elm
Something small. No larger then an 1/8 but I would gofor something like 3/32. Make sure that the hole isn't covered by the gasket.
I was thinking about this some more after I posted. You say it only leaks at start up. Question, are you driving it after start up or is it sitting in the drive way? If you are driving it. It may still be leaking and it may not be noticable because it is being blown away. If it is sitting in the drive and not leaking after the therm opens I would still check the coolant level.
My 49 leaks from the water pump seal just a little at start up and then it stops. I know it will go to full out leaking all the time but I keep a close eye on it.
