STI thread gauges for the receiving thread
After cutting with the STI tap and before fitting the wire thread insert, a check belongs in the sequence. A normal thread gauge does not fit there, because the receiving thread is larger than the nominal thread. VÖLKEL makes 229 precision thread plug gauges for this, as go/no-go, go and no-go gauges: metric coarse and fine threads in tolerance ISO 1 (4H), American STI threads in 2B and 3B, plus Whitworth, British Standard Fine and pipe threads.
Why the standard gauge fails here
The receiving thread has the pitch of the nominal thread but a larger diameter. A 6H gauge for M 8 x 1.25 would not enter at all, and a gauge for the next nominal size up would have the wrong pitch. Only an STI gauge checks what was actually cut. The article checking STI threads correctly sets this out in full.
Why the tolerance is tight
Metric STI gauges are toleranced in the tight 4H class, American ones in 2B or the tighter 3B. That is not an end in itself. If the insert sits in a receiving thread that is too wide, it does not bear on all coils and the joint loses exactly the load capacity the insert was fitted for. Too tight, and the insert cannot be wound in cleanly.
Check twice
Two checks make sense: the receiving thread with the STI gauge before the insert goes in, and the finished nominal thread with the normal 6H gauge once the insert is seated. Only both together prove the repair will hold.
The matching tools are under STI threading tools, normal gauges among the thread plug gauges and thread ring gauges, the handling in the article check internal threads with thread plug gauges.
