two it does nothing or makes it permanently worse, and the difference is visible as soon as you know what you are looking at.
Safety
Step by step
- Understand what a seal is doing.Two hard surfaces never touch perfectly — at a microscopic scale both are rough. A seal is a softer material forced into that roughness so there is no continuous path for fluid to travel along. Everything else follows from that.
- Recognise an O-ring seal.A rubber ring sitting in a machined groove, squeezed between the groove and the bore or face it runs against. Taps, shower diverters, mixer cartridges, filter housings and countless appliance parts use them.
- Understand why an O-ring is not tightened.Its compression is set by the groove dimensions, not by you. Better still, system pressure pushes it harder against the sealing face, so it seals better under pressure. If it leaks, the ring is perished, cut, the wrong size, dry, or the surface it runs on is scored — and none of those is fixed with force.
- Recognise a flat gasket.A sheet of fibre, paper, rubber, cork or PTFE crushed between two flat faces, usually held by bolts or a union nut. Pump flanges, appliance covers, waste traps, immersion heaters and flue joints all use them.
- Get gasket compression even.A gasket needs the same crush all the way round. That is why bolts are tightened in a diagonal or star sequence in stages, rather than one bolt fully at a time. An unevenly crushed gasket leaks at the loose side and is damaged at the tight side.
- Recognise a compression olive.The soft brass or copper ring inside a compression fitting, deformed by the nut against the tapered seat. It is the one seal here that tightening creates — and it creates it once.
- Understand why an over-tightened olive cannot seal.Past a certain point the olive is flattened, the pipe beneath it is deformed, and there is nothing left to make a line contact against the seat. The fix for a weeping compression joint is very often to cut the olive off and fit a new one rather than to apply more force.
- Match the material to the job.Nitrile, EPDM, silicone, PTFE and fibre all suit different temperatures, pressures and fluids, and an O-ring that is right for cold water may perish in weeks on hot water or in contact with oil. Replace like for like from a proper kit rather than from a random assortment.
- Prepare the surface, not just the seal.Clean off old gasket, limescale and debris, and check the sealing faces for scoring, pitting or distortion. A new seal on a damaged face leaks exactly as the old one did.
- Lubricate where the design expects it.Many O-rings are fitted with a smear of silicone grease so they slide into place without rolling or tearing. Use a grease compatible with the material and, on drinking water, one approved for the purpose. Never use oil on rubber that oil attacks.
Tips
- Take the old seal with you when buying a replacement, or measure it. O-rings come in a great many sizes that look identical by eye and are not.
- A cheap assorted O-ring box is worth having. So is a tub of silicone grease — the two together solve a surprising proportion of household drips.
- If a joint has been over-tightened before you got to it, expect to replace the olive or the gasket rather than to rescue it. The damage is already done.
Common mistakes
- Tightening an O-ring joint — Its compression is set by the groove. Tightening deforms the housing or shears the ring and cannot improve the seal.
- Tightening gasket bolts one at a time — It crushes one side before the other has moved, so the gasket seats unevenly. Star sequence, in stages, is not fussiness.
- Wrapping PTFE tape around an olive to stop a weep — PTFE tape is for tapered threads, not for a compression seal. It stops the olive seating properly and turns a fixable joint into a leaking one.
- Reusing a crushed gasket — A gasket seals by permanently deforming into the roughness of both faces. Once crushed and released it will not do it again, and reusing one is a false economy measured in minutes.
- Using petroleum grease on rubber — It attacks several common rubbers and swells or degrades them. Silicone grease is the general-purpose answer, and a water-approved grade for drinking water.
If it doesn't work
A tap leaks from the base of the spout rather than the outlet
Cause: Worn O-rings on the spout, not a worn washer or cartridge — Fix: Different fault, different part. GuideHQ has a guide on this specific leak; it is an O-ring change and it is straightforward.
A compression joint weeps and keeps weeping however hard it is tightened
Cause: The olive is over-compressed, misaligned, or seated on a scored or out-of-round pipe — Fix: Drain, cut the fitting off or remove the olive, and remake with a new olive on clean straight pipe. More force is the thing making it worse.
A flange or cover leaks from one point only
Cause: Uneven compression, a damaged sealing face, or a gasket that has been nipped during assembly — Fix: Undo it, look at the gasket for a witness mark showing where it was and was not crushed, and remake with a new gasket and a proper tightening sequence.
A new seal fails within weeks
Cause: The wrong material for the temperature or fluid, or a damaged sealing surface — Fix: Check what the original was made of and match it, and inspect the face it runs on. Repeated early failure is nearly always the material or the surface, not the fitting.
Questions people ask
What is the difference between a washer and an O-ring?
A tap washer seals by being pressed flat against a seat, and it is a consumable that wears. An O-ring seals radially in a groove and should not be under repeated compression. Both are rubber and they do different jobs.
Can I make a gasket?
For simple flat joints, yes — gasket sheet and a hammer used to tap out the bolt holes against the flange is a traditional and effective method. For anything pressurised, hot or safety-critical, use the manufacturer's part.
What about liquid gasket and sealant?
Anaerobic and silicone jointing compounds have real uses on rigid flanges. They are not a repair for a damaged sealing face or a substitute for the right gasket, and excess squeezed inside can block passages.
Why do seals perish?
Heat, ultraviolet light, ozone, chemical attack and simply being compressed for years all harden rubber until it takes a permanent set and cracks. It is why seals are consumables and why an old one rarely goes back in successfully.