Ben Deutschman of the Slant Six Club of New York/New Jersey wrote that owners should be careful not to put on spark plugs with the "crush washer" (the loose metal washer at the end), but to take off the washer first, if they have a 1960-1974 engine. After 1974, the head was redesigned, the plug tubes were eliminated, and taper seat plugs were used. If you do have a 1974 or earlier slant six, be sure to replace the plug tube seals - they are cheap and easy to replace.