I use #43 .0890 . the tap specifies 2.5 mm .0984 . I use #43 with no probs also it's the same for my 4-40 sae treads . Get a spiral point (gun tap) for non blind holes , works so much better , just a little more $ then your std taper/plug tap . when tapping use some cutting fluid of some sort and keep the tap clean
