I was just about to use some metric taps for the first time (3 and 4mm) when I realised that the set had two sizes of thread pitch for each diameter.These are 3 x 0.5, 3 x 0.6, 4 x 0.7, 4 x 0.75. The wiki on thread sizes wasn't helpful in explaining this difference, and the difference isn't due to coarse or fine thread sizes
The 3 x 0.5 and 4 x 0.7 seem to be the "correct" sizes, so what are the other thread pitches for?
Are you sure they are not 3.2mm x 0.59mm and 4.1mm x 0.73mm? (5BA and
3BA respectively)
It's a metric only set of taps and dies. It's old, but unused (obtained when I cleared out my F-i-L's shed over 10 years ago). Have a look here for a modern version:
The "Product contents" have similar sizes to the set I have.
Ignoring wiki, this says there are two fine pitch sizes as well as one coarse pitch size
M3x0.5 MF3x0.35 MF3x0.6
M4x0.7 MF4x0.5 MF4x0.75
Looks like the wiki is deficient in that it only mentions one fine pitch! It's interesting that there are slightly different tapping drill sizes for coarse or fine pitch threads.
Thanks for that ref - it didn't turn up in the StartPage search I did. I'll use the coarse taps.
Slightly odd that one of each fine thread is actually coarser than the corresponding coarse thread though.
Standards are wonderful things: that's why they have so many of them.
Nick
But BA sizes /are/ metric!
[Though I realise yours aren't BA.]
Yes, I don't claim to know much about obscure threads ...
"Preferred_sizes" is the clue here. M3x0.5 and M4x0.7 are the standard coarse sizes. The other sizes may be used in odd applications. I think the aircraft industry is one where ususal sizes are used.
Hide the M3x0.6 and M4x0.75 taps to stop yourself using them accidentally.
I'd missed it was ISO 262 and not 261. Interestingly, BS ISO 261 at
That's a good point!
My metric set contains two pitches for many of the common sizes, it's quite easy to get caught out. I think one pitch usually dominates though.
Or stick a bit of (coloured) tape around their shanks to differentiate them.
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