24.0 mm dia. Insert the rounded section into one section of the joint and tap firmly to make the shape of the bamboo the same as that of the tool. Turn the tool upside down and set into joint so the rounded part just used sticks up,and then lower the other section of bamboo down onto the rounded section. Tap from below to drive the rounded section of the tool into the second part of the joint. This will make both sections of the joint the same size/ shape. Rust-proof resistant.