I just pulled the heads of both my engines to check out the better one to use on my build. One was the 10mm hex and the other was a 12 mm triple square. I know they should be different hole sizes but I measured the bolt holes with my digital caliper and they are the exactly the same size. I was going to use my 10 mm head on my 12 mm block because it has no cracks between the valves. Is it safe to use it or do I have to modify it to fit the block?