Machine Bolts
Manufacturer
India · Mumbai
Machine Bolts — The General-Purpose Standard of Everyday Engineering
A machine bolt is a hex head bolt made to commercial dimensional and thread tolerance standards — ASME B18.2.1 in inch sizes, DIN 931/933 or ISO 4014/4017 in metric — intended for general machinery assembly, equipment mounting, and fabrication work where a certified, code-governed structural connection (like an ASTM A325 structural steel bolt) is not required. It is, in the simplest terms, the ordinary hex bolt used across the overwhelming majority of mechanical engineering, equipment manufacturing, and general fabrication worldwide.
The defining characteristics are a standard hexagonal head sized to commercial across-flats dimensions, a thread manufactured to commercial 2A/2B (inch) or 6g/6H (metric) tolerance class — looser and less expensive to hold than the tighter thread tolerances sometimes specified for precision assemblies — and availability in either full thread (threaded the entire shank length, DIN 933 / ASME B18.2.1 fully threaded) or partial thread (a smooth unthreaded shank section below the head, DIN 931), selected based on whether shear load will be carried on the smooth shank or the threaded section is acceptable to bear against the clamped material.
Machine bolts are manufactured across the standard commercial property class ladder — Grade 2 (low carbon, general low-stress use), Grade 5 (medium carbon heat-treated, the most commonly stocked general engineering grade, equivalent to metric 8.8), and Grade 8 (alloy steel heat-treated, high-strength general engineering, equivalent to metric 10.9) — giving a straightforward strength selection ladder for the vast majority of machine assembly, equipment frame, and general fastening applications that do not require the certified traceability and larger wrenching dimensions of a structural bolt.
| Property | Value |
|---|---|
| Min Tensile Strength | ≥ 120 ksi (827 MPa) |
| Min Yield Strength | ≥ 92 ksi (634 MPa) |
| Metric Equivalent | Grade 8.8 |
| Thread Tolerance | 2A / 6g Commercial |
| Head Markings | 3 Radial Lines |
| Material | Medium Carbon, Q&T |
| Typical Use | General Machinery Assembly |
Machine Bolt vs Structural Bolt — Why the Difference Matters
Confusing these two categories is one of the most common and consequential procurement errors in general engineering — the two are not interchangeable, and the difference goes well beyond just strength grade.
Machine Bolt (ASME B18.2.1)
Standard hex head dimensions, commercial thread tolerance (2A/2B), and standard property classes (Grade 2/5/8). Manufactured and sold for general machinery, equipment, and non-structural fabrication assembly. Not governed by structural building or bridge codes.
Structural Bolt (ASTM A325 / A490)
Larger heavy hex head and nut dimensions specifically sized for structural steel connection design, manufactured under ASTM structural bolting standards with mandatory lot traceability, and installed to a specified pretension (torque or turn-of-nut method) verified by inspection — requirements that a general machine bolt is not manufactured or certified to meet.
Why Machine Bolts — Versatility Across Every General Application
Full Thread vs Partial Thread — Selected by Shear Requirement
Full thread (DIN 933) maximises adjustability and works well in tapped-hole applications where the full length may need to engage. Partial thread (DIN 931) leaves a smooth shank section specifically for connections where shear load is better carried on an unthreaded shank than on the thread root — a straightforward but important selection criterion for any joint subject to shear (rather than pure tension) loading.
Commercial Tolerance Keeps Cost Down for General Use
Machine bolt thread tolerance (2A/2B or 6g/6H) is looser than precision thread classes, which keeps manufacturing cost and lead time down for the vast majority of applications that do not require tighter fit tolerance. This is a deliberate, correct engineering trade-off for general assembly — not a quality shortcut — since tighter tolerance thread adds cost with no functional benefit in a standard mechanical joint.
Three-Grade Ladder Covers Nearly Every General Strength Requirement
Grade 2 for light-duty and non-critical assembly, Grade 5 for the great majority of general machinery and equipment work, and Grade 8 for higher-load applications — this three-step ladder, mirrored by the metric 4.6/8.8/10.9 system, covers the strength requirement of nearly every general mechanical assembly without needing to step up to exotic alloy or structural-certified fasteners.
