Help on Thread Size

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I sure could use some advice on identifying this thread size.The attached pic is a lift tank we are installing on a friends boat lift.  There are two tapped holes on the tank.  The one hole on the very end is a factory 1 inch straight tap that accepts a standard 1 inch grey plastic elbow or 1 inch PVC nipple fine.  No problem.The other hole marked in red is our challenge.  This is not a factory drilled or tapped hole; however, it is about the same diameter as the factory 1 inch. I do not have calipers with me, so I am measuring this very crude with a tape.  I cannot identify the TPI, but they for sure are not as deep o rpronounced as the factory inlet.   Problem is the hole marked in red will not accept a 1 inch grey elbow or 1 inch PVC plug.  They are too large, not by much but enough that they will not go.  I have not tried a 1 inch NPT nipple, that may go.  I need to plug this after market hole becasue it is nearly straight through the tank and the air pressure from the tank may rupture and leak.If someone has some ideas I should would be grateful. Attached ImagesGeezerPower Mig 255C185 TIGBlue 175 MIGRanger 8 Kohler 20HP1974 5K Lincoln/Wisconsin Powered (Cherry)Victor/Harris O/AK 487 Spool Gun
Reply:Would it be possible to use one of the Expanding plugs meant for stripped pipe thread in the hole?  It could be the difference between straight and taper NPT
Reply:You need to do a couple things:Use a thread pitch gauge (try both imperial and metric... I have had surprises on this a bunch of times.Is the hole tapered? measure the thread near the surface and deeper. If tapered, what is the taper? pipe taper is 1:16 (3/4" per foot) There are other tapers that are not unheard of, such at 1-1/4" per foot (of style boiler fittings) 1/2" per foot, and a variety of  others.What is the diameter as  measured (root diameter)? From this and the thread pitch you can get the nominal
Reply:Get some calipers and a thread pitch gauge. You don't honestly think anybody would be able to tell you what size that is just by looking at a picture, do you?
Reply:Originally Posted by mdlbldrmatt135Would it be possible to use one of the Expanding plugs meant for stripped pipe thread in the hole?  It could be the difference between straight and taper NPT
Reply:Originally Posted by enlpckYou need to do a couple things:Use a thread pitch gauge (try both imperial and metric... I have had surprises on this a bunch of times.As I was driving bsack from the lake I thought about a thread pitch gauge in my tap and die set.  I am headed back there Thursday and I will take it with me and hopefully I can confirm the thread pitch.  Is the hole tapered? No.  It is straight drilled and threaded.  measure the thread near the surface and deeper. If tapered, what is the taper? pipe taper is 1:16 (3/4" per foot) There are other tapers that are not unheard of, such at 1-1/4" per foot (of style boiler fittings) 1/2" per foot, and a variety of  others.I guess we can skip this since it is not tapered, but I sure apreciate the info.What is the diameter as  measured (root diameter)? From this and the thread pitch you can get the nominal
Reply:Originally Posted by slagmaticGet some calipers and a thread pitch gauge. You don't honestly think anybody would be able to tell you what size that is just by looking at a picture, do you?
Reply:Just out of curiosity, what is in the bottom of the hole? It looks like either a blind hole, or more likely, as if it has a spring loaded ball valve, possibly to prevent a vacuum from forming. What did you intend to do with the fitting, plumb a breather pipe and vent from it to a better location? Use it as a return or fill port?
Reply:Originally Posted by Oldiron2Just out of curiosity, what is in the bottom of the hole? It looks like either a blind hole, or more likely, as if it has a spring loaded ball valve, possibly to prevent a vacuum from forming. What did you intend to do with the fitting, plumb a breather pipe and vent from it to a better location? Use it as a return or fill port?
Reply:It's also possible that it is a safety valve(PRESSURE RELIEF) to prevent over pressure of the tank.v   Bill
Reply:Originally Posted by the hatIt's also possible that it is a safety valve(PRESSURE RELIEF) to prevent over pressure of the tank.v   Bill
Reply:Well I appreciate all the inputs, but the owner has decided to leave the hole as is.  I believe that is a mistake but it is not my tank.  Thanks againGeezerPower Mig 255C185 TIGBlue 175 MIGRanger 8 Kohler 20HP1974 5K Lincoln/Wisconsin Powered (Cherry)Victor/Harris O/AK 487 Spool Gun