The guide boss is a description of the material surrounding the valve guide. This material is very important to support the valve guide.When you're using dual valve springs, the inner spring will require extra clearance of the guide boss. Normally you would cut the guide boss to fit the spring shims that are placed underneath the springs, the problem with this is that the shims inner diameter is alot smaller than most dual valve springs' inner diameter. We cut ours thicker to fit the spring and not the shim. The reason why we do that is so we have more material around the valve guide, so in the future if the guides the need to be replaced, we don't have to worry about splitting the guide boss. There is an advantage and disadvantage to this. The disadvantage is that we must make custom shims to fit the guide boss. It costs us more but we are worried more about the reliability than the cost.