On this page
- Threaded Stem Caster Thread Pitch Guide: UNC vs Metric, Decoded
- Quick-answer pitch matrix (the only table most buyers need)
- UNC thread sizes explained (Unified National Coarse)
- Metric thread sizes explained (ISO)
- How to identify an unknown stem before you reorder
- Torque targets and thread-engagement rules
- UNC vs metric — why they do NOT cross-thread
- Replacement matrix by industry
- Frequently asked questions
- Related Engineering Tools & Guides
Threaded Stem Caster Thread Pitch Guide (UNC vs Metric Explained) differ by load capacity, wear behavior, and floor compatibility.
- Match capacity per caster to your total load divided by 3 (one caster may be airborne)
- Polyurethane and rubber wheels favor floor protection; phenolic and steel favor heavy capacity
- Top-plate or stem mount is dictated by the equipment, not preference
- CasterHQ stocks Albion, Hamilton, P&H, Colson, Faultless, and Durastar from Mansfield, Texas
- Call 844-439-4335 for fitment help on any non-standard caster
Threaded Stem Caster Thread Pitch Guide: UNC vs Metric, Decoded
Threaded stem casters in North America use four common thread specs: 3/8"-16 UNC, 1/2"-13 UNC, 5/8"-11 UNC, and metric 10mm-1.5 or 12mm-1.75. Mixing UNC with metric feels tight for about 30 days, then strips. This guide gives the exact pitch, engagement depth, torque, and replacement matrix so your next order matches the old stem exactly.
In this guide
Quick-answer pitch matrix (the only table most buyers need)
Four threads cover ~95% of threaded stem casters sold in North America: 3/8"-16 UNC, 1/2"-13 UNC, 5/8"-11 UNC, and metric 10mm-1.5 or 12mm-1.75. The first number is the outside diameter, the second is threads-per-inch (UNC) or pitch-in-mm (metric). Get those two specs right and the caster will seat flush, torque to spec, and hold.
| Thread spec | OD (in) | TPI / pitch | Typical caster capacity | Common on |
|---|---|---|---|---|
| 3/8"-16 UNC | 0.375 | 16 TPI | 75-250 lb | Office chairs, carts, light shelving |
| 1/2"-13 UNC | 0.500 | 13 TPI | 250-500 lb | Hospital beds, med carts, warehouse carts |
| 5/8"-11 UNC | 0.625 | 11 TPI | 500-1,200 lb | Industrial racks, floor scrubbers, tool boxes |
| 10mm-1.5 metric | 0.394 | 1.5mm pitch | 100-300 lb | European furniture, imported equipment |
| 12mm-1.75 metric | 0.472 | 1.75mm pitch | 250-600 lb | European industrial, MRO replacements |
Engineer tip: If the equipment is U.S.-built, assume UNC. If it was imported from Europe, Japan, or Korea (or the label has a CE mark), assume metric. The two thread systems are NOT interchangeable even when the major diameter looks identical.
UNC thread sizes explained (Unified National Coarse)
UNC (Unified National Coarse) is the standard fastener thread spec in the United States, governed by ASME B1.1. Threaded stem casters use coarse pitch because coarse threads tolerate minor corrosion, paint buildup, and cross-threading better than fine (UNF) threads.
- 3/8"-16 UNC — 0.375" OD, 16 threads per inch. The most common caster stem in North America. Used on office chairs, healthcare carts, AV carts, light warehouse carts.
- 1/2"-13 UNC — 0.500" OD, 13 TPI. The workhorse medium-duty stem. Hospital beds, imaging carts, commercial kitchen carts, standard industrial carts.
- 5/8"-11 UNC — 0.625" OD, 11 TPI. Heavy-duty. Found on powered floor machines, pallet jacks with stem casters, heavy tool chests, and mezzanine carts.
Less common UNC sizes you may see on vintage or custom equipment: 1/4"-20 (very light duty), 7/16"-14 (specialty), 3/4"-10 (extra-heavy).
Field note: On commercial kitchen equipment, 1/2"-13 is almost always correct. We stock same-day replacements in 4" and 5" poly and rubber with stainless NSF-friendly finishes.
