91 civic - tough question about cooling

it shouldn't be machined at all [or at least, not with "traditional"
gear] for the reasons stated. it just needs to be lapped on a properly
flat rigid bed.

 

Radiators are relatively soft. I once saw the neck of the radiator had
become "out-of-round", causing several new radiator caps to blow air bubbles
into the puke can because an effective seal was not formed. A new radiator
fixed the problem. You might check for gunk build-up in the neck which
could cause an incomplete seal.

Later, Ralph