Johansson gauge blocks (‘Jo blocks’) are made of steel and used for precision length measurement. Their surface is very smooth, and two blocks can adhere to each other; however, strong adhesion occurs only after sliding (wringing). Various hypotheses explaining wringing and adhesion mechanisms in the blocks have been suggested in the literature, including the role of intermolecular forces, oil surface tension and air pressure. The authors study the frictional sliding of two Jo blocks against each other to obtain insights into the mechanisms of wringing. The results show an increase in the friction force with the sliding distance, which is consistent with the removal of the oxide film from the steel surface by wringing. This is likely the dominant mechanism of Jo block adhesion.