What are the proper size threads for covering the inlet and outlet holes on the oil cooler? I've tried about every cap I can think of and nothing seems to fit.

 

Why are you trying to cap off the oil cooler? The two openings on the cooler are 18mm, as is the spot on the rear iron. The front cover female side is 16mm. If you're planning on making some braided lines, racingbeat sells the metric to -10 adapters.