There are deep and shallow. In the front holes, I think deep oines hit the cylinder casting. I like the deep opnes. Brass does not grab as well as stell does. They dont corrode, but you dont want them popping out either. The shallow ones work ok too, but the deeper the plug, the more surface area to grab the bore. Also make sure there is no edge or slag from the old plug in the bore walls. It doesnt always come off in the cleaning processes.