Arthur asked: > My heating oil tank contents gauge (at home not on the boat) > has been broken. I'm trying to replace it. However I am > stumped when deciding the nominal size of the threaded hole > in the top of the tank in order to buy a replacement gauge. > The diameter of the hole is approx 45mm. > For reference purposes only, I measured the size of a spare > wash basin waste I happened to have at home against the hole > in the tank top. It is a good bit too small to fit the > threaded hole and would drop into the tank, this waste > fitting has a diameter (outside edge to edge of the thread) > of about 41mm, the locking nut is 45mm (flat to flat of > spanner size) and the internal diameter of the waste is > 37mm. The box it came in claimed it to be a chrome plated > 32mm waste. > I understand that the welded-in threaded flange in the oil > tank is a BSP size, either (I suspect) 1.5 inch or 2 inch, > but from the above (probably > useless) info. which size is it most likely to be? > The gauge I would like to buy comes with a 1.5in BSP thread > size only (and from the photo of it on t'internet it does > look just like the one I'm replacing (vintage 1995)). A > different (and much more expensive and sophisticated (more > to go wrong)) gauge comes in 1.5 inch and 2 inch sizes.
By the sound of it, the threaded hole in your tank is 1.5 in BSP. http://en.wikipedia.org/wiki/ISO_228 gives the major diameter of a 1.5 BSP thread as 47.803mm. This is over the threads on a male fitting, so it seems reasonable that the clear diameter of a female fitting is around 45mm. So all you should need is a simple gauge with a 1.5 male thread, such as the £7.37 version half way down the page at http://www.bes.co.uk/products/077.asp.
