Types of Bolts: Parts of a Bolt, Common Bolt Kinds, Grades and Uses

A bolt is an externally threaded fastener that passes through holes in the parts being joined and is tightened by turning a nut. The main types of bolts are hex, heavy hex, carriage, flange, square, U-bolt, J-bolt (anchor or foundation), eye, T-head, shoulder, plow and elevator bolts, plus the headless stud. Every bolt has the same basic parts: a head, an unthreaded shank, a threaded length and a chamfered end. This page covers the parts of a bolt, each bolt kind and where it is used, metric thread sizes, strength grades such as 8.8 and 10.9, and a worked load example.

Parts of a bolt

Picture a standard hex bolt lying on its side, head on the left. Moving from left to right you meet each of these bolt part names:

A student tip: bolt length is measured from under the head to the tip, not including the head. The one exception is a countersunk bolt, which is measured overall because its head sits inside the part.

Bolt vs screw: what is the difference?

The distinction used in ASME B18.2.1 and by Machinery’s Handbook is about how the fastener is meant to be tightened, not its shape. Broadly:

• A bolt goes through clearance holes in the assembled parts and is tightened by turning a nut.
• A screw goes into a tapped hole, or forms its own thread, and is tightened by turning its head.

The same hex fastener can therefore be a bolt in one joint and a cap screw in another. In Indian workshops the words are used loosely, and a hex cap screw is usually still called a hex bolt. Other threaded fasteners, including screws, nuts, washers, rivets and pins, are covered on the types of fasteners page.

Types of bolts and their uses

Hex and heavy hex bolts

The hex bolt is the default kind of bolt, and the one most people picture. Six flats give a spanner a good grip in tight spaces. Heavy hex versions have a bigger, thicker head for more bearing area and are used in steel structures, including high-strength friction grip (HSFG) joints.

Carriage and plow bolts

Both have a square neck under the head that bites into the hole, so the bolt cannot turn while the nut is tightened. The carriage bolt’s smooth dome leaves nothing to grip from outside; the plow bolt’s countersunk head sits flush, so soil and gravel slide over it.

Flange bolts

The flange acts as a washer that cannot fall off. Serrated flange bolts add teeth on the underside to resist loosening under vibration.

U-bolts and J-bolts

These are bent from round bar and threaded at the ends. A U-bolt wraps round a pipe or axle and takes two nuts. A J-bolt or L-bolt is set in wet concrete, with its hook giving the anchorage, and the threaded end sticks up to hold a base plate.

Eye bolts

Used for lifting. Load them in line with the shank; a sideways pull on a plain eye bolt greatly reduces its safe load. Only use forged, load-rated eye bolts for lifting.

T-head, shoulder and elevator bolts

These are special-purpose bolts. T-head bolts slide into T-slots on machine tables. Shoulder bolts give an accurate pin for a part to turn on. Elevator bolts have a thin head that sits flat against a moving belt.

Studs

A stud has no head. One end screws permanently into a tapped hole, such as an engine block, and a nut goes on the other. Taking the part off does not wear out the thread in the soft casting, which is why cylinder heads and pipe flanges use studs.

Metric thread sizes: coarse and fine

In India, bolts follow ISO metric threads, written as M followed by the nominal diameter in mm. If only the diameter is written, the thread is coarse pitch. Fine pitch is written with the pitch after an “x”:

• M10 means M10 coarse, with a pitch of 1.5 mm
• M10 x 1.25 means M10 fine, with a pitch of 1.25 mm
• M12 coarse has a 1.75 mm pitch; M16 coarse has a 2 mm pitch

Coarse threads are quicker to assemble, resist damage and suit most work. Fine threads have a slightly larger stress area, give finer adjustment and resist loosening better, so they are common in automotive and precision assemblies.

Bolt grades: what 8.8 and 10.9 mean

Steel bolts carry a property class stamped on the head, defined by ISO 898-1 and its Indian equivalent, IS 1367 (Part 3). The number has two parts:

• The first number × 100 = nominal tensile strength in MPa.
• The second number ÷ 10 = the ratio of yield strength to tensile strength.

So 8.8 means a tensile strength of 8 × 100 = 800 MPa and a yield strength of 0.8 × 800 = 640 MPa.

