Disclosed is an apparatus for protecting the walls of a row of cylinders of an engine block from contamination when the walls of the other row of cylinders of the engine block are thermally coated. For this purpose, the apparatus comprises a plurality of inflatable hollow body members that can be moved into the interior of the cylinders whose walls are to be protected and inflated once they are in position in the interior of the particular cylinders. The hollow body members comprise a dimensionally stable inner sleeve member that is enclosed by an inflatable elastic envelope member. The inflatable hollow body members are arranged in two groups located opposite to each other.