Why will my stopcock not turn, and how do I free a seized one?

What actually seizes inside a stopcock, the penetrating-oil and quarter-turn approach that frees most of them, and the point at which forcing it becomes the expensive mistake.

Difficulty: beginner
Time: 25 min
Read: 5 min
Safety: caution

Short answer

Almost every seized stopcock is bound at the spindle threads by years of scale and corrosion, not jammed at the washer. Free it by working the handle a few degrees each way rather than forcing it one way, with penetrating oil on the gland nut and spindle and time between attempts. If it still will not move with hand pressure, replace the valve — do not lever it.

A stopcock seizes because nobody ever turns it. The spindle threads sit in a warm, damp brass body, lime and corrosion build up in the thread form, and after fifteen years the valve is effectively glued in position. The good news is that the binding is almost always in the threads rather than at the seat, so the cure is lubrication and patience rather than force. The bad news is that the same fifteen years have made the brass brittle, and a stopcock that shears is a genuine emergency.

Safety

A stopcock is usually seized because it is old, and old brass snaps. If the handle flexes, the body creaks or you find yourself putting real force on it, stop — a stopcock broken off in the on position turns a stiff valve into an uncontrollable leak in the middle of your house, with the only remaining shut-off buried in the pavement. Never use a pipe as a lever on the handle.

What you'll need

Penetrating oil
A cloth and a bowl
An adjustable spanner (optional)
A torch

Inside a screw-down stopcock, and how it differs from a gate valve

Step by step

Find it and clear the space around it first.Usually under the kitchen sink, sometimes in a downstairs cloakroom, a utility room, under the stairs or in the hall by the front door. Empty the cupboard completely so you can get both hands to it and see what you are doing. Working blind at arm's length behind a waste trap is how people break them.
Check which way it should turn.Clockwise closes, the same as a tap. If somebody has already tried, it may be part-closed rather than fully open, which is worth knowing before you decide it is seized.
Put penetrating oil where it can actually do something.The binding is at the spindle threads inside the gland nut, so the oil goes around the spindle where it emerges from the gland nut, and around the gland nut itself. Oil on the outside of the body does nothing. Leave it half an hour, then apply more.
Work it a few degrees each way — do not push one way.Hand pressure only. Try a small movement clockwise, then back anticlockwise, then a little further each way. The rocking is what breaks the scale in the threads. A valve that will not move at all today often moves after three or four attempts spread over an evening.
If it starts to move, take it all the way and back.Once it turns, close it fully, then open it fully, then close and open again a few times to clear the threads. Leave it fully open and then back it off a quarter turn — a valve wound hard against its stop is the one most likely to seize again.
Expect a drip at the spindle afterwards.Freeing an old gland almost always makes it weep slightly around the spindle. Nip the gland nut up about an eighth of a turn with a spanner — no more — and the weep stops. Over-tightening it makes the valve stiff again.
Stop if it will not go with hand pressure.No spanner on the handle, no pipe extension, no hammer. If firm hand pressure and repeated oiling has not moved it, the valve needs replacing, and replacing it means isolating at the outside stop tap first.
Test it properly once it is free.Close it, then open a downstairs cold tap. The flow should die to nothing within a few seconds. If it keeps running at a trickle, the washer has perished and the valve does not actually isolate — which matters enormously the day you need it.

Tips

Do this in summer, on a dry Saturday morning, not at eleven at night with water coming through a ceiling. A stopcock is only useful if you know it works before you need it.
Label it. A luggage tag on the pipe saying which way closes it is worth a great deal to whoever is in the house when something bursts.
If the whole family cannot find it in the dark, it may as well not exist — show everyone where it is.
Some houses have a second internal valve where the main enters, often under floorboards near the front wall. Worth knowing about if the kitchen one has failed.

If it doesn't work

Handle turns freely round and round without ever shutting off

Cause: This is a gate valve rather than a stopcock, and the gate has corroded off the spindle — Fix: The valve is finished — turning it will never do anything again. Isolate at the outside stop tap and have it replaced, ideally with a proper stopcock or a full-bore lever valve.

It closes but a tap still trickles

Cause: The rubber washer on the jumper has perished or hardened, so the valve no longer seals on its seat — Fix: Treat the valve as unreliable. It can be rewashered with the water off at the outside tap, but on a valve old enough to seize the sensible answer is replacement.

Water weeps around the spindle after you turn it

Cause: The gland packing has dried out and moving the spindle disturbed it — Fix: Nip the gland nut up an eighth of a turn with an adjustable spanner. If that does not stop it, the gland can be repacked with PTFE tape wound into the recess, but a persistent weep on a very old valve is a replacement signal.

Handle flexes or the body creaks under pressure

Cause: Dezincified brass — the zinc has leached out of the alloy over decades and the casting is now weak — Fix: Stop immediately. Do not apply any more force. Isolate outside and have the valve replaced before it fails on its own.

Keeping on top of it

Close and reopen the stopcock fully, then back off a quarter turn (Twice a year)
Check nothing has been stacked in front of it in the cupboard (Whenever you reorganise)

Questions people ask

Can I use a mole grip or a spanner on the handle?

No. The handle is thin, the spindle is brittle and the extra leverage is exactly what snaps them. If hand pressure and oil will not free it, it needs replacing, not more force.

Should I leave it fully open or a quarter turn back?

A quarter turn back from fully open. Fully open jams the spindle against its stop, which is the position most likely to seize. A quarter turn back makes no measurable difference to flow.

How do I isolate the house to change it?

At the outside stop tap in the pavement or verge, using a stop tap key. That valve is stiff and brittle too, so read the guide on using it before you go near it.