The two little drain back holes in a BB Olds cast iron cylinder head look really inadequate for getting oil that's pumped up there back on their way to the pan. Is there any benefit to enlarging them and how big can you go before hitting a water jacket?

 

The only time I've ever found them to be "inadequate" was on a 350 that was so crudded up internally that the drainback holes were almost completely plugged. Certainly deburr and radius the holes, but there's no need to dramatically enlarge them. If you really are worried, use restricted pushrods.

 

Damn holes in my F head were big enough factory I could stick the end of my pinkie finger in them.. I didn’t even radius them as there is no way that hole was restricting drain back. Just my opinion of course

 

If you are having drain back issues, you are pumping too much oil up there. Use restricted pushrods or restrictor plugs in the lifter feed holes if the drain holes are clear and you are experiencing too much oil up top.