BS ISO 7425-1:2021 pdf free.Hydraulic fluid power cylinders一Dimensions and tolerances of housings for elastomer- energized, plastic-faced seals Part 1: Piston seal housings.
5.2 Axial length
For axial lengths for the nominal piston diameters, see Iable3.
If the extrusion gap exceeds the capabilities of the plastic-faced material to bridge such a gap, length L2
should be selected and back-up rings (anti-extrusion rings) used (see Clause 7).
5.3 Radial depth
5.3.1 For radial depths, see Table 3.
NOTE This document includes an alternative seal housing radial depth for bore diameters where D  25 mm.
5.3.2 The larger radial depths shall be selected where higher stresses or wider tolerances are involved.
Consultation with the manufacturer, however, is recommended when making the appropriate selection.
6 Dimensions and tolerances
Seal housing dimensions and tolerances shall be selected from Table 3.
7 Extrusion gap
7.1 The extrusion gap (g, see Figure 1) is determined by the bore diameter and adjacent metal components on either side of the seal.
NOTE 1 The extrusion gap varies considerably depending upon use or non-use of non-metallic bearings on the piston.
NOTE 2 Maximum value for the extrusion gap is achieved when the piston is in contact with one side of the cylinder or bearing (i.e. D-d3 — see Figure 1).
NOTE 3 The extrusion gap is widened by the expansion of the cylinder due to internal pressure.
7.2 It is recommended that details concerning the extrusion gap and need for non-extrusion rings be
the subject of consultation between the housing designer and seal manufacturer.
8 Surface finish
8.1 General statement
The surface roughness of the seal housing and any mating part has a significant impact on the life and sealing performance of the seal.
Where surface roughness measurements are taken, it is recommended that instruments complying with ISO 3274. including an electric wave filter, be used.
8.2 Sliding and static sealing surfaces
8.2.1 Unless otherwise agreed, the roughness values shall be in accordance with Table 1.
8.2.2 Unless otherwise agreed the material ratio, Rmr, of housing surfaces that are in mating contact with the seal should be between 50 % and 80 % at a profile section level (Röc) of 25 % of Rz, from a reference level, C0, of 5 % Rmr (in accordance with ISO 42871997. 4.5.4).
8.2.3 For some seal designs, a minimum surface roughness of 0,1 pm Ra can be required for the sliding sealing surface as the surface otherwise can be too smooth to provide adequate lubrication for the seal.BS ISO 7425-1 pdf download.