What is ISO 12129-2-Plain bearing tolerances about?  
ISO 12129 is an International Standard that discusses plain bearings.
ISO 12129-2 is the second part of the multi-series that is applicable to tolerances of plain bearings.
ISO 12129-2 specifies tolerances on form and position as well as the surface roughness of shafts, flanges and thrust collars as used in plain bearing units.
Note: ISO 12129-2 applicable to journal or thrust plain bearing units or to a combination of both, installed either horizontally or vertically.
Note: ISO 12129-2 is not applicable to crankshaft bearing units in combustion engines.
Who is ISO 12129-2- Plain bearing tolerances for? 
ISO 12129-2 on plain bearings is applicable to:
- Manufacturers of plain bearings
- Mechanical engineers
- Manufacturers of machinery and equipment
- Mechanical engineers
Why should you use ISO 12129-2- Plain bearing tolerances?
Plain bearings are used for sliding, rotating, oscillating, or reciprocating motion. Plain bearings support and guide moving components inside the engine. The objective of plain bearings is to prevent wear of the machine's support points, which would occur if the shafts were to roll directly on the structure. The reliability of a plain bearing is not only a function of the proper selection of material and fit but depends in particular on the quality of the shafts and thrust collars used in the plain bearing unit.
ISO 12129-2 covers various parameters such as lubricating film thickness, runout tolerance, concentricity tolerance, flatness tolerance, and surface roughness and their corresponding accuracy grades.
If you are a manufacturer, you can demonstrate the reliability and quality of your products by meeting the requirements in accordance with ISO 12129-2.
What’s changed since the last update?   
BS ISO 12129-2:2019 supersedes BS ISO 12129‑2:1995, which is withdrawn. BS ISO 12129-2:2019 includes some significant changes with respect to BS ISO 12129-2:1995. These include: 
- The title has been improved
- Normative references have been updated
- The limitation to thick-wall bearings has been removed
- The limitation to ISO 7902 for calculation of minimum lubricant film thickness has been removed
- The case of different accuracy grades of journal und thrust collar has been explicitly added
