50 Cropped Head Aluminium Nuts and Bolts (50 of each) - Square Head are available at the same price, as well as longer bolts - please look at our other listings. M6 x 11mm with 10mm size cropped nuthead. These are high tensile and have been scientifically tested to be 2 1/2 times stronger than standard aluminium bolts. These bolts are the standard bolt size used in the vast majority of greenhouses. Square heads are used when rebuilding greenhouses, cropped are designed to fit into glazing bars once the greenhouse is built and can be used for attaching shelves and accessories.