Metric thread sizes explained (ISO)
Metric threads are designated M<diameter>-<pitch> (e.g., M10-1.5). The diameter is in millimeters. The pitch is the linear distance in mm between thread crests.
| Metric spec | OD (in) | Nearest UNC | Safe to swap? |
|---|---|---|---|
| M8-1.25 | 0.315" | 5/16"-18 | NO — different pitch |
| M10-1.5 | 0.394" | 3/8"-16 | NO — looks close, strips in 2-6 weeks |
| M12-1.75 | 0.472" | 1/2"-13 | NO — different pitch |
| M14-2.0 | 0.551" | 9/16"-12 | NO |
| M16-2.0 | 0.630" | 5/8"-11 | NO |
The biggest trap: M10-1.5 vs 3/8"-16 UNC. The major diameters differ by only 0.019" (about 1/2mm) so a metric stem will start to thread into a UNC receiver and feel tight for maybe 3-4 turns. Under load-cycling vibration, the misaligned threads shave metal from both sides until the nut spins free.
How to identify an unknown stem before you reorder
When the equipment has no part number and the old caster is chewed up, use this three-minute procedure:
- Measure OD with calipers. Clean the threads first. Read to 0.001" or 0.01mm.
- Count threads. Lay a machinist's rule or a thread gauge against the stem. Count the number of full threads that fit within exactly 1 inch (or 10mm for metric).
- Compare to the matrix above. A 0.500" stem with 13 TPI is 1/2"-13 UNC. A 0.472" stem with ~5.7 crests per 10mm (= 1.75mm pitch) is M12-1.75.
No thread gauge? The pragmatic field test: take a known 1/2"-13 UNC nut (any hardware-store hex nut will do) and try to hand-spin it onto the stem. If it threads cleanly to the shoulder, you have 1/2"-13. If it binds at the second or third thread, it is metric.
Never force a nut. If it binds, stop. Forcing a metric nut onto a UNC stem (or vice versa) cuts new threads into the stem and makes the caster unusable.
Torque targets and thread-engagement rules
Thread engagement is the number of full threads above and below the mounting plate that are in contact with the retaining nut. Rule of thumb from ASME and field experience: at least 1.5× the thread diameter of engagement for full rated load.
| Stem | Min. engagement | Main nut torque | Jam nut torque |
|---|---|---|---|
| 3/8"-16 UNC | 0.56" (9 threads) | 25-30 ft-lb | 20 ft-lb |
| 1/2"-13 UNC | 0.75" (10 threads) | 40-50 ft-lb | 30 ft-lb |
| 5/8"-11 UNC | 0.94" (10 threads) | 75-90 ft-lb | 60 ft-lb |
| M10-1.5 | 15mm (10 threads) | 25-30 ft-lb | 20 ft-lb |
| M12-1.75 | 18mm (10 threads) | 40-50 ft-lb | 30 ft-lb |
Always install a jam nut on top of the main nut. Tighten main nut to spec, hold main nut with a wrench, then tighten jam nut against it. This is the only passive anti-loosening solution that holds up to caster vibration without thread-locker.
Thread-locker pick: Use Loctite 242 (blue) for serviceable installs. Skip Loctite 271 (red) unless the stem will never be removed — red is rated for 450°F+ heat to release.
UNC vs metric — why they do NOT cross-thread
The confusion comes from major diameters that look almost identical. A 3/8" UNC stem measures 0.375"; an M10 metric stem measures 0.394" — a 0.019" difference (about the thickness of two pieces of printer paper). But the pitches are different: 16 TPI on UNC means a crest every 0.0625"; M10-1.5 means a crest every 0.059". Across 10 threads, that drift adds up to a full thread's worth of mismatch.
Here is what happens when you force the wrong thread:
- First 24-72 hours: feels tight. Holds under static load.
- Week 1-2: micro-vibration starts to shave the tops off the misaligned threads.
- Week 3-6: the caster begins to wobble, then spins free. Sometimes the stem drops out completely.
This is the #1 reason a threaded stem caster loosens after installation — not jam-nut error, not under-torque, but mixed thread systems.
