Maybe the mating surface of the OFH is warped? Maybe it was overtorqued before, or overheated, or any of the hundred other things that causes aluminum to warp easliy. Could be your didnt clean the mating surfaces well enough? Maybe you put the bolts back incorrectly, I believe they are different lengths. Did you use any sealant on the gasket at all? If so that could be why its leaking. If you did all these things then maybe its something else leaking?