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.
- DIN 9021 (ISO 7093): Often called "penny washers" or "repair washers," these have a much larger outside diameter (typically 3x the internal diameter) to spread the load further.
- 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
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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
|
