Nanthaporn,
These are twin spacers that are cast in Mexico and shipped to Albemarle . Albemarle machines the parting lines, then they tap and thread the hole. After that, they send the parts for shot peen. The bolt is on the threads when the parts are shot peened. After they get them back, they find metal residue in the threaded area, making it difficult for the bolt to thread. They also have a rough surface on the mating casting caused from the shot peen process. This is where the bolt guides in.
Can you pls see attached and advise us on the questions being asked next to the pictures?