I recently changed the blade on a 4" 29-2, it took a .146". The customer service rep at S&W will tell you what you need based on model and barrel length. There is a lot of good information on the gunsmithing forum. If you read some of it and follow the directions that are included, it's not a bad job. Be aware that the new white outline blades have qute a thin line compared to the originals.