How to choose a thread tap and die set correctly

Coarse or fine thread - which do you need?

Metric threads to DIN 13 come as coarse (M) and fine (MF) pitch. On a coarse thread the pitch is fixed to the diameter, for example M8 with 1.25 mm pitch. Fine threads have a smaller pitch such as M8x1 and resist loosening under vibration better.

For repair work you first have to identify the existing thread. Measure the outside diameter with a caliper and the pitch with a thread gauge. Only then can you tell whether an M or MF tap and die is correct.

Which HSS grade is the right one?

Hand taps and dies are usually made of tool steel (WS), HSS or HSS-E. Tool steel is cheap and fine for soft materials. HSS is heat resistant and the standard for most repairs, while HSS-E contains about 5 % cobalt and cuts tough and alloyed steel with far better durability.

• WS (tool steel): for aluminium, brass and plastic, low life in steel.
• HSS: heat resistant to around 600 °C, a good all-round choice for mild steel and cast iron.
• HSS-E (with cobalt): for stainless, high-strength and alloyed steel, longer tool life.
• Check the tolerance class - for most fasteners class 6H is sufficient.

Which tap-wrench equipment belongs in the set?

The tap wrench guides the tap straight, while the die stock holds the round die. An adjustable tap wrench with two jaws covers a whole diameter range, whereas fixed wrenches are stiffer and more accurate per size.

For repairs a complete set of taps, dies, both holders and a matching drill set is the most practical. You can cut new threads and re-cut existing, dirty or slightly damaged threads without replacing parts.

Frequently asked questions