However, the overbore to these blocks resulted in thin cylinder walls. Weatherby Magnum is one of the most overbore cartridges available. Throat erosion is greater in higher chamber pressure overbore cartridges. The engine utilizes an overbore design with two bigger valves. Overbore engines are a 15 lb addition. The higher the O result, the more overbore a cartridge will be. Maximum factory recommended overbore is only , although they are commonly bored . Like the 390, factory recommended overbore is only , but commonly bored by .