Round File Size

• The round file fits into the hook of the cutting teeth on the 137's chain. These teeth have three different angles that form the cutting surface of the teeth. To file all these angles consistently, you'll need a special round file. Most 137 chainsaws will use a chain with the pitch, a measurement of three consecutive rives divided by two, of .325 inch. This pitch will set the angles of the cutting teeth, so the file must correspond to the pitch. The file size needed for a .325-inch pitch is 3/16 inch.
  
  

Flat File Size

• The metal stubs in front of every cutting tooth set the depth of the tooth's cut. Called rakers, or depth gauges, these also need to be filed down regularly with the teeth, usually every four to five filings. The flat file needs to be crosscut about 6 to 10 inches long. The thickness of the flat file for sharpening the 137's depth gauges should be .05 inch. This will allow you to smoothly and uniformly file down the rakers.
  
  

Sharpening Angles

• The last important sharpening specification for filing refers to the angle at which the file should pass over the teeth. The most important angle aligns with the top plate of the cutter, which sets the degree of the cut. Use a rough guide of about 30 degrees to start with, and increase in sharpness if necessary. If possible, use a file guide to ensure this angle stays consistent for every tooth; otherwise, the chain will cut to the left or the right. Hold the file handle 10 degrees below the cutter and file at a very slight upward angle.
  
  

Sharpening Tips

• To uniformly sharpen every tooth, count the number of strokes used and start sharpening at the most heavily damaged tooth. If the teeth aren't the same size, the chain will cut poorly. If the angles aren't set properly, the chain will dull more quickly and cut poorly. Replace the chain if teeth are missing or heavily damaged, or if several teeth are much smaller than the rest. Use sharpening guides in the beginning to ensure that these specifications remain consistent.