Replacing the screw drive rack on a Lift-master Model 2000SD 1/2 HP. The part number I ordered from Amazon was a
“Liftmaster 81C168 Garage Door Screw Drive Carriage Rack” The link to detailed instructions that I followed (more or less) from Chamberlain.com are:
https://www.chamberlain.com/CatalogResourcesV3/en-us/shared/files/tucmanuals/114A2728.pdf
Video on how to identify different grades of bolts. thanks for watching