Quick Answer: A non standard screw is any fastener that falls outside published international standards (ISO, ANSI/ASME, DIN). This includes non-metric screws (UNC/UNF inch series), specialty drive screws, custom-dimension fasteners, and proprietary security screws. Stainless steel screws resist corrosion well but can strip more easily than alloy steel and should be avoided in certain structural, high-heat, or galvanic environments. For sheer tensile strength, Grade 8 alloy steel screws outperform stainless steel in most head-to-head comparisons.
What Are Non-Metric Screws Called?
Non-metric screws are most commonly called Unified Thread Standard (UTS) screws, or simply imperial screws or inch-series screws. The two main thread families are:
- UNC (Unified National Coarse): Fewer threads per inch, faster assembly, better grip in soft materials like aluminum, plastic, and wood. Example: a ¼-20 screw has 20 threads per inch at ¼-inch diameter.
- UNF (Unified National Fine): More threads per inch, greater tensile strength and vibration resistance, used in precision machinery and automotive applications. Example: ¼-28.
Beyond UNC and UNF, other non-metric thread forms include:
- UNEF (Unified National Extra Fine): Used in thin-walled tubing, optics, and instrumentation.
- BSW / BSF (British Standard Whitworth / Fine): Older British imperial standard, still encountered in vintage machinery and classic motorcycles.
- BA (British Association): Very small screws used in electronics and watches.
- NPT (National Pipe Taper): Tapered thread for fluid and gas pipe fittings, not interchangeable with standard fastener threads.
In everyday workshop language, people also refer to non-metric screws simply as "SAE screws" (after the Society of Automotive Engineers), particularly in the United States automotive and hardware trades.
Do Stainless Steel Screws Strip?
Yes—stainless steel screws strip more readily than many users expect, and understanding why helps you avoid costly rework.
Why Stainless Steel Is Prone to Stripping
Stainless steel (typically 304 or 316 grade) has a lower hardness than alloy steel. On the Rockwell B scale, 304 stainless typically measures around HRB 70–80, while Grade 8 alloy steel sits at HRC 33–39—a significantly harder material. The softer head material means drive recesses (Phillips, hex, Torx) deform more easily under torque, especially if the driver bit is even slightly worn or misaligned.
Galling: The Hidden Stripping Risk
A related problem unique to stainless is galling (also called cold welding). When stainless steel threads contact another stainless surface under pressure and rotation, the oxide layer that gives stainless its corrosion resistance can break down, causing the threads to microscopically weld together. The result looks like a stripped thread but is actually the fastener seizing solid mid-installation. Galling risk is highest with:
- Stainless screws driven into stainless tapped holes or nuts
- High-speed driving with impact drivers
- Dirty or dry threads (no lubrication)
- Fine-thread fasteners with larger contact surface area
How to Reduce Stripping and Galling
- Use a Torx (star) drive instead of Phillips; Torx distributes torque over six contact points rather than four, dramatically reducing cam-out.
- Apply anti-seize compound (nickel-based for high heat, copper-based for general use) to threads before installation.
- Drive at low speed with steady axial pressure—avoid impact drivers for stainless-into-stainless assemblies.
- Use fresh, correctly sized driver bits. A worn Phillips bit in a stainless head is a reliable recipe for stripping.
When Not to Use Stainless Steel Screws
Stainless steel is not a universal solution. There are several situations where it is the wrong choice:
| Situation | Why Stainless Fails | Better Alternative |
|---|---|---|
| High-strength structural connections | 304/316 stainless has a tensile strength of ~70,000–85,000 psi; Grade 8 alloy steel reaches 150,000 psi | Grade 8 or Grade 10.9 alloy steel, zinc-plated or hot-dip galvanized |
| High-temperature environments (>400°C / 750°F) | Standard 304/316 loses strength significantly above 400°C; can also sensitize and lose corrosion resistance | High-temp alloys (A286, Inconel) or ceramic-coated fasteners |
| Contact with carbon steel in wet conditions | Galvanic corrosion: stainless acts as cathode, accelerating rust on the carbon steel it contacts | Matching material fasteners, or use isolation washers and coatings |
| Contact with aluminum in marine/outdoor environments | Large electrochemical potential difference causes accelerated corrosion of the aluminum | Aluminum rivets, coated fasteners, or use sealant as a barrier |
| Pressure-treated lumber (ACQ/CA treated) | Modern copper-based preservatives corrode standard 304 stainless; 304 screws fail within years outdoors | 316 stainless (higher molybdenum content), hot-dip galvanized, or silicon bronze |
| Magnetic applications | Austenitic stainless (304, 316) is non-magnetic and cannot be used where magnetic properties are required | 410 or 430 martensitic/ferritic stainless, or carbon steel |
| Budget-constrained, non-exposed interior work | Stainless costs 3–5× more than zinc-plated steel with no performance benefit indoors | Zinc-plated or yellow chromate steel screws |
Which Screw Is Stronger?
