A fastener can look right, fit through the clearance hole and still be wrong for the assembly. For purchasing teams, the critical task is to identify the complete connection before comparing supplier prices. Diameter alone does not describe a thread, and converting a drawing from inches to millimetres does not make two thread systems interchangeable.
This guide is a practical checklist for sourcing bolts, screws, nuts and related fasteners. Use it to organise the information your engineering team has approved, then request quotations against the same specification.
Read the thread designation before comparing dimensions
A metric callout such as M8 × 1.25 identifies an 8 mm nominal diameter and a 1.25 mm pitch. An inch callout such as 1/4-20 UNC identifies a nominal quarter-inch diameter and 20 threads per inch in the Unified coarse series. These examples explain the notation; they are not equivalent replacement parts. Include the applicable thread standard and fit class when the drawing requires them.
| Purchasing field | Metric example | Inch example |
|---|---|---|
| Thread designation | M8 × 1.25 | 1/4-20 UNC |
| Pitch information | Distance between adjacent threads | Number of threads in one inch |
| Length | State the dimension and measurement reference | State the dimension and measurement reference |
| Mating component | Matching specified internal thread | Matching specified internal thread |
Define the whole joint in your RFQ
Make one line item for each approved size. Attach the drawing revision or recognised part designation, and identify the nut, insert or tapped component that receives it. A photo helps communicate head style, but it cannot establish a thread tolerance or material grade.
For custom threaded parts, a dimensioned drawing is especially helpful for thread depth, unthreaded portions and controlled interfaces. RapidDirect describes using a 2D drawing alongside the model to communicate threads and special tolerances. Apply that communication principle to your own RFQ; its service specifications are separate from your order requirements. Read the drawing reference.
- Specify head type, drive style and tool-access restrictions.
- Record the required material and strength designation separately.
- Identify the finish, appearance requirements and any restricted substances.
- List quantities by size rather than giving one combined total.
- Identify which assembly features require inspection evidence.
Check length, finish and packaging as separate decisions
Use the length reference defined for the selected head style. Do not let a purchasing description substitute overall length for under-head length without engineering agreement. For partially threaded bolts, show both the required threaded section and the grip region.
Ask the supplier to confirm whether the quoted condition is the finished, ready-to-assemble condition. Where a coating or treatment affects an inspected feature, specify when acceptance is measured. Avoid a vague colour-only instruction such as “silver finish”: it leaves the actual treatment unresolved.
Packaging is also part of a usable order. Specify separate bags or cartons by item number, readable labels, lot identification if required, and protection needed for the expected storage and transport conditions. A mixed box of visually similar fasteners creates avoidable sorting work.
Compare quotations with an exception list
Create a comparison sheet with one column per supplier and one row per requirement. Record proposed alternatives explicitly: a different grade, a changed thread length, a substitute coating or a different packing method. A lower price is not comparable until those differences have been reviewed.
For a first order, identify the sample approval criteria before authorising the production lot. Ask for confirmation of the actual thread designation, assembly fit, identification and any requested inspection records. A successful hand-fit check alone does not approve strength or performance.
Ready to prepare the line items? Review our fastener RFQ guide, or send the drawing, quantities and mating-part information for project review.
Frequently asked questions
Can I replace a metric bolt with a similar-size inch bolt?
Do not approve a substitution from nominal diameter alone. The mating thread, pitch, fit, head clearance and required mechanical properties must all be checked against the assembly specification. Keep substitutions documented on the approved order.
What should I do if the existing sample has no drawing?
Provide clear photographs, measured dimensions and information about the mating part. Identify whether the sample is worn or damaged. Treat the sample as a starting point and agree a controlled specification before ordering replacements.
Should bolts, nuts and washers be quoted together?
Quoting the joint components together can make comparison easier. Keep each component on its own BOM line with its own dimensions, material, finish and quantity, and explain which items are assembled together.
What information makes repeat orders easier?
Retain the approved part number, drawing revision, accepted exceptions, sample reference and packaging instructions. State any changes explicitly instead of relying only on a previous purchase order number.
