Talked with customer service this week to see why my new Exmark has red blades with the splined bushing. He said that as of 2005 1/2 they were going to the new blades to keep the blades from self tightning after striking an object and that the new blades only needed to be torqued to 50 lbs. Some dealers had to cut the old blades off with a torch after striking an object such as a metal stake because it would tighten the blade to the point that an impact wrench would not remove them and the new splined washer or bushing would prevent that. The new blades are red instead of black. Not sure which models this affects but mine is a regular Z. Just sharing info that I thought would be useful.