Buying taps - which pitch diameter limit?

[Birdpath]

Hi folks:
Starting to replace some of my old locally sourced taps with better spiral point taps. When I look at Greenfield taps, I see I have a lot of choices regarding pitch diameter limit (i.e. "H" value). So much for thinking there is only one 1/4-20 spiral point tap! Is there one (e.g. H1?) that is considered standard or basic for home shop use? Tapping holes the in the usual random steel and aluminum. Thanks for any insight or direction. Best, Daniel

 

[projectnut]

The most common sizes are H2 and H3. Each H number is .0005" over the nominal size. An example for a1/4-20 tap would be: an H1 would tap to .2505, an H2 tap would be .251", a H3 tap would be .2515" . The H numbers go up to H11 if I remember correctly. Be careful when selecting taps. There are also designations like 1/4-20 STI. The STI refers to "special thread insert" These taps are used for Heli coils and other thread inserts.

H1 is a fairly tight fit. It will work fine with high quality fasteners, however typical hardware store fasteners are not precision quality and may have interference problems.

ON EDIT: Typo in my original post that has been corrected. An H3 will will tap to .2515", an H5 tap will tap to .2525"

 

[MrWhoopee]

I believe H3 is the most commonly provided if you just order a tap without an H specification.

I always thought STI meant spiral thread insert, but apparently not.
"S.T.I. (Screw Thread Insert) Taps are special taps for helical coil wire screw thread inserts, which provide positive means for protecting and strengthening tapped threads in any material. These STI taps are correctly sized to produce an internal thread that accommodates a helical coil wire screw thread insert."

 

[Birdpath]

Thanks for the help!