Threading Inserts and Holders, Decoded: How to Read 16ER-150ISO and Match It to a Tool

The lathe is stopped, there is a print calling for M20 × 2.5, and in your hand is an insert stamped 16ER-250ISO and a holder stamped SER0750K16. Do they go together? Will that insert cut the thread on the print? This is the code, position by position, and the rule that tells you whether an insert and a holder belong to each other—because on a threading tool, unlike a turning tool, the insert is the thread.

A threading insert is not a turning insert with a sharp corner.

In turning, the insert removes material, and the toolpath decides the shape. In threading, that is reversed: the toolpath only repeats down the length of the part, and the finished thread form is ground into the insert itself. Flank angle, root radius, crest—all of it lives on the cutting edge before the tool ever touches the workpiece.

That single fact drives everything else. It is why a threading insert is specified by a thread standard and a pitch rather than by a nose radius, why you cannot "make do" with a close insert, and why the catalog for a threading line is so much deeper than for turning. We stock 249 threading inserts, 204 threading tool holders and 52 threading boring bars, and that depth is not variety for its own sake—it is what it takes to cover the thread forms a job shop actually meets.

If you have not read it yet, the companion piece on turning insert designations covers the ISO code for general turning inserts. Threading uses a different system entirely.

Reading the code: 16ER-250ISO, position by position

Four pieces of information, in a fixed order. Take 16ER-250ISO:

So 16ER-250ISO is a size-16 insert, external, right-hand, ISO metric 2.50 mm pitch. And 06NL-075ISO is a size-06 insert, internal, left-hand, ISO metric 0.75 mm. Once you can read it, the whole shelf becomes searchable.

Full profile or partial profile: the decision that sets your inventory

This is the real fork in the road, and it is the one that decides whether you buy six inserts or sixty.

A full-profile insert is ground for one specific pitch. It cuts the flanks and forms the crest of the thread, which means the thread comes off the machine complete and to the standard—no separate operation to bring the major diameter down, and a controlled crest radius that resists cracking. The price is inventory: one pitch, one insert.

A partial-profile insert grinds only the flank angle—55° or 60°—and leaves the crest alone. One insert then covers a range of pitches in that flank angle, because the flanks are the same 60° whether the pitch is 1.0 mm or 2.0 mm. The price is that you have to turn the outside diameter to size first, since the insert will not form the crest for you.

You can see the trade-off in our own catalog without reading a word of theory. Of the 245 coded threading inserts we stock:

• 96 ISO metric references, covering pitches from 0.30 mm to 8.00 mm—one reference per pitch, per size, per hand, per side.
• 90 Whitworth references, covering 4 to 56 TPI—same story.
• 57 partial-profile references in total: 29 at 55° and 28 at 60°.

Three-quarters of the references exist because a full profile is pitch-specific. The 57 partial-profile references cover, between them, a far wider span of threads. If you run one or two repeat threads, buy a full profile and get a better thread. If you take in whatever walks through the door, a partial profile is what keeps the drawer sane.

Thread form: what the letters after the pitch mean

The form code is where the print gets satisfied or missed. These are the families we carry:

• ISO—ISO metric, 60° flank, specified by pitch in millimeters. M20 × 2.5 means a 2.50 mm pitch, so you want a -250ISO insert. Our range runs 0.30 mm to 8.00 mm.
• W—Whitworth, 55° flank with rounded crest and root, specified in threads per inch. Ours runs 4 to 56 TPI. This is the family that covers BSW, BSF and the British pipe threads still found on older equipment and hydraulics.
• A60—partial profile, 60° flank. One insert across a pitch range for metric, UN and other 60° threads.
• A55—partial profile, 55° flank. The same idea for the Whitworth family.

Two practical notes. First, 55° and 60° are not close enough to substitute: a 5° flank error shows up as a thread that will not gauge. Second, a metric thread and a UN thread share the same 60° flank but are specified differently—metric by pitch in millimeters, UN by threads per inch—so a 60° partial-profile insert covers both, while a full-profile insert does not.

