1990 leaking coolant from head

ok guy here's one for you
i recently rebuilt the top end my 90 rm 125
have had no problems from it up until last weekend or so,
my problem is that the oring gasket that goes on the head
is leaking coolant could i have just got a bad gasket?
seems like the gaskets esp the outer one are to thin or was it my error and maybe i tightened the head down to tight

do they make a one piece gasket instead of the o-ring type?

yeah buddy looked at it last night said it was crushed just slap a new one
was hoping they made one piece flat gasket