Bronze VS Factory Steel Valve Guides on a 2JZ Engine Rebuild – Factory guides are all steel guides ,  they're put in very tightly.  You'll notice when you drive them out how extremely tightly they are . The new bronze guides will actually measure about a half thousand larger than the ones that came out but they'll slide right in .The expansion rate of the bronze is different than the steel and the factory puts these in really tight because the steel doesn't expand as much as the aluminum.