You cannot use the locknut that came with your grinder to tighten two blades! You can use the manufacturer's locknut to tighten one blade only. Their locknut has a 22mm shoulder which will not allow it to tighten two blades. We designed the Universal Nut with its flat base and hexagon head specifically for the purpose of holding any double blade combination. Using a 13/16(21mm) spanner, socket or crescent wrench, its much easier than manufacturers two pin spanners. It can also be used to tighten other accessories.