The most common screw defects are head cracks and bursts, seams and laps, poor head fill, damaged drive recesses, coating faults, wrong length or diameter, poor thread fit and hidden heat-treatment problems. Most of them can be caught at incoming inspection with good light, a magnifier, a calliper, thread gauges and a test drive with the correct bit. ISO 6157-1 sets the acceptance limits for surface discontinuities on bolts, screws and studs, while ISO 898-1 and ISO 3506-1 cover mechanical properties. This guide shows what each defect looks like, how to check for it, and when to accept, sort or reject a batch.
Common Screw Defects at a Glance
| Defect | What you see | Likely cause | Quick check | Usual action |
|---|---|---|---|---|
| Head cracks and bursts | Fine or open lines across the head or at the head-to-shank radius | Cold heading set-up, material inclusions, over-working | Rotate under strong light, use a magnifier | Reject open cracks; judge small marks against ISO 6157-1 |
| Seams and laps | Long thin lines along the shank or thread | Wire rod defects, rolling or heading folds | Look along the thread; check depth with a fingernail | Reject if it reaches the thread root or head fillet |
| Poor head fill or eccentric head | Thin, uneven or off-centre head | Worn dies, tooling set-up | Compare with a gauge or approved sample | Sort or reject, depending on drawing tolerance |
| Drive recess defects | Shallow recess, rounded ribs, burrs | Worn punches, poor heading control | Seat the correct bit and drive a few screws | Reject if the bit cams out or will not seat |
| Coating faults | Bare patches, blisters, pits, white or red rust | Poor cleaning, bath control, handling damage | Visual check, coating thickness reading | Reject or re-coat by agreement |
| Wrong length or diameter | Parts do not match the drawing or label | Mixed part numbers, set-up error | Calliper on length under head and thread crest | Sort 100% or reject the lot |
| Thread pitch or fit errors | Cross-threading, tight or loose fit | Wrong rolling dies, worn dies | GO/NO-GO ring gauges, thread comb | Reject if gauges fail |
| Heat-treatment or embrittlement | Brittle snapping, stretching, delayed fracture | Hardness out of range, hydrogen after plating | Hardness test, torque test, wedge test by supplier | Quarantine the lot and investigate |
Why Screw Defects Matter
Small defects can shorten the life of a screwed joint. A crack in the head can grow under cyclic load, a rough or oversize thread can seize during tightening, and a poor coating can start corrosion early. In construction, transport and outdoor structures, these are safety and warranty problems, not just cosmetic ones. For a distributor, one bad batch also means returns, site complaints and damage to your own brand.
Key Standards and Basic Inspection Tools
- Surface discontinuities: ISO 6157-1 covers bolts, screws and studs for general use. ASTM F788 gives similar guidance for inch-series parts.
- Mechanical properties: ISO 898-1 for carbon and alloy steel, ISO 3506-1 for stainless steel.
- Product standards: for the AU and NZ market, AS 3566 covers self-drilling screws for building, including corrosion classes. See our AS3566 Bugle Head Batten Screw page for a typical example.
Tools worth having at receiving inspection:
- GO/NO-GO thread ring gauges for the sizes you stock
- Vernier calliper or micrometer for length and diameter
- Magnifier and a bright, even light source
- Correct driver bits and a drill or torque wrench for test driving
- A coating thickness gauge, if you buy zinc or coated screws in volume
Visual Defects You Can Spot Quickly
Head cracks and bursts
Look for fine lines or open splits across the head, radiating from the centre, or at the head-to-shank radius. A burst is where the metal has visibly opened. These act as stress raisers and can lead to sudden fracture. Reject any open crack. Very small surface marks should be judged against ISO 6157-1 or your drawing. Our guide on how to avoid head cracking in screws explains the causes in more detail.
Seams, laps and folds
Seams and laps are long, thin imperfections that run along the shank or across the thread. Shallow seams may be within standard limits. Deep seams that reach the thread root, or that show in the head fillet, reduce fatigue strength and give corrosion a place to start. Treat those parts as suspect.
Poor head fill, undersized and eccentric heads
Undersized heads give less bearing area and can pull through soft materials. Eccentric heads load the joint unevenly. Thin head fill on countersunk or bugle head screws weakens the drive and can strip during installation. These problems usually point to worn dies or tooling set-up. Comparing parts with an approved sample is the fastest check. On decking screws with coloured heads, head shape also affects appearance once installed.
