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Patching a tube

A tube is not patchable when it has a large gash or seam split. In that case, forget about patching and throw the tube away.

If the point of leaking is not obvious, pump up the tube with a little air. Listen or feel around the tube to detect escaping air. If you cannot find the leak that way, you could submerge sections of the tube in a bowl of water; bubbles will indicate air coming out of the puncture. Pouring water on the tube might work when you're out on the road without a bowl available.

Check for leaks around inner tube, using bowl of water

You can't detect the leak, or air leaks from the stem? In either case, it's easiest to discard the tube and use a new one.

Otherwise, the tube seems patchable, so get your patching kit.

Parts of a bicycle inner tube patch kit

Check whether your rubber cement has dried in the container. You might need to buy more cement at a bike shop or arts & crafts store before patching.

Typically the kit's instructions will tell you to roughen the area around the hole with the sandpaper (or other tool) in the kit. Then apply cement to an area slightly bigger than the patch. You wait for the cement to cease being liquidy and to become tacky (semi-dry).

Rubber cement placed on a bicycle inner tube to patch it

Then apply the patch and press it down.

A bicycle inner tube patch just after being applied Pressing down on a bicycle inner tube patch

After the cement has dried for several minutes, you can peel off that clear plastic cover.

A completed bicycle inner tube patch