Beyond those, API threads for oilfield connections are their own world, and they get their own tooling: our API threading holders take 22ER and 27ER inserts, and the API threading boring bars take 22NR and 27NR. Those are the two largest insert sizes we stock, which is not a coincidence—API threads are coarse and deep, and the insert has to survive it.

Internal or external is a different tool, not a setting.

An external thread is cut with a square-shank holder working on the outside of the part. An internal thread is cut with a bar that has to reach down a bore, which brings in every rigidity problem covered in boring bar nomenclature: overhang, minimum bore, and chatter.

There is a pattern in our catalog worth knowing before you order. Sizes 06 and 08 exist only as internal inserts. Not one external reference in either size—49 references, all N. That is not an oversight: the small sizes exist so that a small bar can reach into a small bore. If you are threading the outside of a part, your range starts at 11.

Size 16 is the workhorse, with 89 of the 245 coded references, split almost evenly between internal and external. If you are standardizing a shop on one size, that is the one with the deepest shelf behind it.

Matching the insert to the holder: the holder already tells you

This is the part that causes the most wasted orders, and it has the simplest answer. Every threading holder on our site names the insert it takes in its own title.

Read SER0750K16: a threading tool holder, right hand, 0.750" shank, and the title finishes with "16ER Inserts". It takes size 16, external, right hand. Put a 16EL in it, and the hand is wrong. Put an 11ER in it, and the insert is too small for the seat.

The available pairings are not arbitrary—insert size and shank size track each other, because a bigger insert needs a bigger seat and a bigger seat needs a bigger shank:

One bonus that comes with the same seat: the lock-ring grooving inserts in the LG series—16ER-100LG through 16ER-250LG, in groove widths from 0.047" to 0.104"—drop into the same size-16 laydown pocket as the threading inserts. If you already own the holder, you can cut a retaining-ring groove without buying another tool. That is why you will find them alongside the threading inserts on the site.

Size 16 spans six shank sizes, which is another way of saying it is the size most machines are set up for. Sizes 22 and 27 start at a 1" shank because the cut they take needs the section behind it.

Browse threading tool holders →

Hand, and the case for a left-hand tool on a right-hand thread

Hand seems obvious until it isn't. A right-hand thread is normally cut with a right-hand tool feeding from the end of the part toward the headstock. Straightforward.

But there are jobs where that approach is the problem: a thread that runs up to a shoulder with no relief groove, or a part too slender to push toward. The answer is to cut it in reverse—start at the shoulder and feed outward, using a left-hand tool with the spindle reversed. The thread that comes out is still right-hand; only the direction of travel changed.

That is why we carry left-hand references at all—72 of the 245—and why RMC's line includes a dedicated reverse-hand threading holder in the STZO style. If you have ever run a thread into a shoulder and scrapped it on the last pass, this is the tool you were missing.

Threading inside a bore: three different tools

Internal threading is where the tooling gets specific, and our range splits three ways:

• Standard internal threading bars take the N-series inserts—06N, 08N, 11N, 16N—and are chosen by minimum bore and reach.
• API threading boring bars take 22NR and 27NR inserts for oilfield connections. These are heavy bars: 1.5", 1.75" and 2" diameters.
• Vertical threading boring bars take triangular negative TNMC inserts rather than the laydown N-series—a different clamping geometry for a different rigidity problem.

On the RMC side the logic is different again: the MTVO and STVO styles are built for O.D. threading and shallow grooving with triangular inserts, the MTHO and STHO gang styles handle shallow grooving or I.D. threading, and the NS and NE series run NG inserts for threading and grooving together. All 156 of them now live in one place: RMC threading & grooving tool holders, made in the USA.

