i don't know to be honest. i'd like to get others opinions on this but i'm thinking anything over 12.5:1 starts pushing the limits of composition gaskets especially ones without wire in them. not that it can't and hasn't been done (often) just that the reliability starts to go down. questions like these are the reason for the post. imho the 1039-1105 are thicker than needs be but maybe this is a manufacturing requirement althought the 1009 isn't that thick but won't fit many bore sizes.