This elbow is part of a set of pipe fittings that are designed to connect to Evergreen #224 tubing. This tubing has an Nominal Outside Diameter (O.D.) of 0.125″/3,2mm and an Nominal Inside Diameter (I.D.) of 0.069″/1,8mm.
(I use the term ‘Nominal’ for the Evergreen tubing dimensions. While the stud on the elbow is designed to fit the ID of the tubing – do not force the stud into the tubing if it fits poorly. Simply use a drill bit to ream the tubing for a fit)
By gluing a length of tubing to the studs on the elbow,
you end up with two flanged pipes bolted together (each elbow has two flanges on either end).
O Scale 1:43.5 (4.5″/5″ pipe)
O Scale 1:48 (5″/6″ pipe)
O Scale 1:64 (7″/8″ pipe)
Nominal pipe size is not generally the pipe O.D. For NPS ⅛ to 12, the NPS and OD values are different. For example, the OD of an NPS 12 pipe is actually 12.75 inches. For NPS 14 and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches OD.
To a large extent such variations in size can be ignored by we modelers as the slight difference at the scale we model is scarcely noticeable. The chart shows that for example if you are modeling in 1:48 then that 0.125″ O.D. of the tubing would be 6″. Look down the column and this is in the 5″ to 6″ (nominal) pipe O.D. – actual being 5.563″ and 6.625″. All the way down and the flange has 8 bolts. These fittings all have 8 bolt flanges.