How To Measure A Slack Adjuster

Count the Splines

Locate the spline area where the slack adjuster connects to the camshaft. Carefully count the number of splines (teeth) on the camshaft opening.

• Common spline counts: 10, 28, or 37 splines
• This count determines compatibility with the camshaft

Measure the Arm Length

Using a measuring tape or ruler, measure the distance from the center of the spline opening to the center of the clevis pin hole. This is the arm length and it is the most critical measurement for finding the right replacement.

• This is Measurement A in the diagram below
• Some slack adjusters have multiple clevis pin holes for different applications

Measure the Spline Diameter

Use a caliper or ruler to measure the diameter of the spline opening. Precision matters here because even small differences affect fitment.

• This is Measurement C in the diagram below
• Typical sizes: 1.25" or 1.50"

Manual or Automatic?

Determine which type of slack adjuster you have. The two types are not interchangeable, and it is easy to tell them apart:

• Manual slack adjusters have a simple, flat body with a hex-head adjusting bolt visible on the front. You turn this bolt with a wrench to take up brake shoe clearance. Common on older trailers and some specialty applications.
• Automatic slack adjusters have a bulkier, rounded housing because they contain an internal clutch mechanism that self-adjusts during brake use. They typically have a pull-back collar or release sleeve, and many have a small window or wear indicator. If the body looks chunky with a visible internal mechanism, it is automatic.

If your vehicle was manufactured after 1994, it almost certainly came with automatic slack adjusters — FMCSA has required them on all new highway vehicles since then.

Note the Manufacturer and Style

Look for branding, part numbers, or date codes stamped on the adjuster. Common manufacturers include Haldex, Meritor, Bendix, and Gunite. Having the original part number makes finding a direct replacement much faster.

• Check both sides of the adjuster body for stampings
• If you find a part number, search it on our site for a direct cross-reference