Six things that ruin a threaded part

1. The flank angle is wrong. 55° insert on a 60° thread, or the reverse. The part looks threaded and will not gauge. Check the form code before the pitch.
2. Partial profile without pre-turning the diameter. A partial-profile insert does not form the crest. If the outside diameter was not brought to size first, the major diameter comes out wrong every time.
3. Hand mismatch between insert and holder. An R insert in an L holder does not seat correctly. The title of the holder tells you which it takes—read it before ordering.
4. Too much overhang on an internal thread. The bar deflects, the pitch diameter wanders, and the finish goes. Pick the largest bar the bore will accept and the shortest reach that will do the job.
5. Taking the whole depth in too few passes. A threading insert cuts on a form, so the engaged edge grows with every pass. Follow the pass and infeed schedule the insert maker publishes for the pitch—it is not the same as a turning depth of cut.
6. Buying the pitch instead of the print. M20 × 2.5 and 20 × 2.5 UN are not the same thread. Confirm the standard, then the pitch, then the hand.

Ordering without guessing

Four questions, in this order, and the code writes itself:

1. Inside or outside? N or E.
2. Which standard and pitch? From the print: ISO metric in millimeters, Whitworth in TPI, or a 60°/55° partial profile if you want range instead of a finished crest.
3. Which hand? R or L—and remember reverse-hand cutting is an option when the geometry is awkward.
4. Which size? Decided by the holder or bar you already own. If you are buying both, size 16 has the deepest range behind it; 06 and 08 are internal only.

Everything above is on the shelf: threading inserts, threading tool holders, threading boring bars, and the full threading range in one place. Not sure which one your job needs? Send us the print or the thread callout, and we will match it.

FAQ

How do I read a threading insert code like 16ER-150ISO?

Four pieces in a fixed order: size, internal or external, hand, then form and pitch. 16 is the insert size, E means external (N would be internal), R means right-hand, and 150ISO means ISO metric with a 1.50 mm pitch. So it is a size-16 external right-hand insert for a 1.5 mm metric thread.

What is the difference between a full-profile and a partial-profile threading insert?

A full-profile insert is ground for one specific pitch and cuts both the flanks and the crest, so the thread comes off complete and to standard. A partial-profile insert grinds only the flank angle, 55 or 60 degrees, and covers a range of pitches, but it does not form the crest, so the outside diameter has to be turned to size first. A full profile gives a better thread; a partial profile gives you fewer inserts to stock.

Can I use a 60-degree insert on a 55-degree thread?

No. A five-degree flank error produces a thread that will not gauge. The 60-degree family covers ISO metric and UN threads; the 55-degree family covers Whitworth and the British pipe threads. Check the form code before you check the pitch.

Does the N in a threading insert code mean neutral?

No. On a threading insert, 'N' means 'internal', for threads cut inside a bore, and 'E' means 'external'. The letter N in a turning holder code means something different entirely, a zero-degree clearance angle, which is why this is the most commonly misread character in the code.

How do I know which insert a threading holder takes?

The holder title states it. SER0750K16 is a right-hand threading tool holder with a 0.750-inch shank that takes 16ER inserts. Insert size and shank size track each other because a larger insert needs a larger seat: Size 11 runs on 0.312 and 0.375 inch shanks, size 16 spans 0.375 through 1.250 inch, and sizes 22 and 27 start at a 1-inch shank.

Why are there no small external threading inserts?

Because the small sizes exist to reach inside small bores. In our range, sizes 06 and 08 are internal only, with no external references at all. External threading starts at size 11.

Can I cut a right-hand thread with a left-hand tool?

Yes, and sometimes you should. Cutting in reverse, starting at the shoulder and feeding outward with the spindle reversed, still produces a right-hand thread. It is the usual answer for a thread that runs up to a shoulder with no relief groove or a part too slender to push toward. RMC makes a dedicated reverse-hand threading holder in the STZO style for exactly this.

See the full threading range →

Insert sizes, thread forms, pitch ranges and holder pairings in this article are taken from the FM Carbide catalog as stocked.