why did I blow a head gasket?

What is your question?

I tightened my rocker arms after replacing the lifters according to the manual, but there were times that I had to take the drive belt loose after that to get it to crank over. I loosened the problem rocker arm and put it back together again with new gaskets, went two blocks and blew the head gasket. Are they separate issues or did I make a mistake somewhere else?