Until a Moderator takes Bob's "Hint, hint" and links
This thread to this current one, may I suggest using threaded Allen Plugs? 3/4"UNC ought to do the job fine (Or should that be Coarse?).
Also, I don't think these can be regarded as "Frost plugs" as once the sump pan is secured in place, these plugs would have nowhere to go IF the coolant in the waterjackets did freeze over.