This page covers M20 Plain Washer to GB/T 858. M20 is the nominal bolt size; the dimensional row below is reproduced from the product table, followed by the tolerance, material and selection notes that apply to this size.
M20 Dimensional Row (GB/T 858)
For bolt size
Inner diameter d1 (mm)
Outer diameter d2 (mm)
Thickness t (mm)
Reference standard
M20
21.0
37.0
3.0
ISO 7089 / GB/T 97.1
Common Applications for M20 Plain Washer — GB/T 858 (Series 071)
M20 Plain Washer — GB/T 858 (Series 071) are commonly specified for high-load load-spreading on structural joints.
Mating Parts for M20
A M20 flat washer pairs with a M20 bolt/screw and matching M20 nut.
Technical Drawing
Customize the parameters below — every spec field is editable, and the drawing redraws to match the values you choose. Changing the size will snap the other fields to that size's standard values; you can then override any individual value.
Sends the selected values to the quote form below.
Request a Quote
The form below follows the current technical drawing configuration. Change the drawing values above and the request details update automatically.
Dimensions & Specifications (GB/T 858)
For bolt size
Inner diameter d1 (mm)
Outer diameter d2 (mm)
Thickness t (mm)
Reference standard
M3
3.2
7.0
0.5
ISO 7089 / GB/T 97.1
M4
4.3
9.0
0.8
ISO 7089 / GB/T 97.1
M5
5.3
10.0
1.0
ISO 7089 / GB/T 97.1
M6
6.4
12.0
1.6
ISO 7089 / GB/T 97.1
M8
8.4
16.0
1.6
ISO 7089 / GB/T 97.1
M10
10.5
20.0
2.0
ISO 7089 / GB/T 97.1
M12
13.0
24.0
2.5
ISO 7089 / GB/T 97.1
M14
15.0
28.0
2.5
ISO 7089 / GB/T 97.1
M16
17.0
30.0
3.0
ISO 7089 / GB/T 97.1
M18
19.0
34.0
3.0
ISO 7089 / GB/T 97.1
M20
21.0
37.0
3.0
ISO 7089 / GB/T 97.1
M24
25.0
44.0
4.0
ISO 7089 / GB/T 97.1
M30
31.0
56.0
4.0
ISO 7089 / GB/T 97.1
M36
37.0
66.0
5.0
ISO 7089 / GB/T 97.1
Materials & Grades
Select a material class to see the grades we supply it in. Each designation is listed with its ISO/EN, AISI/SAE, JIS and GB equivalents.
Third-party video, selected automatically for this topic. It is not produced by Engineering Hardware and is not a specification source — use the tables on this page for dimensional data.
Inside the plant
Manufactured in our own plant
This part is cold-formed, threaded, heat-treated and finished in-house. The stages below are where its class and finish are actually decided.
01
Cold heading
Wire rod is drawn to size and cold-formed into blanks. Forming rather than cutting keeps the grain flow continuous around the head, which is where a bolt is most likely to fail.
Head dimensions and under-head radius checked at set-up and per shift.
02
Thread rolling
Threads are rolled between hardened dies rather than cut. Rolled threads are stronger in fatigue than cut threads and hold pitch and flank angle far more consistently across a batch.
Go / no-go ring gauges to ISO 965 tolerance class 6g.
03
Heat treatment
Quench-and-temper brings carbon and alloy steel to the property class ordered — 8.8, 10.9 or 12.9 — with the furnace charge tracked so a finished lot can be traced back to it.
Core hardness and tensile tested per lot to ISO 898-1.
04
Surface treatment
Zinc, zinc-nickel, hot-dip galvanizing, black oxide, phosphate and Dacromet, selected against the service environment rather than against the price list.
Coating thickness and neutral salt spray hours to ISO 9227.
Frequently Asked Questions
What clearance and tapping drill suit M20?
Use a 22 mm clearance hole (ISO 273 medium) for through-fixing, or a 17.5 mm tapping drill for a coarse M20 thread.
Should M20 Plain Washer — GB/T 858 (Series 071) be tightened by torque or by angle?
For critical structural joints at M20, angle-controlled or stretch-controlled tightening is more repeatable than pure torque — the friction scatter at large diameters can dominate the torque signal.
What grip range is typical for M20 structural bolts?
Grip ranges are highly application-dependent at M20. As a guide, choose a length that gives at least 1× the thread diameter engagement in the nut plus 2–3 thread pitches projecting.
Which washer dimensions matter most for M20?
For a flat washer, the outside diameter (the bearing surface) and the thickness control how the load is spread under the head and how much elastic preload is preserved. The inside diameter must clear the bolt body but not be so large that the head bears mostly on the washer's outer rim.
How do I request a quotation?
Use the technical drawing on this page to pick the exact configuration, then submit it with the quote form. We respond within one business day.
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