Size: The spring measures approximately 1 1/4 inches long with a 3/4 inch diameter. Material: It is typically made of high-carbon steel with an anti-rust coating for durability and corrosion resistance. Replaces older part numbers: It replaces part numbers such as 791-BQ01642, 791-610317R, 610317, and others. The MTD 791-610317B Spring-spool is a genuine OEM replacement part made of metal. It is installed between the inner reel and the outer spool assembly to keep pressure on the bump head knob assembly of your string trimmer.