5C collets: dimensions, sizes and one expensive trap

5C is the dominant American standard for holding workpiece stock on small precision lathes — not for holding cutting tools. It is governed by ASME B5.26 and is interchangeable across manufacturers, with one important exception described at the bottom of this page.

5C dimensions

Where published figures differ between makers, both are shown. See the note below before machining to any of them.
Parameter	Value	Notes
Capacity range	1/16" – 1-1/8" (1.6 – 28.6 mm)	Agreed across sources
Body outside diameter	1.250" (31.75 mm)	Parallel body, front and back
Head diameter	1.485"	The flange at the nose
External thread	`1.238"-20` / `1.242"-20`	Right-hand; the draw tube engages here
Internal thread	`1.041"-24` / `1.042"-24`	Used for a work stop or ejector
Overall length	3.25" – 3.33" (82.6 – 84.6 mm)	Published values range across makers
Slots	3 (some 6)	Three is standard on modern production
Typical runout when new	0.0005" – 0.0015" TIR	Degrades with wear — see below

One published figure we could not reconcile: the taper angle. Some references describe the 5C collet seat as a 5° per side / 10° included angle, while at least one major collet manufacturer lists the taper angle as 20°. These are not the same measurement described loosely — we have not resolved which convention each is using. Don't machine a collet closer from this page. Use it to identify and order, then confirm against the standard or the closer manufacturer's drawing.

Standard fractional increments

5C sets don't step uniformly — the increment widens as the size grows:

Size band	Increment
1/16" – 1/8"	1/64"
1/8" – 1/2"	1/32"
1/2" – 1-1/8"	1/16"

That means a full fractional 5C set is a specific, countable number of collets rather than a simple range — worth knowing before you compare two suppliers' "complete set" claims.

The C series: 3C through 7C

5C is one size in a family. The designation tells you the nominal bar capacity:

Collet	Nominal bar capacity	Metric	Typical application
3C	5/8"	16 mm	Instrument work, light bar stock
4C	1"	25 mm	Medium bar work
5C	1-1/8"	28.6 mm	General production — the de facto standard
6C	1-1/2"	38 mm	Larger bar stock, heavier production
7C	1-7/8"	47.6 mm	Largest of the standard series

Collet types worth knowing

Round, hex and square. Round is the default; hex and square hold pre-formed stock without marking it.
Step collets. An internal step registers the work on a turned shoulder, giving repeatable axial position.
Dead-length (push-out) collets. Used for second-operation work where the part must stop at a fixed axial position instead of being drawn back as the collet closes. If you've ever lost a length dimension between operations on the same part, this is usually the fix.
Emergency / soft collets. Machinable blanks you bore to a non-standard size.

Checking a used 5C collet

5C is common on the second-hand market, and worn collets are the usual reason a supposedly precise lathe won't hold tolerance. A quick test:

Clean the collet and its seat with solvent and a lint-free wipe.
Mount a precision test bar and indicate at roughly 1" from the collet face.
Indicate again at 2" and at the back of the collet to catch bell-mouthing.
Anything beyond about 0.002" TIR at 1" should be retired to rough work or replaced.

Also inspect the draw tube threads — a stripped or damaged draw tube won't close the collet fully, and the symptom looks exactly like a worn collet.

The Hardinge trap. Collets stamped Hardinge or Cataract are dimensionally and functionally distinct from later generic 5C collets. The draw-tube thread, nose projection, slot geometry and step angle are Hardinge-specific, and a collet of the same outside diameter is not interchangeable. Generic and import 5C collets will physically close to the same diameter but won't necessarily deliver the same gripping force or runout. If you have a Hardinge machine, source Hardinge-pattern collets specifically.