STAINLESS STEEL A4 (316)

The A4 designation refers primarily to the material grade of the washer—specifically Type 316 stainless steel, which is often called "Marine Grade" due to its high resistance to corrosion in salt water and acidic environments.

Dimensional Standards

While "A4" tells you what it's made of, the dimensions are defined by separate engineering standards. The most common standards for metric A4 washers are:

  • DIN 125 (ISO 7089): The most widely used standard for a "standard" flat washer.
  • BS 4320: A British standard that categorizes washers into "Forms" based on thickness and diameter ratios.

Understanding "Forms" (BS 4320)

In the UK and Europe, A4 washers are often classified into "Forms" which determine their specific proportions:

Form

Description

Use Case

Form A

Normal diameter, normal thickness

General purpose engineering

Form B

Normal diameter, light thickness

Light-duty applications

Form C

Large diameter, normal thickness

Marine decks and cleats

Form G

Largest diameter and extra thickness

Heavy-duty load spreading
STAINLESS STEEL A4 (316)