If it happens, it might caused by the abrasion of the taps. It could be noticed by observing if there has abrasion in the bottom of the shank. Change the taps often; or using more wear-resistant material or increasing coating if not satisfy with the life of taps.

After the introduction of ISO into JIS in 1982, it became the new JIS by some amendments .And the specifications can be seen as followings:

1)      Marks indicate

Old JIS        New JIS

PT = = = = = R (External thread) Rc (Internal thread)

PS = = = = = Rp

Pf = = = = = G

2)  Precision

Old JIS      New JIS

PF Grade A Internal Thread = = = G Internal Thread

PF Grade B Internal Thread = = = Abolition

3)      Can the taps be applied generally?

Yes (But the precision can not be guaranteed)

4)      Can the gauge be applied generally?

No (The tolerance is different)

     No. The precision of the gauges is made by the precision of external and internal threads, which should be made for the precision of  taps.