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#11311
efr215
Participant

The old tap is almost certainly 1/8” BSP, the dimensions, allowing for a wear and small measurement errors is too close to be anything else.

The adaptor is more of a problem, 0.427” is a “nothing” size but it is not far off 7/16” (0.437”). The trouble is that it is also almost exactly 11mm but that is also not a “preferred” size.

After a good deal of head scratching I suspect that what we have here is a hole opened up for a 3/8” BSP Helicoil insert, (the 0.036” pitch is almost exactly 28 tpi which is correct for 1/8” BSP), it would seem that the Helicoil insert has for one reason or another failed and has then had the insert made.

Does that make sense to you?