Brake Drum: Testing and Inspection
VISUAL INSPECTIONInspect the drum for cracks. If any large, through-the-drum cracks are located the drum must be replaced.
NOTE: Cracks in drums are often difficult to locate. To quickly determine if a drum is cracked lightly drop the drum (from a height of 4-5 inches) onto a flat hard surface. A cracked drum will make a dull thud sound while a good drum will make a ringing sound.
Inspect for signs of overheating. An overheated drum will often be discolored (blue/gold), warped, or heat checked. Heat checks are small cracks in the drum friction surface. Heat checks can be removed by resurfacing.
NOTE: The cause of the overheating should be determined prior to replacing the shoes/linings or drums.
Inspect for scoring. Any grooves or scores in excess of 0.008 inches should be resurfaced.
SPECIFICATIONS - Specifications
MAXIMUM DIAMETER
Purpose
The thickness of the drum friction surface is directly proportional to the drums ability to absorb and release heat during braking. As the drum becomes thinner due to normal wear and resurfacing it looses its ability to absorb and release heat and is more prone to brake fade, distortion, and cracking.
As the drum wears the inside diameter of the drum increases. The amount of drum thickness lost to wear is equal to 1/2 the increase in diameter.
There are two specifications related to drum thickness.
Discard Diameter - This is the maximum diameter at which it becomes unsafe to operate. A drum which has reached this thickness should not be machined and should be discarded.
Maximum Refinish - This is the maximum diameter to which a drum may be machined or resurfaced to. This diameter is smaller (thicker) than the Discard diameter because it assumes the drum will need to have a sufficient thickness left, after machining, to allow for further wear from a new set of shoes/linings.
Which One Should Be Used?
If you install a new pair of shoes with the drum diameter at or less than the "Maximum Refinish" specification the drums should have a sufficient thickness to last the normal life of the new shoes/linings.
The "Discard Diameter" should be used to determine if a drum needs to be replaced at the present time of inspection.
If you install a new pair of shoes with the drums machined at or near "Discard Diameter", within a short time the drums will be too thin (unsafe) and the new shoes/linings will be subject to overheating and brake fade.
How To Measure
NOTE: For additional information on how to use a drum micrometer see Fundamentals and Basics. Fundamentals and Basics
- A drum micrometer is used to measure the drum diameter.
- The micrometer scale should be initially set to the original (new) thickness of the drum.
- The base of the micrometer should be placed in the deepest groove in the drum and held steady while the measuring point is swiveled to find the maximum diameter.
- The base and measuring point should be kept at the same depth in the drum.
Drum Micrometer:
- The measurement should be repeated at 3 or more places around the circumference of the drum to determine if the drum is out of round. If the measurements differ by more than 0.004 inches the drum should be resurfaced.
NOTE: A drum which is out of round will cause a pedal pulsation to be felt upon braking.
- The measurement should also be repeated at various depths of the drum to check for a bellmouth condition. If the measurements differ by more than .010 inches the drum should be resurfaced.
NOTE: 1 inch = 25.4mm, so if your micrometer measures in inches and the specified thickness on the disc is in millimeters, convert millimeters to inches by dividing the specified number of millimeters by 25.4.
Example: Specification on disc is 17.8 mm.
Dividing 17.8mm by 25.4mm/in = 17.8/25.4 = 0.70 inches.