Simply select the options for the aluminum head that you wish to purchase. First select the size you would like, followed by the thread, then the direction you would like the thread to run. Total for that head will appear after you select all options. Note: Some of the options are unavailable for purchase, such as 6" heads with larger than 5/8" bore. This is because hub size prevents this from being able to be an option. When selecting a smooth bore, there is no need to select right or left in the thread directions option, instead select the set screw option. If you have any questions about which aluminum head is the one for you, feel free to call us at (909) 793-6636.