some shops measure at different locations so you get different #s and their some variations in the casting process of the carbs from the manufacturer so sometimes the amount you can remove is different. also, some shops do not like to bore out the carb so much that it leaves the walls paper thin. 41.xx mm will be about the max before you need to build up the walls such as with some form of epoxy cement like ice man suggested but at that point you can just buy a larger carb