Threaded joints are probably the most common form of mechanical assembly. They allow for the inexpensive but effective joining of parts. This allows for less expensive manufacturing and easier maintenance, repair or servicing of a finished assembly.
To take advantage of threaded joints, you need to understand was standard sizes of threads exist, what types of bolts and hardware exist and what are the implications of different grades of hardware – in terms of size, mechanical properties, corrosion resistance and fastening strength.
Unified Screw Threads – Fine (UNF)
Unified Screw Threads – Course (UNC)
National Pipe Thread (NPT)
Metric Thread
ACME Thread
Socket Head Cap Screw (SHCS)
Hex Head Cap Screw (HHCS)
Button Head Cap Screw (BHCS)
NPT Tap Drill Sizes When tapping NPT threads, the drill size determines the minor diameter of the internal thread. Because NPT threads are tapered, the drill creates a straight hole that the tapered tap gradually engages — you’ll get full threads near the entrance and partial threads at depth. NPT Size TPI Tap Drill (in) … Read more
What is NPT? NPT (National Pipe Thread) is the US standard for tapered pipe threads. The taper is 1°47′ (3/4 inch per foot), which means the threads wedge tighter as you screw them together — creating a seal. NPT threads require thread sealant (Teflon tape or pipe dope) for a leak-free connection. NPT vs. NPTF: … Read more
Matching Nut Grades to Bolt Grades Using the wrong nut with a high-strength bolt is a surprisingly common mistake — and it’s dangerous. A Grade 5 nut on a Grade 8 bolt will strip the nut threads before the bolt reaches its rated clamp load. Always match (or exceed) the nut grade to the bolt … Read more
How to Identify a Hex Bolt’s Grade Every hex bolt has markings on its head that tell you its material grade and mechanical properties. Those radial lines, numbers, and letters aren’t decorative — they’re a specification. Knowing how to read them means you can verify the right bolt is in the right application without pulling … Read more
Standard Flat Washer Sizes Flat washers distribute load under a bolt head or nut, protect the surface from damage, and span oversized or slotted holes. The two common types in the US are: Type A (Narrow) — general purpose, standard series Type B (Wide) — larger OD for better load distribution on soft materials Dimensions … Read more
Standard Hex Nut Sizes These are the basic dimensions for standard hex nuts per ASME B18.2.2. The “width across flats” is what you need for wrench/socket sizing. Nominal Size Threads/in (UNC) Width Across Flats (in) Width Across Corners (in) Height (in) 1/4″ 20 7/16 0.505 7/32 5/16″ 18 1/2 0.577 17/64 3/8″ 16 9/16 0.650 … Read more
Standard Hex Cap Screw Dimensions Hex cap screws (also called hex cap bolts or hex head bolts) are the workhorse fastener of mechanical assembly. This table covers dimensions per ASME B18.2.1 — the data you need for designing bolt holes, counterbores, clearances, and grip lengths. Key Dimensions Explained Width across flats (F) — your wrench/socket … Read more
Metric Tap & Clearance Drill Sizes This table covers standard coarse pitch metric threads (the default — if a drawing says “M10” without specifying pitch, it means M10×1.5 coarse). For each thread size, you get the tap drill (for cutting internal threads) and clearance drill (for a bolt to pass through freely). Thread Pitch (mm) … Read more
UNF Tap Drill Sizes UNF (Unified National Fine) threads have more threads per inch than UNC for the same diameter. They’re stronger in thin-walled parts, provide finer adjustment, and resist vibration loosening better — but they’re less forgiving and easier to cross-thread. Thread Size TPI Tap Drill Decimal (in) 75% Thread Clearance Drill #0-80 80 … Read more
UNC Tap Drill Sizes UNC (Unified National Coarse) is the default thread series in the US. If a drawing says “1/2-13” — that’s UNC. Coarse threads are stronger, more forgiving of dirty conditions, and easier to assemble than fine threads. Thread Size TPI Tap Drill Decimal (in) 75% Thread Clearance Drill #1-64 64 #53 0.0595 … Read more