OK.. I'll just measure it out.. .. -6mmI do not recall the number for the front, but it is the same as the rear. Look at the insert. See the little bump tabs? The whole idea is to have those bump tabs inside the OEM strut housing with a little room to spare. If you leave too much housing then you run the risk of pulling the threads on the insert/bolt and/or making removal of the insert difficult (in the circumstance of a warrantee). Structurally I do not believe it adds much accept the insert will move around inside the housing without the tabs. Just my two cents.