Big Block Valve Cover Bolt Question

[mssl72]

Recently a good friend of mine asked me about the bolts that secure the valve covers to the head on big blocks. He's been looking at 1966 L72's and seeing 2 different bolts styles being used. One that has a 3/8" head and one that has a 7/16" head. The 3/8 bolt has a raised ridge around the top and the 7/16 bolt is pretty much flat. The majority have been 3/8". If anybody with a '66 big block doesn't mind, take a look at your car and let me know what 'cha got on yours.