IS0-6431 Air Cylinders

In Europe the specification often used to select larger air cylinders is ISO-6431.

In North America, the comparable specification for similarly sized air cylinders is the NFPA .

I do not suggest that the two sets of standards are the same, for they are not. The ISO-6431 is a metric standard, the NFPA use imperial measure in their set of air cylinder standards.

The cylinders covered by ISO-6431 are usually of the tie-rod type and include bore sizes from 32 mm (about 1 1/2" inch) up to 100 mm (or about 4").

These cylinders are of metric dimension, meaning that the cylinder bores, the rod strokes, the port sizes, the rod threads, and the mounting accessories are all in metric measure.

Since an ISO-6431 a cylinder must meet the ISO standard dimensions for that size of cylinder, if you have a cylinder of this type on your equipment now, you should be able to purchase a substitute ISO-6431 air cylinder of the same bore and stroke size from any other manufacturer and have it drop-in as a replacement; good news if you do not know where your present cylinder was obtained.

ISO Air Cylinder

Tie Rods

Many ISO-6431 have integral tie rods as shown in the picture, unlike older NFPA style cylinders that had tie rods that were not integral to the barrel body. The older models of air cylinders had loose rods that were inserted through the cylinder end caps and attached to the cylinder with nuts at both ends, effective, but cumbersome.

The tie rods in ISO-6431 type cylinder are extruded as part of the barrel. The tie rod bolts to hold the cylinder end caps in place are screwed through the caps into mating threads in the cylinder barrel.

Current designs of the ISO-6431 cylinder include unthreaded holes in the barrel and what were through-bolts in earlier versions of the cylinder used to attach the end caps, are now self tapping metal screws in current versions of the ISO-6431 air cylinder.

The ISO-6431 piston rod diameter will meet the ISO standard for that particular bore size, and typically comes with a male metric thread. A variety of other rod-end thread configurations are available within the ISO standard.

Mounting Styles

ISO-6431 cylinders have a wide variety of mounting styles and accessories, including:
  • Basic mount - threaded holes on the sides of the caps for mounting the cylinder flat on a surface
  • Front and rear flange mounts
  • Rear clevis and pin mount
  • Trunnion Mounts
  • Rod clevis
  • Rod alignment couplers
  • Double end rod
  • Female rod thread
  • Cushions
  • Position sensing

Shock Absorbing

To improve cylinder life it is important to try to prevent the cylinder piston from bottoming on or slamming into either the rod end or rear end caps when the cylinder in operation.

By designing the tooling that is attached to the cylinder rod to have an external, shock-absorbing stop will stop the rod before the piston inside the cylinder hits the end cap. This will limit the cylinder-end-cap-damaging, inside-the-cylinder, impact.

I have seen pistons damaged and the cylinder rod actually sheared from the piston face due to inside-cylinder impact damage.


Sideloading of the cylinder rod must be avoided if you wish to have longer ISO-6431 air cylinder life.

If the cylinder installation has the potential for sideloading of the cylinder rod by the end-of-rod tooling an alignment coupler should be used.

If an alignment coupler is insufficient to eliminate sideload, the cylinder mounts must be such that the cylinder can "give" as sideloading force is applied to the cylinder rod. Perhaps a rear clevis, or a trunnion mount might be in order to accomplish this.

Or, consider guiding and carrying the work load on exterior rails, completely divorcing the cylinder rod from any function or loading but the basic powering of linear movement which, after all, is what ISO 6431 air cylinders are designed to do.