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Both aluminum and steel bungs feature 10AN male (7/8-14) threads and have a large 12.4mm (1/2") internal diameter for optimal flow.
The 14-0274 aluminum fitting can be used as a weld-in or press-in fitting (specifically for the USDM Mitsubishi EVO 8-9 magnesium valve cover).
The press-in portion is 16.6mm diameter. 
It replaces the PCV valve's bushing so installation does NOT require valve cover removal. The fitting has extremely close tolerances to make a tight fit. NOTE: The JDM EVO 4-9 valve cover uses a different PCV valve and is NOT compatible.
The 14-0293 weld-on fitting (shown below) is made from steel and is commonly used on oil pans. To prevent rust, this raw (uncoated) fitting is soaked in oil.