# Cylinder bore-Cylinder tubes

**Diameter tolerances**

Metseals piston seals are always matched to the tolerance of the cylinder bore. The match is made with the seal’s outside diameter slightly bigger than the biggest diameter for the actual cylinder bore and tolerance grade. In Europe, H8 tolerance is the predominate standard for hydraulic cylinders. The Metseal types that are suitable for use in hydraulic cylinders are therefore always, as a standard, adapted to the H8 tolerance. Seals with 80 mm diameters and above are in most cases also available in H10 tolerance. Choice of bigger tolerance will give higher pre-stress and when the cylinder bore is close to the minimum diameter, always a higher friction. For seals in the upper diameter range, the maximum pre-stress and friction is low, despite the H10 tolerance and a H10 tolerance seal can be used even if the cylinder bore tolerance is H9 or H8. Cylinder tubes which are drawn over mandrel and not further machined on the surface, very often, at first glance, have a very poor diameter tolerance due to the elliptical form of the cylinder bore. The minimum and maximum diameters being located about 90° from each other. To get the pre-stress, Metseals piston seals are not dependent on the diameter, but solely the circumference of the cylinder bore. The circumference of an ellipse is proportional to the mean value of the minimum and maximum - diameter. Thin wall tubes that are roller burnished will also become elliptical and out of round but even they, like the drawn tube, have in general a circumference that is much closer to the circumference of a truly round tube than is suggested by the diameter tolerance. The pre-stress for Metseals piston seals will thereby correspond to the mean value for the minimum and maximum diameter of the cylinder bore. This is often equal to the middle value in a narrower tolerance. In general it is more common for the guiding ring and not the piston seal to establish if elliptical and out of round cylinder bore can be accepted. One useful method to establish the quality of the cylinder bore is to fit a Metseal seal on a piston with an outside diameter corresponding to the smallest acceptable cylinder diameter and then draw the piston with the seal along the bore. At a too small diameter the friction will be too high and if in any part the circumference is too big, the sealing friction will be to low. One can then approve the cylinder bore within a certain range of frictional resistance.