6mm is just a C-hair under one quarter inch. The spools have been designed to work with 6mm fasteners and to support the weight of the intended bike class. What is the brand and advertised application of the spools? Where they universal or intended for dirt bikes, racing, sport bikes, cruisers, dressers...etc.
If you are worried about shearing the stock 6mm bolts, replace them with a higher property class fastener. Metric bolt grades and strengths vary to include tensile strength, minimum breaking torque for screws made of carbon steel and alloy steel with property class designations 4.6, 4.8, 5.8, 8.8, 9.8, 10.9 and 12.9. The higher the class number, the higher the tensile strength.
