I have a problem with one of my drain plug. The part number is 127951 and it says (plug, water jacket drain) but does not tells the type of treads. It is 1/2 inch but I would like to know if it is NPT or straignt treads? The one on starboard side works fine but the one on port side is very hard to tighten and scrap the treads so I want to buy a tap to retap the hold in the motor but want to be shure to buy the right tap. Thanks for your help. Jean