Throttle bores are prone to wearing out in Holley carburetors much more so than the throttle shafts. They normally wear because of operation in dirty environments or by not using a throttle stop. AED makes a special reverse cut machining and reaming tool (sold separately) that allows you to properly align worn out throttle bores and machine them to the correct size so that you may install these special bronze bushings. This is a much better solution than having the shafts ride on cast aluminum.