Not suggesting that what you stated was incorrect, Stuart, just saying that such a tap configuration does not seem to be popular in UK industry.

Doing a bit of digging around, it seems that 2 flute taps are more used in the US for machine tapping certain materials - all the references to 2 flute taps were on US machinist websites & US manufacturers & most refered to their use for machine tapping. Maybe not the best type to use as a hand tap ?

In the UK machine tapping is usually done with spiral flute (for blind holes - swarf ejected backwards out of the hole) or spiral point (for through holes- swarf pushed forwards through the hole). A 2 flute tap has more space to accumulate swarf due to the larger gullets, where our usual 3 or 4 flute taps require a reverse rotation frequently to break the chips & stop them packing up in the smaller gullets - which usually leads to the tap breaking.

Still maintain that using a new HSS tap from a reputable manufacturer is the best way forward if you are having tapping problems. Unkown provenenace old taps - particularly those that have been rattling around for decades in an old tobacco tin ! - "no-name" autojumble specials in large sets made from "tungsten steel" (whatever that is !) and Ebay Chinese cheapies are far from guaranteed to give decent results. The cutting edges should feel sharp & easily make fine shavings with minimal pressure from your thumbnail - if a tap doesn't feel "proper sharp" when you touch it it is unlikely to cut well.

Taps can be resharpened like any other cutting tool & you may be able to improve one by hand using a small diameter round slip stone or "reamer" stone in the gullet. But for small taps they are best treated as disposable & replaced when they start to get blunt. Depending on the material you are tapping, life can be very short - I had one customer tapping 1/4" holes in a nimonic alloy casting who used a new tap on very hole. A new tap was almost guaranteed to complete one hole without issue, but it was 50:50 as to whether or not it would complete 2 without breaking. Given the casting concerned cost around £50k before machining (and that was around 1990), a couple of dozen taps per component was small beer compared to scrapping a part.

Nigel B.