Drive recess defects
For Phillips, Pozi, square and Torx drives, check for a shallow recess, poorly formed ribs or corners, and burrs inside the recess. These cause cam-out, damaged bits and low installation torque. Always test samples with the correct bit and make sure it seats fully.
Coating and surface faults
Bare patches, blisters, pits, rough plating and early white or red rust all point to problems in cleaning, bath control or handling. For roofing, decking and coastal projects, agree the coating system and corrosion class in writing before production. Our articles on inspecting coating thickness and plating peeling problems cover these checks step by step.
Dimensional and Thread Defects
Wrong length or diameter
These are simple errors but common when a project has several similar part numbers. Measure length under the head and diameter on the thread crest, and confirm both against the drawing or standard. On structural connections, a small length error changes embedment depth and holding power.
Wrong thread pitch or form
The wrong pitch or profile leads to cross-threading and poor engagement. A thread gauge or comparator chart sorts this out quickly. If you are not sure which tolerance class to specify, see metric thread classes explained.
Oversize or undersize threads
Threads that are too tight can seize or gall. Threads that are too loose give play and lower strength. Use GO/NO-GO ring gauges: the GO gauge should run on smoothly, and the NO-GO gauge should not enter more than two turns.
Hidden Defects Found by Testing
Some defects only show up in testing or in service. Hardness that is too high gives brittle fracture. Hardness that is too low lets the screw stretch and the joint relax. High-strength screws that are electroplated can suffer hydrogen embrittlement, which causes delayed cracking after installation. Read more in avoiding hydrogen embrittlement failures. Internal cracks and inclusions come from the steel or from overloading during cold heading, and are controlled through material checks and process control at the factory. For high-strength screws, ask for hardness and mechanical test results for each batch.
A Practical Incoming Inspection Checklist
- Documents: purchase order, batch number, drawing revision, stated standard and property class.
- Labels and packing: cartons or small packs clearly marked and undamaged; lot numbers traceable.
- Sampling: take parts from the top, middle and bottom of several cartons, not just one box.
- Visual: head cracks, bursts, seams, coating coverage, discolouration, recess burrs.
- Dimensions: length and diameter with a calliper; thread pitch and fit with gauges.
- Function: drive or tighten a few screws into the real material at normal torque.
- Records: photograph any non-conformity and describe it against the standard or drawing.
For bulk shipments, our guide to incoming inspection for bulk fasteners covers sampling in more detail.
Common Mistakes When Inspecting Screws
- Checking only one carton, so a mixed or bad lot is missed.
- Judging cracks and seams without a standard or drawing limit, which leads to arguments with the supplier.
- Testing recesses with a worn or wrong-size bit, then blaming the screw for cam-out.
- Skipping a test drive into the real material, so coating and torque problems are found on site.
- Reporting a problem without photos, batch number and the relevant standard clause.
What to Send With a Screw Enquiry
- Drawing or a clear sample photo, with the standard you work to (for example AS 3566, ISO or DIN)
- Size, length, head and drive type, thread type
- Material, property class and coating or corrosion class
- Application, such as timber, steel framing, concrete or roofing
- Quantity, packing format and destination port
- Any special inspection points you want checked before shipment
Working With Linkworld Fastener
Linkworld Fastener has 18 years of fastener export experience. We run our own cold heading workshop and our own inspection room and QC team, which check parts, including hardness, before every shipment. We make standard screws and customized fasteners to your drawing or sample, pack them under your own brand, and can offer free warehouse storage with staged shipments. Browse our screws range or send your requirements through the enquiry form below.
FAQ
What are the most common defects in screws?
Head cracks and bursts, seams and laps, poor head fill, damaged drive recesses, coating faults, dimensional errors, poor thread fit and hardness or heat-treatment problems.
Which standard covers surface defects on screws?
ISO 6157-1 covers surface discontinuities on bolts, screws and studs for general use. ASTM F788 is the inch-series equivalent.
How do I check thread fit on a screw?
Use GO/NO-GO ring gauges. The GO gauge should run on smoothly by hand, and the NO-GO gauge should not enter more than two turns.
When should I reject a batch of screws?
Reject when you find open head cracks, seams that reach the thread root, failed thread gauges, recesses the correct bit will not seat in, or brittle fractures in a torque test. Record the defect with photos and the batch number.
Send your drawing, a sample photo or the specification through the enquiry form below, and our team will reply with the options that fit your project.
Send an Enquiry
Send a drawing, sample or clear reference photo. Add the quantity and application if available, and we can start from the information you already have.