M20-1.5 x 80mm Class 10.9 Hex Head Cap Screws: Specs Explained
By Mutual Screw & Supply | Published: 2026-11-01
Category: Product Reviews
Get clear answers on M20-1.5 x 80mm Class 10.9 hex head cap screws: thread fit, torque, coating, and when to choose Class 10.9 over 8.8.
Quick answer
For most machinery and equipment assembly, M20-1.5 x 80mm Class 10.9 hex head cap screws give you a high-strength, fine-thread fastener with a partially threaded shank that aligns and clamps well. Verify torque from your joint design or an engineering source, not a generic chart.
Why the 1.5 mm pitch matters on an M20 bolt
The 1.5 mm pitch is a fine thread for M20, not the coarse 2.5 mm pitch. Fine threads provide more threads per inch of engagement and can offer better resistance to vibration loosening, but they are more sensitive to cross-threading and require careful alignment during installation. They are common in machinery and equipment assembly where precise clamping is needed.
What partially threaded means for your joint
On this 80 mm screw, the unthreaded shank portion sits under the head. That shank can act as a dowel or alignment feature in a close-fit hole, and it keeps the shear plane out of the threaded section. If your joint relies on the screw to locate parts, a partially threaded screw is often the better choice than a fully threaded one. However, if you need thread all the way to the head for a nut or tapped hole near the surface, this screw will not work.
Class 10.9 vs 8.8: when the higher grade pays off
Class 10.9 screws have a higher minimum tensile strength than Class 8.8, so you can use fewer or smaller fasteners for the same load, or gain extra safety margin in a highly loaded joint. The trade-off is that 10.9 is more sensitive to hydrogen embrittlement and generally costs more. For static, well-controlled joints, 8.8 may be sufficient; for critical or dynamic loads, 10.9 is often worth the premium. Always confirm that your mating parts and washers can handle the higher preload.
Zinc plating: what it protects and what it does not
The zinc plating on these screws provides basic corrosion protection for indoor and occasional outdoor use. It is not a substitute for hot-dip galvanizing or stainless steel in coastal, chemical, or high-moisture environments. Also, plated fasteners can experience hydrogen embrittlement if not properly baked after plating; for critical Class 10.9 applications, ask your supplier about the coating process and baking.
M20-1.5 x 80mm Class 10.9 hex head cap screw FAQ
- What does the 1.5 in M20-1.5 mean?
- It is the thread pitch: 1.5 mm between adjacent threads. This is a fine pitch for M20, which provides more threads per inch and can improve resistance to vibration loosening compared to coarse threads.
- How much of the 80 mm length is threaded?
- The screw is partially threaded, so a portion under the head is unthreaded shank. The exact thread length is not specified in the catalog; for critical grip calculations, measure a sample or request a dimensional drawing from the supplier. Do not assume a standard thread length without verification.
- What torque should I use for M20-1.5 Class 10.9?
- There is no universal torque value. Torque depends on lubrication, joint stiffness, and the required preload. Use a torque calculated for your specific joint, or consult an engineering reference. As a starting point, many tables for lubricated Class 10.9 M20 fine thread list values in the 400–500 N·m range, but you must confirm for your application. Always validate torque with a qualified engineer for critical joints.
- Can I use these screws outdoors?
- Zinc plating offers moderate corrosion resistance, suitable for sheltered outdoor use. For direct exposure to rain, salt, or chemicals, consider stainless steel or a more protective coating.
- What is the difference between Class 10.9 and 8.8?
- Class 10.9 has a higher minimum tensile strength (1040 MPa vs 800 MPa for 8.8) and a higher yield strength. This allows higher preload and potentially smaller fasteners for the same load, but 10.9 is more prone to hydrogen embrittlement and typically costs more.
- Are these screws reusable?
- Steel screws can be reused if they are not damaged, but for critical applications, replace fasteners after disassembly, especially if they have been torqued to yield or exposed to fatigue. Inspect threads and coating before reuse.
Product facts
- M20-1.5 X 80mm Hex Head Cap Screws | Class 10.9 Steel Zinc Plated | Partially Threaded (25 / Pkg.): M20-1.5 x 80mm, Class 10.9 steel, zinc plated, partially threaded, sold in packs of 25. This combination gives you a high-strength fine-thread fastener in a convenient pack size for assembly work.
M20-1.5 X 80mm Hex Head Cap Screws | Class 10.9 Steel Zinc Plated | Partially Threaded (25 / Pkg.)
