Plug gauges for parallel pipe threads
A G pipe thread seals on a face or a ring, not on the flanks. For that to work the thread has to be within size, otherwise the sealing face sits crooked. VÖLKEL makes 29 precision thread plug gauges for this, as go/no-go gauges and as individual go and no-go gauges, from G 1/8 x 28 to G 4" x 11.
Go and no-go
The go side must screw in over the full thread length, otherwise the thread is undersized or the profile incomplete. The no-go side may engage two turns at most, otherwise it is oversized. On a go/no-go gauge both sit on one handle and the no-go side carries a red ring. In the large nominal sizes the go and no-go sides come separately, because one shared handle would be unwieldy. The handling step by step is in the article check internal threads with thread plug gauges.
Tolerance classes A and B
For parallel pipe threads, the DIN EN ISO 228 series defines classes A and B, A being the tighter one. Which applies follows from the drawing or the application; on sealing-face joints A is usual.
Not for tapered threads
These gauges check parallel G threads only. Tapered pipe threads to Rc (BSPT) or NPT are not checked with a parallel plug gauge but by engagement depth on a tapered gauge. The differences are explained in the article pipe thread types explained.
Matching taps are under G (BSP) pipe thread, gauges for other systems among the thread plug gauges, and external threads are checked with thread ring gauges.
