The 3 X's I have read and almost every time is a marking for the HO blocks , I have a 84 from a RS Capri that is marked this way also . Another I D for 302 to 351 is the .5 inch deck height that is easy to see at water pump . No clearance to head on a 302 and .5 or so clearance on the 351 .

Quote Originally Posted by rspears View Post
There's a lot of folks who believe the story of higher nickle content giving a stronger block, and it applies to some 302's, too. The mark on the 302 is in the valley area between the lifter bores, and is "XXX". My engine guy swears that he has talked to people at Ford who say it is true, but as Dave says there are others who say it is simply a myth. For me, the XXX block is either stronger due to better metalurgy or is the same as all the rest so it cannot hurt to have the mark, which by the way can be seen on my 302 block