Not recommended for turbo use! These guides are made by us custom by the leading bronze valve guide manufacturer in the world using a proprietary broze alloy. These shouldered guides fit properly in the iron 2.3 Ford head and some aluminum heads.  Valve guide sizing (honing) is required and we recommend .002" valve to guide clearance for the exhaust side. We recommend these for N/A engines only. For turbo engines we install and recommend iron valve guides to withstand the high EGT's.