Proof stress is the stress the bolt must carry without any permanent stretch. It is slightly below yield and is what bolt load calculations use. Classes 8.8 and above are quenched and tempered alloy or medium-carbon steel. For M16 and larger, class 8.8 values rise slightly (minimum tensile 830 MPa, proof 600 MPa).

Worked example: loads on an M12 class 8.8 bolt

A bolt breaks through its threads, not its shank, so strength is worked out on the tensile stress area, A_s:

A_s = (π/4) × (d − 0.9382p)²

For M12 coarse, d = 12 mm and p = 1.75 mm:

1. d − 0.9382p = 12 − 0.9382 × 1.75 = 12 − 1.642 = 10.358 mm
2. A_s = 0.7854 × 10.358² = 0.7854 × 107.29 = 84.3 mm²
3. Proof load = 84.3 × 580 = 48.9 kN
4. Load at nominal yield = 84.3 × 640 = 53.9 kN
5. Minimum breaking load = 84.3 × 800 = 67.4 kN

Preload. A common rule of thumb for reusable joints is to tighten to about 75 percent of proof load:

F_i = 0.75 × 48.9 = 36.7 kN

The tightening torque is then estimated with T = K × F_i × d, where the nut factor K is about 0.2 for dry, plain steel threads. T = 0.2 × 36,700 × 0.012 = 88 N·m. K changes a lot with lubrication and plating, so use the fastener maker’s torque table for real assemblies.

Bolt materials and coatings

• Carbon and alloy steel: classes 4.6 to 12.9. Plain finish, or with a black oxide coat, rusts quickly outdoors.
• Zinc plated (electro-galvanised): a thin bright coat for indoor and light outdoor use.
• Hot-dip galvanised (HDG): a thick zinc coat for towers, bridges and outdoor structures. Nuts are tapped oversize to fit over the coating.
• Stainless steel A2-70 and A4-70: A2 is 304-type and A4 is 316-type austenitic stainless. The 70 means a tensile strength of 700 MPa. A4 resists chlorides better, so it suits coastal and marine sites.

Very high-strength bolts (10.9 and 12.9) should not be electroplated without de-embrittlement baking, because plating can cause hydrogen embrittlement and sudden fracture.

How to specify a bolt

A complete order callout gives the type, standard, size, length, grade and finish. For example:

Hex bolt ISO 4014 – M12 x 60 – 8.8 – zinc plated

This reads as a partly threaded hex head bolt, M12 coarse thread, 60 mm long under the head, property class 8.8, zinc plated. A fine thread would be written M12 x 1.25 x 60. Always order a nut of matching class (class 8 nut with an 8.8 bolt, class 10 with 10.9).

Common bolt failures

• Fatigue: most bolt fatigue cracks start at the first thread engaged in the nut, where load transfer is most concentrated. The next most common places are the thread run-out and the head-to-shank fillet. Correct preload is the best defence, because a well-tightened bolt feels only a small part of each load cycle.
• Self-loosening: transverse vibration lets the threads slip and the nut back off. Remedies are higher preload, longer and thinner (more elastic) bolts, prevailing-torque nuts, wedge-locking washers or thread-locking adhesive.
• Overload and thread stripping: too much torque, a short nut engagement or a soft nut.
• Corrosion and hydrogen embrittlement: especially with high-strength plated bolts in wet conditions.

Threaded joints are part of the design of machine elements course in B.Tech mechanical engineering; NPTEL has free lectures on bolted joint design.

FAQs

What are the main types of bolts?

The common types are hex, heavy hex, carriage, flange, square, U-bolt, J-bolt (anchor or foundation), eye, T-head, shoulder, plow and elevator bolts, along with studs. Hex bolts are the most widely used.

What are the parts of a bolt called?

A bolt has a head, a bearing face under the head, an unthreaded shank, a threaded length with a given pitch, and a chamfered point. Grip length is the thickness of material it clamps.

What does 8.8 mean on a bolt head?

It is the property class. The first 8 means a nominal tensile strength of 800 MPa, and the second 8 means yield is 80 percent of that, 640 MPa.

What is the difference between a bolt and a screw?

A bolt passes through clearance holes and is tightened with a nut. A screw goes into a threaded hole or forms its own thread and is tightened by turning its head.

What is the tensile stress area of an M12 bolt?

For M12 coarse thread (1.75 mm pitch), the tensile stress area is 84.3 mm², from (π/4)(d − 0.9382p)².

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