Screw strength is measured primarily by tensile strength (resistance to being pulled apart) and proof load (maximum stress without permanent deformation). The grading systems differ between metric and imperial standards.
Imperial (Inch-Series) Screw Grades
| Grade | Material | Tensile Strength (psi) | Common Use |
|---|---|---|---|
| Grade 2 | Low/medium carbon steel | 74,000 | Light-duty general hardware |
| Grade 5 | Medium carbon steel, quenched & tempered | 120,000 | Automotive, machinery |
| Grade 8 | Medium carbon alloy steel, Q&T | 150,000 | Heavy structural, suspension components |
| 304 Stainless | Austenitic stainless steel | ~70,000–85,000 | Corrosion-resistant applications |
Metric Screw Property Classes
| Property Class | Material | Tensile Strength (MPa) | Common Use |
|---|---|---|---|
| 4.8 | Low carbon steel | 420 | Non-critical general use |
| 8.8 | Medium carbon steel, Q&T | 800 | Standard structural/mechanical |
| 10.9 | Alloy steel, Q&T | 1,040 | High-load bolted joints |
| 12.9 | Alloy steel, Q&T | 1,220 | Aerospace, motorsport, critical assemblies |
| A2-70 (stainless) | 304 stainless | 700 | Corrosion-resistant, moderate load |
Verdict: For raw tensile strength, Grade 8 (imperial) or Class 12.9 (metric) alloy steel screws are the strongest commercially available standard fasteners. Stainless steel occupies a middle tier—stronger than Grade 2 or Class 4.8, but substantially weaker than Grade 8 or Class 10.9. When both corrosion resistance and high strength are needed simultaneously, A4-80 stainless (316 grade, 800 MPa) or titanium fasteners are the practical options, at considerably higher cost.
Non-Standard Screws: When and Why They Are Used
Standard catalogued fasteners cover the vast majority of applications, but non-standard screws are specified for several legitimate engineering and business reasons:
- Custom dimensions: A shaft diameter, thread pitch, or head geometry that falls between standard sizes—common in medical devices, aerospace brackets, and consumer electronics where space is tightly constrained.
- Proprietary security: Tamper-resistant screws with pentagon, tri-wing, or spanner drives that prevent disassembly without manufacturer-supplied tools. Widely used in electronics (Apple products, game consoles) and public infrastructure (elevator panels, utility meters).
- Special materials: Titanium, Inconel, PEEK (polymer), or ceramic fasteners for extreme temperature, weight, or chemical environments that off-the-shelf stainless or alloy steel cannot satisfy.
- Hybrid thread forms: A fastener may combine a metric thread pitch with an imperial shank diameter, or use a thread form optimized for plastic (AB, B, or PT thread) that differs from standard machine screw threads.
- Legacy equipment maintenance: Older machinery built to BSW, BA, or early DIN standards requires non-modern fasteners for authentic repair without modifying original tapped holes.
Identifying a Non-Standard Screw
If you pull an unknown fastener from a machine and need to identify it, follow this process:
- Measure the shank diameter with calipers. Metric diameters (M3, M4, M5, M6, M8…) will be clean whole or half millimeter values. Imperial sizes (No. 4, No. 6, No. 8, ¼", 5/16"…) convert to specific decimal inches.
- Count threads per inch or measure thread pitch using a thread gauge comb. Pitch is mm between threads for metric; TPI for imperial.
- Check the head markings. Metric bolts show property class digits (8.8, 10.9); SAE Grade 5 shows three radial lines; Grade 8 shows six radial lines on the head.
- If measurements fall between standard values—for example, a 4.5 mm diameter with 0.75 mm pitch that matches no ISO standard—you are likely dealing with a non-standard or proprietary fastener that needs to be sourced from a specialty supplier or manufactured to print.
