Anybody go to the trouble of tapping their freeze plugs?
Looks like 1 5/8" is common size, would then if going typical to O ring boss #24, would need a 1 7/8"-12 thread. How tough to do and worth it? Make it easy for future reverse cooling if desired seems to me.

1 5/8 is a common size, but there isn't enough meat in there for the o ring groove. at most you might have 1 - 1 1/2 thread and that would be marginal to hold in the freeze plug

This is how its done.....

Skip to 6:32

https://www.youtube.com/watch?v=mEsuTX6Empc

Cool video ! Thanks for posting that !!

I guess if you have a CNC machine to threadmill the threads, then you can make whatever threads you want. That is a cool way to do it if your machine has a lot of Z travel. My HAAS only has 16 inches of Z, so I don't think I could get the block on the table and have any room for a tool.

I might suggest a 1-11/16-16 UN threads. The 1-5/8" through hole would work great with that thread and leave 75% threads.

http://www.efunda.com/designstandards/screws/unified.cfm?start=397&finish=472

In Canada we just put a block heater in every freeze plug...