How much water in the oil? Did you cause a scratch with that damn file? A very tiny scratch can cause small bubbles and a water leak.
I would first check my sparks for a bit of rust on the internal end to see if rust or liquid was on one of them.
Most cracks are on the head.
Did you replace the thermostat with a new one when you did the head job?
Why did you use a FILE on the head surface? Seems a little harsh.
Did you put any sealant on the head gasket when installing? THAT alone could be your problem. Head gaskets should be installed with all surfaces DRY.
Did you follow torque sequences when installing the head bolts?
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />