Gewindetypen

Picking the wrong thread system means parts that don't mate, leaks in fluid connections, or expensive rework. This page covers which thread to use, tap drill sizes for the common sizes, how to specify threads on drawings, and the mistakes that cause rejected parts.

Welche Gewindeart benötigen Sie?

Start here. Your application determines the thread system. Don't default to metric just because you prefer it — if the mating part is NPT, you need NPT.

Application	Use This	Why
General fastening (EU/Asia/global)	Metric M (coarse)	Most common worldwide. Cheapest taps and fasteners. Default for new designs outside North America.
General fastening (North America)	UNC	Standard in US/Canada. Every hardware store stocks UNC fasteners.
Dünnwandiges Rohr, vibrationsanfällig	UNF	Feingewinde = mehr Gewinde pro Zoll = stärker in dünnen Querschnitten, widersteht Lockerung unter Vibration.
Hydraulik-/Pneumatikfittings (US-Ursprung)	NPT	Konisch, selbstdichtend mit Gewindedichtstoff. Standard für US-amerikanische Hydraulikgeräte.
Hydraulic/pneumatic fittings (EU/UK/Asia origin)	BSP (BSPP/BSPT)	55-degree thread angle. Standard on European and Asian equipment. Parallel (BSPP) needs O-ring or sealant; tapered (BSPT) is self-sealing.
Spindeln, Schraubstöcke, Linearaktuatoren	ACME (inch) or Trapezoidal Tr (metric)	Starkes, viereckiges Gewindeprofil, das für Bewegungsübertragung und Axiallasten ausgelegt ist. Standard für Kraftübertragung.
Hochpräzise Einstellung, Mikrometerspindeln	Metrisches Feingewinde	Feinere Steigung = feinere Einstellung pro Umdrehung. M10x0,75 statt M10x1,5.
Electrical conduit, pipe fittings (legacy)	BSW / BSF	Whitworth threads. Still found on legacy British equipment and some electrical conduit.
Food/medical sanitary fittings	BSP parallel + O-ring or Tri-clamp	Smooth, sealable, cleanable. NPT's tapered threads trap debris.

Gewindesysteme im Überblick

System	Standard	Angle	Pitch Range	Sealing	Typical Use	Cost / Availability
Metric M	ISO 261/262	60°	0.25–6.0 mm	Parallel — none	General fastening (global)	Lowest cost, widest availability
UNC	ASME B1.1	60°	4–32 TPI	Parallel — none	General fastening (North America)	Low cost in US, limited outside
UNF	ASME B1.1	60°	12–56 TPI	Parallel — none	Thin-wall, vibration, aerospace	Low cost in US, limited outside
BSW	BS 84	55°	4–48 TPI	Parallel — none	Legacy British equipment	Medium — declining availability
BSP (BSPP)	ISO 228	55°	11–28 TPI	Parallel — needs seal	Plumbing, hydraulics (EU/Asia)	Low in EU/Asia, medium in US
BSPT	ISO 7	55°	11–28 TPI	Tapered — self-seal	Pipe connections (EU/Asia)	Low in EU/Asia, medium in US
NPT	ASME B1.20.1	60°	8–27 TPI	Tapered — self-seal	Pipe connections (North America)	Low in US, medium elsewhere
NPTF	ASME B1.20.3	60°	8–27 TPI	Dry seal — no sealant	Fuel lines, refrigeration	Medium — special taps
ACME	ASME B1.5	29°	2–16 TPI	None (motion, not sealing)	Lead screws, vises, jacks	Medium — special tooling
Trapezoidal (Tr)	ISO 2901	30°	1.5–44 mm	None (motion, not sealing)	Lead screws (metric designs)	Medium — special tooling

Cost rule of thumb Metric M taps and fasteners are the cheapest globally. UNC/UNF are cheapest in North America. Pipe threads (NPT/BSP) cost 20–40% more than standard fastener threads because the taps are specialized and used less often. ACME and Trapezoidal require custom tooling — expect 2–3x the cost of a standard thread. If your design doesn't require ACME, don't use it.

Metrische Gewinde (ISO)

The most widely used thread system worldwide. Designated as M[diameter]x[pitch]. If no pitch is specified, coarse pitch is assumed. Fine pitch threads exist for each diameter and are used when you need finer adjustment, better vibration resistance, or higher strength in thin-walled sections.

Häufige metrische Gewinde & Kernlochbohrungen

Designation	Diameter (mm)	Pitch (mm)	TPI	Tap Drill (mm)	Minor Dia. (mm)	Type
M2×0.4	2.0	0.40	63.5	1.60	1.467	Coarse
M2.5×0.45	2.5	0.45	56.4	2.05	1.913	Coarse
M3×0.5	3.0	0.50	50.8	2.50	2.359	Coarse
M3×0.35	3.0	0.35	72.6	2.65	2.521	Fine
M4×0.7	4.0	0.70	36.3	3.30	3.141	Coarse
M5×0.8	5.0	0.80	31.7	4.20	4.019	Coarse
M6×1.0	6.0	1.00	25.4	5.00	4.773	Coarse
M8×1.25	8.0	1.25	20.3	6.80	6.466	Coarse
M8×1.0	8.0	1.00	25.4	7.00	6.773	Fine
M10×1.5	10.0	1.50	16.9	8.50	8.160	Coarse
M10×1.25	10.0	1.25	20.3	8.80	8.466	Fine
M10×1.0	10.0	1.00	25.4	9.00	8.773	Fine
M12×1.75	12.0	1.75	14.5	10.20	9.853	Coarse
M12×1.5	12.0	1.50	16.9	10.50	10.160	Fine
M14×2.0	14.0	2.00	12.7	12.00	11.546	Coarse
M16×2.0	16.0	2.00	12.7	14.00	13.546	Coarse
M16×1.5	16.0	1.50	16.9	14.50	14.160	Fine
M20×2.5	20.0	2.50	10.2	17.50	16.933	Coarse
M20×1.5	20.0	1.50	16.9	18.50	18.160	Fine
M24×3.0	24.0	3.00	8.5	21.00	20.319	Coarse
M30×3.5	30.0	3.50	7.3	26.50	25.706	Coarse

Grob vs. Feingewinde — Wann was verwenden

Factor	Coarse Pitch (default)	Fine Pitch
Kosten	Niedriger — Standardgewindeschneider, breiteste Verfügbarkeit	10–30 % höher — seltenere Gewindeschneider, engere Bearbeitung
Geschwindigkeit	Schneller zu schneiden (weniger Umdrehungen)	Langsamer zu schneiden (mehr Umdrehungen pro Tiefe)
Vibrationsbeständigkeit	Ausreichend für die meisten Anwendungen	Besser — kleinerer Steigungswinkel widersteht Lockerung
Dünnwandfestigkeit	Weniger Gewindeeingriff in dünnen Querschnitten	Mehr Gewinde pro mm = stärker in dünnen Wänden
Adjustment precision	Coarser adjustment per turn	Finer adjustment per turn (M10x1.0 moves 1mm/rev vs 1.5mm)
Ausrissrisiko	Größerer Zahn = verteilt Last über weniger, größere Gewinde	Kleinerer Zahn = mehr Gewinde teilen sich die Last, aber jedes Gewinde ist schwächer
Schadenstoleranz	Mehr Toleranz gegenüber Kerben und Schmutz	Weniger Toleranz — beschädigte Gewinde quergreifen

Wann Feingewinde verwenden Verwenden Sie Feingewinde, wenn: (1) die Wanddicke zu dünn für ausreichenden Grobgewindeeingriff ist, (2) die Verbindung Vibrationslockerung widerstehen muss, (3) Sie feinere Axialverstellung benötigen, oder (4) die Anwendung Luftfahrt oder hochpräzise ist. Andernfalls bleiben Sie bei Grob — es ist günstiger, schneller und weniger wahrscheinlich Probleme während der Montage zu verursachen.

Gewindetiefe-Regeln nach Werkstoff

Thread engagement depth depends on the material. Softer materials need more engagement to develop full thread strength. Harder materials need less.

Material	Min. Thread Depth	Recommended Depth	Why
Aluminum (6061, 7075)	1.5×D	1.5–2.0×D	Soft — needs more threads to avoid stripping under load
Steel (mild, 4140)	1.0×D	1.0–1.5×D	Stark genug mit Standard-Eingriff
Stainless steel (304, 316)	1.0×D	1.0–1.25×D	Similar to steel. Galling risk means don't over-tighten.
Titanium (Ti6Al4V)	0.75×D	0.75–1.0×D	Sehr stark — tiefe Gewinde sind verschwendete Zerspanungszeit und -kosten
Brass / Bronze	1.5×D	1.5–2.0×D	Soft — strips easily. Consider helicoil if high load.
Plastics (nylon, Delrin)	2.0×D	2.0–2.5×D	Sehr weich — Grobgewinde verwenden, selbstschneidend oder Einsätze in Betracht ziehen
Cast iron	1.0×D	1.0–1.25×D	Spröde — tiefere Gewinde helfen nicht, da Versagen durch Bruch auftritt, nicht durch Ausriss

D = nominal thread diameter. Example: M6 in aluminum needs minimum 9mm thread depth (1.5 × 6).

Kostenauswirkung übermäßiger Gewindetiefe Deep threads in hard materials (stainless, titanium) dramatically increase cycle time. Going from 1.0×D to 2.0×D on an M10 thread in 316 stainless adds 5 full turns of tapping — that's significant. If you don't need the extra strength, don't specify it.

Unified-Gewinde (UNC/UNF)

The standard thread system in North America. UNC (Unified National Coarse) for general use; UNF (Unified National Fine) for thin walls, vibration, and precision applications. Same 60-degree thread angle as metric but sized in inches.

Häufige UNC/UNF-Größen & Kernlochbohrungen

Designation	Diameter (mm)	Diameter (in)	TPI	Pitch (mm)	Tap Drill (mm)	Tap Drill (in)	Type
#0-80 UNF	1.524	0.060	80	0.318	1.25	#56 (1.18)	Fine
#1-64 UNC	1.854	0.073	64	0.397	1.50	#53 (1.51)	Coarse
#2-56 UNC	2.184	0.086	56	0.454	1.80	#50 (1.78)	Coarse
#4-40 UNC	2.845	0.112	40	0.635	2.35	#43 (2.38)	Coarse
#6-32 UNC	3.505	0.138	32	0.794	2.95	#36 (2.97)	Coarse
#8-32 UNC	4.166	0.164	32	0.794	3.60	#29 (3.45)	Coarse
#10-24 UNC	4.826	0.190	24	1.058	4.20	#25 (3.96)	Coarse
#10-32 UNF	4.826	0.190	32	0.794	4.40	#20 (4.06)	Fine
1/4-20 UNC	6.350	0.250	20	1.270	5.35	#7 (5.11)	Coarse
1/4-28 UNF	6.350	0.250	28	0.907	5.50	#3 (4.70)	Fine
5/16-18 UNC	7.938	0.3125	18	1.411	6.80	F (5.49)	Coarse
5/16-24 UNF	7.938	0.3125	24	1.058	7.00	I (5.69)	Fine
3/8-16 UNC	9.525	0.375	16	1.588	8.30	5/16 (7.94)	Coarse
3/8-24 UNF	9.525	0.375	24	1.058	8.60	Q (5.90)	Fine
7/16-14 UNC	11.112	0.4375	14	1.814	9.80	U (6.81)	Coarse
1/2-13 UNC	12.700	0.500	13	1.954	11.10	27/64 (10.72)	Coarse
1/2-20 UNF	12.700	0.500	20	1.270	11.60	29/64 (11.51)	Fine
5/8-11 UNC	15.875	0.625	11	2.309	14.00	17/32 (13.49)	Coarse
3/4-10 UNC	19.050	0.750	10	2.540	17.00	21/32 (16.67)	Coarse
7/8-9 UNC	22.225	0.875	9	2.822	20.00	49/64 (19.45)	Coarse
1"-8 UNC	25.400	1.000	8	3.175	22.50	7/8 (22.23)	Coarse
1"-12 UNF	25.400	1.000	12	2.117	23.50	15/16 (23.81)	Fine
1"-14 UNF	25.400	1.000	14	1.814	24.00	61/64 (24.21)	Fine

Wann UNC vs. UNF verwenden

Condition	Use	Why
Default / don't know	UNC	Cheaper taps, faster machining, more available, more forgiving during assembly
Wanddicke < 0,5×D	UNF	Mehr Gewinde pro Zoll = mehr Eingriff in dünnem Querschnitt
Vibration / dynamische Belastung	UNF	Kleinerer Steigungswinkel widersteht Lockerung. Mit prevailing-torque-Mutter oder Loctite für kritische Verbindungen verwenden.
Luftfahrt / Militär	UNF	AN/MS-Standard-Hardware ist UNF. Erforderlich für viele MIL-SPECs.
Schneller Prototyp / Einzelexemplar	UNC	Jeder Baumarkt hat UNC. Sie können die Schraube heute kaufen.
Thread depth limited by design	UNF	More threads in less depth = higher strength in shallow holes

Rohrgewinde (NPT / BSP)

Rohrgewinde sind für Fluid- und Gasverbindungen. Sie müssen abdichten, was grundlegend anders ist als bei Standardbefestigungsgewinden. Zwei Systeme dominieren: NPT (amerikanisch, 60°) und BSP (britisch/europäisch, 55°). Sie sind nicht austauschbar.

NPT vs. BSP — Hauptunterschiede

Property	NPT	BSP (BSPP / BSPT)
Thread angle	60°	55°
Standard	ASME B1.20.1	ISO 228 (parallel), ISO 7 (tapered)
Taper	1:16 (3/4 in/ft)	BSPT: 1:16 — similar taper but different TPI
Parallel variant	NPSC (rarely used)	BSPP (very common — uses O-ring or sealant)
Sealing method	Gewindeverformung + Dichtstoff (Teflonband, Gewindedichtpaste)	BSPP: O-Ring oder Bonded Seal. BSPT: Gewindeverformung + Dichtstoff
Common regions	North America, Taiwan, Philippines	Europe, UK, Asia, Middle East, Australia
Nominal sizing	Bezieht sich auf ungefähre ID des Rohrs, nicht OD des Gewindes	Gleiche Konvention — 1/2 BSP ist NICHT 0,5 Zoll am Gewinde

Do NOT mix NPT and BSP They have different thread angles (60° vs 55°) and different TPI at the same nominal size. A 1/2 NPT male will thread into a 1/2 BSP female for 1–2 turns before jamming. It will leak, and forcing it will damage both parts. Always verify the thread system before machining pipe threads — check the mating fitting or ask for a sample.

Häufige Rohrgewindegrößen

Designation	System	Nominal	TPI	Tap Drill (mm)	Angle
1/8-27 NPT	NPT	1/8"	27	8.80	60°
1/4-18 NPT	NPT	1/4"	18	11.80	60°
3/8-18 NPT	NPT	3/8"	18	15.20	60°
1/2-14 NPT	NPT	1/2"	14	18.80	60°
3/4-14 NPT	NPT	3/4"	14	24.30	60°
1-11.5 NPT	NPT	1"	11.5	30.50	60°
1/8-28 BSPT	BSPT	1/8"	28	8.60	55°
1/4-19 BSPT	BSPT	1/4"	19	11.40	55°
3/8-19 BSPT	BSPT	3/8"	19	15.00	55°
1/2-14 BSPT	BSPT	1/2"	14	19.00	55°
3/4-14 BSPT	BSPT	3/4"	14	24.50	55°
1-11 BSPT	BSPT	1"	11	30.70	55°

Tapered vs parallel pipe threads Tapered threads (NPT, BSPT) seal by the threads themselves deforming against each other. Parallel threads (BSPP) cannot seal by the threads alone — they require an O-ring, a bonded seal (washer), or thread sealant. BSPP is preferred for connections that are frequently assembled and disassembled, because the sealing element (O-ring) is reusable — unlike tapered threads which need fresh sealant every time.

NPTF (dry seal) NPTF is a variant of NPT where the threads are designed to deform and create a seal without any sealant. Used in fuel systems, refrigeration, and applications where sealant contamination is unacceptable. NPTF taps are not interchangeable with NPT taps. The "F" stands for "Fuel" — historically used in fuel lines.

ACME- und Trapezgewinde

These are power transmission threads, not fastening threads. They have a flat-topped profile (29-degree for ACME, 30-degree for Trapezoidal) that provides high strength and low friction for converting rotary motion to linear motion.

Wann ACME / Trapezoidal benötigt wird

Application	System	Why
Lead screw (inch design)	ACME	Standard for US-made vises, CNC lead screws, jacks
Lead screw (metric design)	Trapezoidal (Tr)	Metric equivalent of ACME. Used in EU/Asian machinery.
Vise jaws, clamps	ACME	High axial load capacity, self-locking under most conditions
Linear actuator	ACME or Tr	Efficient motion conversion. ACME more common in US.
Valve stems	ACME	Standard in industrial valves. Resists galling.

Häufige ACME-Abmessungen

Designation	Major Dia. (in)	TPI	Pitch (mm)	Thread Depth (in)	Tap Drill (in)
1/4-16 ACME	0.250	16	1.588	0.109	0.188
3/8-12 ACME	0.375	12	2.117	0.146	0.292
1/2-10 ACME	0.500	10	2.540	0.175	0.400
5/8-8 ACME	0.625	8	3.175	0.219	0.500
3/4-6 ACME	0.750	6	4.233	0.292	0.594
1"-5 ACME	1.000	5	5.080	0.350	0.844
1-1/4-5 ACME	1.250	5	5.080	0.350	1.094
1-1/2-4 ACME	1.500	4	6.350	0.438	1.313

Häufige Trapezoidal (Tr)-Abmessungen

Designation	Major Dia. (mm)	Pitch (mm)	Thread Depth (mm)	Tap Drill (mm)
Tr8×1.5	8	1.5	0.825	6.50
Tr10×2	10	2.0	1.100	8.00
Tr12×3	12	3.0	1.650	9.50
Tr14×3	14	3.0	1.650	11.50
Tr16×4	16	4.0	2.200	12.00
Tr20×4	20	4.0	2.200	16.00
Tr24×5	24	5.0	2.750	19.00
Tr28×5	28	5.0	2.750	23.00
Tr30×6	30	6.0	3.300	24.00
Tr36×6	36	6.0	3.300	30.00

ACME vs Trapezoidal Functionally identical — both are 29–30 degree flat-topped power transmission threads. Use ACME for inch designs, Trapezoidal for metric designs. Don't mix them. ACME has a 29-degree included angle; Trapezoidal has 30 degrees. The 1-degree difference is enough to prevent proper mating.

Gewinde-Bezeichnungsformat

How you specify a thread on a drawing matters. The format must be unambiguous — the machinist and the inspector both need to understand exactly what thread to cut and what tolerance to hold.

Metrische Gewindebezeichnung

Format


          M[diameter]x[pitch] - [tolerance class]   (internal: H, external: g)

Examples:
M10 — M10 coarse (default pitch 1.5mm), no tolerance specified = 6H/6g assumed
M10x1.5-6H — M10, pitch 1.5mm, internal thread, tolerance class 6H
M10x1.0-6g — M10 fine pitch 1.0mm, external thread, tolerance class 6g
M8x1.25-6H/6g — M8 paired thread (both internal and external specified)

Unified-Gewindebezeichnung

Format


          [size]-[TPI] [series] - [tolerance class]   (internal: B, external: A)

Examples:
1/4-20 UNC-2B — 1/4 inch, 20 TPI, Unified National Coarse, internal, class 2B
3/8-16 UNC-2A — 3/8 inch, 16 TPI, coarse, external, class 2A
1/4-28 UNF-3B — 1/4 inch, 28 TPI, Unified National Fine, internal, class 3B
#10-32 UNC-2B — Number 10 size, 32 TPI, coarse, internal, class 2B

Rohrgewindebezeichnung


          Examples:

          1/4-18 NPT — 1/4 inch NPT, 18 TPI

          1/2-14 NPTF — 1/2 inch NPTF (dry seal), 14 TPI

          G 1/2 BSP or G 1/2" — 1/2 inch BSPP (parallel), ISO 228

          R 1/2 BSPT — 1/2 inch BSPT (tapered), ISO 7

          Rc 1/2 — 1/2 inch BSPT internal tapered (ISO 7 notation)

ACME / Trapezoidal-Bezeichnung


          Examples:

          1/2-10 ACME-2G — 1/2 inch, 10 TPI, general purpose, class 2G

          1-5 ACME-3C — 1 inch, 5 TPI, centralizing, class 3C

          Tr20x4 Tr 8e — Trapezoidal, 20mm diameter, 4mm pitch, tolerance class 8e

Gewindeklassen

Thread classes define the tolerance (how loose or tight the fit is). A tighter class costs more to manufacture because the tap, the hole size, and the pitch diameter all need tighter control.

Metrische Gewindeklassen (ISO 965)

Class	Type	Fit	Typical Use	Cost Impact
4H / 4g	Tight	Minimal clearance	Precision instruments, aerospace	High — requires custom gages
5H / 5g	Medium-tight	Small clearance	Precision machinery, gauging	Medium-high
6H / 6g	Standard	Normal clearance	General purpose — this is the default for 95% of work	Baseline — no extra cost
7H / 7g	Loose	Large clearance	Hot-dip galvanized parts, rough conditions, easy assembly	Lower (easier to manufacture)

H = internal (nut/hole). g = external (screw/bolt). The number is the tolerance grade — lower = tighter. If no class is specified, 6H/6g is assumed.

Unified-Gewindeklassen (ASME B1.1)

Class	Type	Fit	Typical Use	Cost Impact
1B / 1A	Loose	Wide clearance	Rough assembly, quick-disconnect, tolerating dirty conditions	Lowest
2B / 2A	Standard	Normal clearance	General purpose — the default for 95% of work	Baseline
3B / 3A	Tight	Minimal clearance	Precision assemblies, aerospace, locknuts	High — requires go/no-go gages

B = internal (nut). A = external (bolt). If no class is specified, 2B/2A is assumed.

When to specify a tighter class Only when the application requires it: (1) precision location (e.g., a threaded mandrel that must position a part accurately), (2) the joint must not have perceptible play, or (3) a locking feature relies on zero-backlash thread engagement. For standard fastening, 6H/6g or 2B/2A is correct. Tighter classes increase tap cost, require gauging, and increase scrap rate — don't over-specify.

Gewindetiefe und Eingriff

Thread depth is one of the most common sources of drawing ambiguity. These rules eliminate the guesswork.

Minimale Eingriffslänge

Rule	Value	Notes
Steel/SS into steel/SS	0.8–1.0×D	Standard rule of thumb. M6 in steel = 5–6mm minimum.
Steel bolt into aluminum	1.5–2.0×D	Aluminum threads strip at lower loads. M6 bolt into AL = 9–12mm.
Steel bolt into plastic	2.0–2.5×D	Plastic is weakest. Consider threaded inserts (heli-coil) for repeated assembly.
Maximum useful depth	1.5×D (steel)	Going deeper than this adds almost no strength — the load is carried by the first few threads. Deeper just wastes machining time.

Sackloch-Grundfreiraum

When tapping a blind hole, the tap cannot cut threads all the way to the bottom. Account for this in your design:

Factor	Value	Why
Tap chamfer / lead-in	2–3 pitch lengths	The first 2–3 threads from the tap's point are incomplete. They don't count as full thread engagement.
Bottoming tap uncut	1–2 pitch lengths	Even a bottoming tap leaves 1–2 pitches uncut at the very bottom of the hole.
Total unthreaded bottom	3–5 pitch lengths	Add this below your required thread depth. For M10x1.5: 4.5–7.5mm below the last full thread.

Blind hole depth callout Don't call out the thread depth as the total hole depth. Call out the thread depth separately from the hole depth. Example: "M10x1.5-6H THRU 15, DRILL 22 DEEP". The machinist drills to 22mm, threads to 15mm, and the remaining 7mm is clearance for the tap point. If you only specify "M10x1.5-6H DEEP 15", the machinist has to guess — and may drill too shallow.

Gewindefasen

Feature	Specification	Purpose
External thread chamfer	0.5–1.0mm × 45°	Helps the bolt start into the nut. Always add this.
Internal thread chamfer	0.5–1.0mm × 120° (countersink)	Countersink at the thread entry. Prevents the first thread from being damaged during bolt insertion.
Undercut (if needed)	0.5mm wider than major dia. × 1–2mm deep	Provides clearance for a thread-cutting tool runout. Needed when a threaded section meets a shoulder.

Häufige Anwendungsfehler

Mistake	What Happens	Correct Approach
Specifying M10 thread without pitch	Coarse (M10x1.5) is assumed. If the mating part is M10x1.25 fine pitch, it won't thread in — or will cross-thread.	Always specify the pitch explicitly: M10x1.5 or M10x1.25. Never rely on "default" when mating with an existing part.
Mixing NPT and BSP	1/2 NPT male threads into 1/2 BSP female for 1–2 turns then jams. Leaks under pressure. Damages both parts if forced.	Verify the thread system on the mating fitting. If uncertain, ask for a sample or measure the thread angle (55° vs 60°).
Blind hole too shallow for thread depth	Tap bottoms out before reaching full thread depth. Incomplete threads = weak joint. Tap can break.	Specify hole depth = thread depth + 3–5 pitch lengths. For M8x1.25 deep 12: drill at least 16–18mm.
Using UNC bolt in UNF nut	Bolt threads in loosely, then binds. The first few turns feel OK because the pitch difference is small. Cross-threading and damage.	Match the series: UNC bolt with UNC nut, UNF with UNF. If you're unsure, count the threads per inch on both parts.
No entry chamfer on internal thread	Bolt's first thread catches the sharp edge of the hole. Misalignment, cross-threading, damaged threads.	Always add a 120° countersink or chamfer at the thread entry. This is a 5-second operation that prevents hours of rework.
Over-tapping soft material (AL, brass) without inserts	Threads strip on first or second assembly/disassembly cycle. Common on M6 and smaller in aluminum.	For joints that will be assembled/disassembled more than 3–5 times, use helicoil or thread inserts. Cost: ~$0.50–1.00 per insert.
Specifying class 4H/3B when 6H/2B suffices	Requires custom taps, go/no-go gages, and tighter hole control. Machining cost increases 30–50%. No functional benefit for standard fastening.	Use 6H/6g (metric) or 2B/2A (unified) unless the application genuinely needs tighter tolerance. Tighter class = more expensive, not "better."
Not accounting for thread coating thickness	Hot-dip galvanized bolts are significantly oversized. A 1/4-20 UNC galvanized bolt may not fit in a 2B nut. Same for thick anodize on aluminum.	After galvanizing, use class 2A on the bolt (oversized) and class 2B on the nut. For anodized parts, specify thread tolerance after plating or mask the threads during anodizing.
Using thread-locking compound on pipe threads that need future disassembly	Thread sealant bonds the threads. Disassembly destroys the threads. Common on NPT hydraulic fittings.	Use Teflon tape (removable) for joints that need future disassembly. Use anaerobic sealant (Loctite) only for permanent connections.
Calling out left-hand thread without LH notation	Right-hand thread is assumed. Parts arrive with wrong-hand threads. Scrap.	Explicitly note left-hand threads: `M10x1.5-6H LH` or `1/4-20 UNC-2B LH`. Never assume the machinist will guess.

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