DIN/ISO A2 stainless steel is a corrosion resistant that is also known as ASTM-304.

A2 / ASTM-304 is an 18/8 stainless steel : this designates a metallurgical content of 18% Chromium and 8% Nickel.

A bolt marked A2-70 is a 304 stainless steel bolt with a 700 N/mm2 tensile strength (see below table).
The -70 representing the tensile strength divided by 10.
-50 and -80 grades are also available but are not all that common.

A2 / 304 stainless steel is whats known as an austenitic stainless steel : it is non-magnetic.

      Though some magnetism can be left residual after cold-working.

A2 / 304 stainless steel is amongst the most highly corrosion-resistant materials available to the designer and engineer.

Tensile strength denotes the load at which the material snaps.

0.2% Yield Strength denotes the load that will PERMANENTLY deform / stretch the material by 0.2% of its original size.
This is sometimes referred to as the elastic limit.

A2-70 can be safely used to hold car body panels and bumpers in place.
Wing bolts, hinge bolts, handles, locks, light fittings etc.

A2-70 can be safely used on non-mechanically loaded engine components where a clamping action is required.
Manifold attachment, ancilliary attachment, pipe clamps, flanges etc.

A2-70 can also be used for securing access panels, cover plates and accessory items to the vehicle.

A2-70 should not be used in heavily loaded mechanical or structural areas.

• Areas of direct loading on the suspension.
• Areas of direct loading on the steering column and linkages.
• Active engine componets (con-rods, crank shaft, clutch & power off-takes)
• Active gearbox and power-train components.

A general rule of thumb is that it's OK to replace 8.8 and lower rated nuts and bolts.

Any nut or bolt stamped with a 10:9 or 12:9 should be replaced with an identically rated (or higher) component AND NOT an A2-70 component.

The below table gives a rough guide to the maximum torque wrench settings to be used on the most popular metric sizes.

A4 grades of stainless steel are usually reserved for use in highly caustic environments where improved resistance to pitting corrosion is required, i.e Marine & exhausts.
Pitting corrosion occures once the chromium becomes exhausted through self-passivation after caustic attack - the effects as usually local to the exposed areas. The pitting forms as the Iron atoms become exposed to Oxygen and form 'rust'.

The improved resistance comes from the addition of a higher proportion of Chromium, which helps to form a protective oxide layer on the iron - the more chromium you have the longer the protection it offers lasts.

However, whilst some properties are improved, others deteriorate. The extra Chromium makes the metal more succeptable to fatigue and cracking : OK if your rolling serenly around on the high seas : not so OK if you driving a rickety old banger like a VW.!

This extra protection is not necessary for automotive use as the component life of A2-70 grades is generally massive (often lifetime guarantees are given on items made from these materials).
A2-70 components will outlive your vehicle.

Professional Engineers agree that A2 Stainless is the optimum selection for automotive use.