IS0-6431 Cylinders

In Europe the larger cylinder specification is ISO-6431; in North America, the comparable specification of similarly sized air cylinders is NFPA .

That is not to suggest that the two sets of standards are the same, for they are not. The ISO-6431 is a metric standard, the NFPA is an imperial measure set of 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 can include unthreaded holes in the barrel and what were through-bolts used to attach the end caps, are now self tapping metal screws.

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

Typically, to improve cylinder life, it is important to try to prevent the cylinder piston from bottoming on or slamming into either the rod or rear end caps when the cylinder is in operation. By designing the tooling reach end of stroke and an external, shock-absorbing stop before the piston hits the end of the cylinder loads can be stopped without relying on cylinder end-cap damaging, inside the cylinder, impact.

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


Sideloading of the cylinder rod is an absolute no-no if you want longer ISO-6431 air cylinder life.

If the cylinder application has the potential for sideloading on the end-of-rod tooling and therefore sideloading the rod itself, 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.

Or, consider guiding the load on rails, divorcing the cylinder rod from any function but powering linear movement.