Onan 10,000 Diesel coolant drain plug ??

Went to do a 2 yr. coolant change/flush. The manual says the coolant plug is 1/4 hex ( e.g. allen_) . I put a 1/4 in and it was pretty sloppy- the plug seems really tight and I didn't want to strip it. 5/16 is too big . Tried metrics , 6 mm too small, 8mm too big. I'll see if I can find a 7mm bit

Anyone have a similar issue and what did you do ?

I'll go back to the storage lot and spray it with Kroil a bit later to get it ready to break loose.