A tap adapter that does not match the thread
I have forced a hose adapter onto an outdoor tap because the first two turns went on and I thought the rest would follow. They did not. The adapter was cut for a thinner indoor thread. I kept turning. The plastic collar went oval, the tap thread picked up a burr, and the join wept as soon as I opened the valve. I blamed the washer. I changed the washer. The drip stayed. The thread was wrong from the start.
A tap adapter is the short fitting that converts the tap you have into the connector your hose uses. It is not the hose. It is not the click collar. It is a thread match. If the thread is wrong, nothing downstream will seal, and Hose fittings leak at the washer, not the hose will not save you. A new washer in a crooked adapter is a new washer in a leak you made.
UK outdoor bib taps I have fitted take a 3/4 inch BSP thread. A lot of indoor mixer taps take 1/2 inch. Cheap hose kits arrive with one adapter, or a universal rubber cone, and no size on the bag. The cone will go on if you lean on it. Then it weeps, so you lean more, and the plastic splits. I would buy the adapter as its own part and match it to the tap I can see, not to the photo on the hose box.
I offer the adapter up by hand. It should start square and turn freely for several turns. If it goes on crooked, or it binds after one turn, I stop. That is a mismatch or a crossed thread. Forcing it will strip the tap. A 1/2 inch fitting looks eager on a 3/4 inch tap. It is not seating. It is rattling on the first few threads.
A brass adapter I can put a spanner on, gently, will outlast a plastic one I pinch with pliers. Hand tight, then a little. Thread tape on a metal-to-metal join can help. It will not fix the wrong size. I have wrapped tape around a mismatch and called it sealed. It held until I knocked the hose. Then I had a wet wall and an adapter I had to cut off.
The other failure is leaving last year's adapter on the tap all winter. The metal tap survives. The plastic does not. I will not invent a frost date. A clear, still night is enough. I now take the adapter off with the rest of the fittings, the same week I drain the hose. The can stays. A watering can still earns its hook next to a hose, and a can does not care what thread your tap is.
If the tap itself is old and the thread is already chewed, a new adapter will not save it. You will feel the play before you feel a seal. Then the job is the tap, or you water from the butt. A sagging gutter is usually the clips is how rain actually reaches the butt. If the outdoor tap is a fiddle and the gutter feeds the butt, I fill the can there.
I would also check the washer face on the adapter, not only the thread. A nicked cone will weep even when the thread is right. Run the tap, then run a dry finger around the join. A drip at the adapter is a thread or a face. A drip at the hose tail is a collar.
Skip any adapter sold with no thread size. Skip universal if you already know the tap is a standard outdoor thread. Skip metal-look plastic. When it splits you cannot mend it. Skip a kit that gives you three adapters and no hint which one is which. I have stood at the tap with all three in my hand. That is how you cross a thread you already had.
This shows in April, when you put the hose back on and the first soak weeps down the wall. It shows again in a dry June week, when a mismatch that only dripped becomes a puddle on the slab. Autumn is when I should take it off and sometimes do not. Winter is when the plastic left on the tap goes. I treat the first cold week as the week the adapter comes indoors. Winter tools need a wipe and a film of oil. The adapter does not need oil. It needs to come off the tap.
I keep the adapter that actually fitted in a tin with the washers. The ones that almost fitted have left the shed.
If I later add a buy link here, it will be a paid promo.