The reason you think it is bigger than 1/4 BSPP is because you’re thinking that it is 1/4” in outer diameter, but that is not true. The BSP fittings were originally sized for the internal diameter of a steel pipe (1/4” ID pipe = 1/4” BSP thread), so that is where the confusion comes in. If you look up dimensions for 1/4 BSPP (remember the second P for parallel) you will see that the OD of the threads is 0.518” or just over 1/2”. Trust me, it is 1/4 BSPP. You will need a 1/4 BSPP adapter to screw something else in there, 1/4 BSPT (Tapered) or 1/4 NPT.Here is my blanking plug. Looks way bigger than 1/4" to me. There must have been a change at some point.?
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