[SGTGunn]

Believe it or not, in my 4 years on the M1 I never once saw any published specs on armor thickness - presumably they were/are classified above my level. That being said, it's pretty easy, standing in the loaders hatch, to see how thick the turret roof armor is - which is to say not very. 40mm seems about right to me, though it may have actually been thinner. When the M1 was designed, there was no top attack ATGM threat, so I honestly don't think roof armor was a serious consideration. Just thick enough to keep out artillery I suspect. Sorry I don't have a more definitive answer.

Adrian