They were cracked a lot right from the factory, I have fooled with magnums a bunch and never seen a cracked one cause any problem. I have only re-used one pair with cracks but I have pulled many from running good motors and was suprised to find cracks. They are caused from the induction hardning process aparently. 318 tend to have less cracks, they are the same casting as a 360 mag but made in different factorys so mabey that is the reason or it could be the lower compression 360 makes more ex heat

If the cracks really bother you get some EQ or RHS magnums, they have pressed in hardned seats to prevent cracking.