Milled heads? what spark plugs?

No you didn't change the heat range of the plug unless you physically changed the plug. The heat range of a plug is the difference in the amount of openess in the bottom of the plug which is changed by the size of the porciline(sp) and cant be changed by the end user. The heat range is determined at the time of manufacture and then would have a different plug # like 64,65,66,67 are all autolite plugs and the 64 would be the coldest and 67 would be the hottest, all are extended tip plugs. they also have a range like this in retracted tip plugs. Different manufactures have different systems for rating there plugs and this is why I said that you have to get into the catalog of the manufacture that you want to use and find the plug that you want or need.