Universally Available Across Every Material and Finish
Because machine bolts are the highest-volume, most broadly manufactured bolt category, they are available in the widest range of materials (carbon steel, stainless, brass), finishes (black, zinc plate, HDG), and sizes of any bolt type — making them the practical default specification whenever a project’s fastening requirement does not have a specific reason to demand a specialised or code-governed alternative.
Machine Bolt Grade Selector — SAE and Metric
SAE grade (US inch system) and metric property class run in parallel — select by required strength, with head markings indicating grade at a glance.
Machine Bolt Configurations & Related Hardware
Full thread and partial thread across the full grade and material range, plus matching nuts and washers. M4–M36.
Full Thread Machine Bolt
Threaded the entire shank length. M4–M36, standard and custom lengths. The most common configuration for general machinery assembly, tapped-hole applications, and connections where thread engagement anywhere along the length is beneficial.
Partial Thread Machine Bolt
Smooth unthreaded shank section below the head with threading only near the tip. M4–M36. The correct choice for shear-loaded connections where the smooth shank bearing against the clamped material provides better shear performance than a threaded section.
Hex Nuts
Standard hex nuts matched to machine bolt thread size and grade, manufactured from the same certified material for consistent property class assembly documentation across the complete bolt-and-nut set.
Washers
Plain flat washers (DIN 125) and spring lock washers (DIN 127) matched to machine bolt thread size for complete assembly sets — standard practice for distributing clamping load and providing vibration resistance in general machinery connections.
SAE vs Metric Grade Equivalence Table
A practical cross-reference for procurement teams working across US inch and metric drawings.
| SAE Grade | Metric Equivalent | Min Tensile (approx.) | Head Markings |
|---|---|---|---|
| Grade 2 | 4.6 / 4.8 | 74 ksi / 510 MPa | No markings |
| Grade 5 | 8.8 | 120 ksi / 827 MPa | 3 radial lines |
| Grade 8 | 10.9 | 150 ksi / 1034 MPa | 6 radial lines |
| — | 12.9 | 176 ksi / 1214 MPa | 12 radial lines |
Equivalences are approximate — always confirm against the specific ASTM/ISO/SAE standard referenced on the engineering drawing.
Machine Bolts Application Areas
General machinery, equipment manufacturing, and fabrication across every industrial sector — the everyday fastener of mechanical engineering.
General Machinery
equipment mounting
Agricultural Equipment
general farm machinery
Appliance Manufacturing
panel assembly hardware
Material Handling
rack & shelving hardware
General Fabrication
custom equipment fabrication
HVAC & Industrial Systems
ductwork support hardware
Automotive Aftermarket
non-structural body bolts
General Construction
equipment installation
Machine Bolt Standards Coverage
Same ISO 9001:2015 quality system as our complete fastener range. Grade compliance and thread tolerance verified against ASME B18.2.1 / DIN 931/933 on every production lot.
Complete Documentation Package
Grade / Standard Stated
Tensile · Hardness
Thread Class · Length
Conformance
On Request
Related Bolt Types & Materials
Machine bolts for general-purpose assembly. Full bolt range for the complete fastening schedule. SS 316L for corrosive environments. Carbon steel for the full strength ladder.
Full Bolt Range
Complete bolt manufacturing capability at Akbar Fasteners — machine bolts, hex flange, carriage, shoulder, hollow, J-bolts, and all specialty forms across carbon steel through exotic alloy, all from a single Mumbai facility.
View Bolt Range →Hex Flange Bolts
For applications benefiting from an integral washer flange to eliminate a separate washer and provide anti-vibration serration — the flanged alternative to a standard machine bolt for high-volume assembly.
View Flange Bolts →Carbon Steel Fasteners
For the complete strength ladder and surface finish options (HDG, zinc plate, mechanical zinc) covering the full range of general engineering carbon steel fastener requirements beyond standard machine bolt grades.
View Carbon Steel →SS 316L Fasteners
For machine bolt applications requiring corrosion resistance beyond carbon steel and coating — SS 316L machine bolts for chemical, marine, and food-grade equipment assembly.
View SS 316L →Need Machine
Bolts?
Thread type (full/partial) · grade · size · material · finish · quantity. Akbar Fasteners Pvt. Ltd. responds within 24 hours.
Export: +91 8696 32 4646 · Support: +91 800 9315 333 · export@akbarfasteners.in