Round Die and Die Stock Size Chart

A round die cuts an external thread. The thread it cuts — M10 × 1.5, 1/4-20 UNC — and the outside diameter of the die itself are two separate things, and mixing them up is the most common ordering error we see.

The die stock grips the die by its outside diameter, not by the thread. Order a die stock from the thread size alone and you will usually get the wrong one.

This page gives the standard die stock sizes, the die outside diameter normally supplied for each thread, and the rod diameter to turn before you start threading.

Start here: a die stock is sized by the die, not the thread

A round die is stamped on its face with two pieces of information:

• the thread it cuts, for example M10 × 1.5 or 1/4-20
• its outside diameter and thickness, for example 38 × 14 (mm) or 1.1/2 × 1/2 (inch)

The second figure is the one that decides which holder you need. Two dies that cut exactly the same thread can be made in different outside diameters — one may fit a 38 mm holder, another needs a 45 mm — so the thread alone never tells you which stock to buy.

The rule: read the OD off the die, then find that OD in the chart below.

DIN 223 metric-OD solid die stocks

Solid die stocks hold one-piece round dies made to a metric outside diameter. Some diameters are supplied in more than one die thickness, which is why a single stock size appears twice below.

Working in inches for the same range? 20 mm ≈ 0.787 in · 25 mm ≈ 0.984 in · 30 mm ≈ 1.181 in · 38 mm ≈ 1.496 in · 45 mm ≈ 1.772 in · 55 mm ≈ 2.165 in.

These stocks are not adjustable in the same way as split stocks. A solid die stock clamps the die firmly; thread fit is set by the die, not by the holder. If you need to open or close the thread slightly, you want a split die and a split stock instead.

BS 1127 inch-OD split die stocks

Split die stocks hold circular split dies made to an inch outside diameter. They clamp the die with three screws, which also lets you open or close the die slightly to fine-tune the thread fit — useful when the thread has to match an existing part.

Split or solid: which die stock do you need?

The two types are built around different die standards and, as a rule, are used in matched pairs.

A solid die can often be clamped in a split holder where the OD matches, but a split die needs a split holder to get any benefit from the adjustment screws. If you are buying both together, match the types — and match the OD.

Die outside diameter by thread

The table below lists the die outside diameter most commonly supplied for each thread. Dies for the same thread are sometimes produced in more than one OD, so treat these as typical sizes and always confirm against the OD marked on the die you actually have.

Metric threads

Fine-pitch dies normally use the same outside diameter as the coarse die of the same nominal size — match on the nominal diameter, then confirm the pitch. M6 × 0.75, M8 × 1.0, M10 × 1.0 and M10 × 1.25, for example, follow the same 13/16 in and 1 in ODs as their coarse equivalents.

UNC and UNF threads

BSW and BSF threads

What diameter rod do I turn before threading?

A die forms the thread by removing material from the outside of the blank, so the diameter you turn the rod to becomes the major diameter of the finished thread.

Turn the blank to the figure in the tables below, then chamfer the leading end so the die starts square. The figures are the maximum major diameter for a standard tolerance external thread — for general work, aim just under the number shown. A blank that is even slightly oversize makes the die hard to start and tears the crests of the thread.

Metric coarse

Metric fine

Unified coarse (UNC)

Unified fine (UNF)

Whitworth (BSW and BSF)

For Whitworth threads the maximum major diameter is the nominal size, so the figure below is both the nominal diameter and the diameter to turn the rod to. Sit a touch under.

Thread-cutting dies vs re-threading dies

Not every round die is meant to cut a new thread, and the difference matters when you are repairing rather than manufacturing.

• Thread-cutting (solid or split) dies have a lead chamfer and cut a full thread from a plain blank. This is what you need to make a thread from scratch.
• Re-threading / repair dies are made to clean up or restore a damaged thread rather than cut a new one. They are commonly marked as such, and they work best on a thread that already exists.

If you are chasing a damaged thread on a vehicle or a machine, a repair die is usually the right tool, and it is far more forgiving of an undersize or slightly oval blank.

Common ordering mistakes

Ordering the die stock from the thread size. The holder is sized by the die OD. Always read the OD off the die face first.

Assuming the OD is the same across brands. The same thread can be made in different outside diameters. A die that fits your holder from one supplier may not fit one from another.

Forgetting the thickness. A 38 mm stock is available for 10 mm and 14 mm thick dies. If you match the OD but not the thickness, the die will not seat properly.

Turning the blank to the nominal diameter. Turning a 10.00 mm blank for an M10 × 1.5 thread makes the die bind. Turn it to just under the figure in the chart.

Skipping the lead chamfer. The die needs a chamfered start to bite squarely. Without it the die starts crooked and the first threads of the run are out of line.

Mixing split and solid. A split die only adjusts in a split holder. Putting it in a solid holder throws away the adjustment you paid for.

Frequently asked questions

What is the difference between a round die and a die nut? A round die is a circular die held in a die stock, which gives you leverage to turn it by hand or in a machine. A die nut is a one-piece hexagon that you turn with a wrench, normally for cleaning up an existing thread rather than cutting a new one.

How do I know which die stock size I need? Match the stock to the outside diameter of the die, not to the thread. Find the OD stamped on the die face, then look it up in the chart above.

Can I use a solid die in a split die stock? Often yes, if the outside diameters match, though the split holder's adjustment screws will do nothing useful. Going the other way — a split die in a solid holder — is not recommended, because the die needs the clamping pressure of a split holder to hold its setting.

What size rod do I need for an M10 thread? Turn the blank to just under 10 mm — around 9.97 mm for M10 × 1.5. A full-diameter 10.00 mm blank will bind the die and tear the thread crests.

Why does my die say 38 × 14? That is the die's outside diameter and thickness in millimetres: 38 mm OD, 14 mm thick. It tells you which holder the die fits — a 38 mm die stock that takes 14 mm dies.

Does the die outside diameter depend on the thread? Not strictly. The thread and the die OD are independent specifications, and the same thread is often available in more than one OD. That is why the charts above are labelled as typical sizes — the die marking is the authority.

Need round dies or a matching die stock?

We manufacture round dies, tap and die sets and die stocks, in carbon steel and HSS, in metric and inch thread systems. Die outside diameter, thickness and thread tolerance can be specified to match the holder you already use, and custom sizes and OEM marking are available.

If you tell us the thread and the holder you have — or the die OD stamped on the part you are replacing — we will confirm the die and stock combination that fits.

Request a quotation with the thread size, die outside diameter and thickness, quantity and destination.