Replacement matrix by industry
| Industry / equipment | Almost always | Sometimes | Rare |
|---|---|---|---|
| Office chairs / task chairs | 3/8"-16 UNC | 7/16" press-fit grip-ring | Metric M10 |
| Hospital beds / med carts | 1/2"-13 UNC | 5/8"-11 UNC | Expanding adapter |
| Commercial kitchen (U.S.) | 1/2"-13 UNC | 3/8"-16 UNC | Plate-mount |
| European imported equipment | M12-1.75 | M10-1.5 | M16-2.0 |
| Warehouse / tilt carts | 5/8"-11 UNC | 1/2"-13 UNC | Plate-mount |
| AV / broadcast carts | 3/8"-16 UNC | Grip-ring stem | M10-1.5 |
| Industrial floor scrubbers | 5/8"-11 UNC | Plate-mount | M16-2.0 |
When in doubt: call with the OEM brand name of the equipment. Most of the time we can match the original stem from the model number alone.
Key takeaways
- Four thread specs cover 95% of threaded stem casters: 3/8"-16 UNC, 1/2"-13 UNC, 5/8"-11 UNC, M10-1.5, M12-1.75.
- UNC and metric are NOT interchangeable. Mixing them looks fine for 30 days, then strips.
- Minimum thread engagement = 1.5× the thread diameter. Fewer than 9 threads in contact = premature failure.
- Always use a jam nut tightened against the main nut. Never rely on a single nut and vibration alone.
- If the equipment has a CE mark or was built outside North America, assume metric until proven otherwise.
Frequently asked questions
How do I know if my caster stem is UNC or metric?
Measure the major diameter with calipers and count the threads per inch (or per 10mm for metric). A 0.500" stem with 13 threads per inch is 1/2"-13 UNC. A 0.472" stem with 1.75mm pitch is M12-1.75. The quick field test: try threading a known 1/2"-13 hardware-store nut. If it spins on cleanly to the shoulder, it's UNC. If it binds at the second or third thread, it's metric.
Can I use a 3/8"-16 UNC caster in a hole tapped for M10-1.5?
No. The major diameters differ by only 0.019" so it will feel tight at first, but under caster vibration the mismatched thread crests will shave down and the fastener will strip out — usually within 30-60 days. Use the correct thread spec or re-tap the hole to match the caster you want to install.
What thread is on a typical office chair caster?
Most North American office and task chairs use a 7/16" press-fit grip-ring stem, NOT a threaded stem. European chairs use an 11mm grip-ring. If your chair does have a threaded stem, it is almost always 3/8"-16 UNC.
How much thread engagement is enough?
Minimum engagement is 1.5× the thread diameter. For a 1/2"-13 UNC stem, that's 0.75" of threads in contact with the nut (roughly 10 full threads). Less than that and you lose static capacity. For full dynamic load rating, aim for 2× diameter of engagement when the mount allows.
Should I use thread-locker on caster stems?
Use Loctite 242 (blue, medium-strength, removable with a wrench) on any threaded stem caster that sees vibration or load-cycling. Skip red Loctite 271 unless the caster will never be removed — red is permanent and requires ~450°F of heat to break. The belt-and-suspenders approach: jam nut plus blue thread-locker.
Is a fine thread (UNF) ever used on caster stems?
Very rarely. Caster stems use coarse thread (UNC) because coarse threads tolerate paint, corrosion, and minor cross-threading better than fine. If you find a fine-thread stem, it's almost always on a specialty or military-spec application. Replace with the same spec — do not substitute coarse.
Need the Right Stem? We'll Match It From a Photo.
CasterHQ stocks 3/8"-16, 1/2"-13, 5/8"-11, M10-1.5, and M12-1.75 threaded stem casters in poly, rubber, steel, and nylon. Text a photo of your old stem and we'll identify the thread, size, and nearest match — usually within an hour.
References & Standards Cited
- ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form)
- ISO 262 — ISO general-purpose metric screw threads
- ICWM — Industrial Caster & Wheel Manufacturers Association caster stem specification guidelines
- Machinery's Handbook 31st ed. — Thread engagement and torque tables
- Field data — CasterHQ Mansfield fulfillment center, 2023-2026 replacement-caster tickets
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Shop All CastersCall 844-439-4335Jordan Wilson
Founder of CasterHQ.com. Works directly with engineers, MRO buyers, and procurement teams across material handling, healthcare, food service, aerospace, and OEM. CasterHQ stocks Albion, Hamilton, P&H, Colson, Faultless, and the in-house Durastar series from a Texas warehouse and retrofits OEM fitments from dimensional drawings when brands discontinue parts.