Identify Truck Brake Drums, Rotors and Pads Before Ordering
Back to the ATP Garage guide libraryAir supply & brakes / Friction component identification
Quick answer: Start with the VIN, axle position and actual brake manufacturer/model, then record the complete installed markings. For a drum, identify the braking surface and mounting dimensions. For a rotor, identify its design, dimensions and hub interface. For pads, match the caliper, pad identity, backing plate and specified friction material. A familiar shape or one matching dimension leaves the request incomplete.
Application: Commercial-truck and trailer friction-component identification. This is chassis/axle guidance: an engine model such as DD15 does not identify the brakes. The dated ConMet and Bendix examples below explain what to record; confirm the current application through the vehicle, axle and brake manufacturers. Have a qualified brake service provider obtain measurements that require wheel-end access.
Which vehicle and axle details should come first?
Put the truck or trailer VIN, make, model and year at the top of the request. Name the axle position and wheel end: steer, drive, tag or trailer, with left/right as applicable. Add the axle label, hub identity and brake manufacturer/model wherever available. Describe any known axle, hub or brake conversion so the supplier can reconcile the current installation with the original vehicle record.
Photograph each accessible marking in context, then take a readable close-up. Keep the complete number, suffix and label fields rather than transcribing only the largest characters. State which component carries each marking. A rotor casting mark, caliper label and carton label should remain separate entries in the record.
What dimensions identify a replacement brake drum?
ConMet's January 2018 manual starts replacement selection with the drum manufacturer and part number; its identification section distinguishes the drum marking from the nearby date code. It describes a VIN-based dealership check if identity is missing and a dimension record if the drum still cannot be identified. [1]
| Field | Record clearly |
|---|---|
| Braking surface diameter | Diameter of the internal braking surface; identify this separately from the outside of the casting. |
| Braking surface width | Width of the surface contacted by the lining. |
| Overall depth and flange thickness | Two separate dimensions, using the reference faces in the manufacturer drawing. |
| Pilot diameter | The locating interface identified in the matching drawing; include a view of its profile. |
| Bolt circle and holes | Bolt-circle diameter, number of holes and hole diameter, with units. |
These are the fields in ConMet's Figure 22, rather than a list of interchangeable drum sizes. Label measurements as taken from a used component and ask the supplier to compare them with the specified replacement drawing. [1]
A pilot needs more than a photograph of the outer opening. ConMet's November 2023 bulletin applies specifically to steer brake drums: a stepped pilot has a smaller guide portion and a larger portion that matches the hub's drum pilot. Identify which surface was measured; do not substitute the smaller opening for the mating diameter. [2]
Use a simple annotated sketch or the manufacturer drawing to show where each measurement begins and ends. Record the units beside every value. If the dimensions and readable part identity disagree, send both records and leave the discrepancy open for confirmation.
Which rotor details matter besides diameter?
Bendix's September 2023 replacement-parts sheet lists rotor application and caliper alongside dimensions and shape. Its entries distinguish flat, U-shaped and splined designs, and use thickness or offset fields for different configurations. That is why diameter alone is an incomplete request. [3]
Send the rotor markings, hub identity, actual axle position and caliper model together. Ask the supplier which drawing defines the required diameter, thickness, offset or height and mounting interface for that rotor. Photograph the side profile and attachment arrangement when the service provider has safe access. Keep an observed used-part measurement separate from a catalog's new-part dimension.
Name the requested service assembly explicitly: rotor alone, a defined rotor kit, or a hub-and-rotor assembly. Ask what hardware and other components are included in that exact package. Do not assume that a catalog illustration of an assembly describes everything supplied with an individual replacement.
Can pad outline or an FMSI number confirm the replacement?
Bendix's September 2023 sheet records pad FMSI identity, caliper model, length, width and application distinctions. Retain all available fields rather than treating the outline as the entire selection. [3]
Bendix S-1441, revised March 2017, shows brake labels near the guide-pin housing and identifies fields including the brake designation and caliper/carrier number. Its pad guidance distinguishes backing-plate thickness, original lining material and different retainer designs. Within that document's application, Bendix strongly recommends replacement as an axle set. Confirm the current brake-specific requirements rather than extending this kit example to every caliper. [4]
Ask the service provider to name the specified pad/lining and backing-plate configuration. Include the readable pad markings and any known wear-sensor arrangement. Confirm the correct retaining hardware and sensor contents with the requested kit. A supplier should resolve a missing specification before the order is treated as a confirmed match.
What should the final parts request contain?
| Record | Include |
|---|---|
| Application | VIN, actual axle position, axle/hub labels and current brake model; disclose known conversions. |
| Component identity | Complete markings, label photographs and the component carrying each number. |
| Measurements | A labeled drawing, units and whether each value is observed on the used part or specified for the replacement. |
| Requested scope | Named drum, rotor, pad set or defined assembly; quantity and package contents to confirm. |
| Service finding | The recorded inspection conclusion and the current application instructions used by the service provider. |
For example, replace “rear brakes for a DD15 truck” with a request naming the truck VIN, actual drive-axle position, brake/caliper identity, component markings and the attached dimension record. If the label cannot be read, say so; do not fill the gap with a guessed number. Ask for written confirmation of the proposed component and package before ordering.
Identification and serviceability are separate questions. A dimension recorded for selection does not establish a wear limit, authorize machining or confirm that the vehicle is ready for service. Keep the inspection result and the applicable manufacturer limits with the repair record. For valve, circuit and chamber identity, use the related air-brake guide below.
Common brake drum, rotor and pad questions
Will the engine serial number identify my brake drums or pads?
Send the vehicle and axle/brake records. The engine identity alone leaves the chassis brake application unresolved.
Is the outside diameter of a drum enough?
Record the braking surface and mounting fields in the matched manufacturer drawing. A single outside measurement leaves the request incomplete.
What if the pilot opening has two diameters?
Photograph the profile and label both measured surfaces. The cited ConMet November 2023 steer-drum bulletin distinguishes a guide step from the larger mating portion; confirm the applicable drawing.
Can I order a rotor by diameter and truck make?
Also supply the actual rotor/hub identity, axle position, caliper, design and required drawing dimensions. Ask the supplier to resolve the complete application.
Does an FMSI number replace the pad specification?
Keep it with the brake/caliper identity, readable markings and the current specified pad configuration. Confirm the backing plate, friction material and kit contents.
What should I do when the old part and VIN lookup disagree?
Disclose the disagreement and known repair or conversion history. Have the actual installation reconciled with the proposed replacement before the order is confirmed.
Manufacturer sources and application limits
Dated examples explain the decision and its limits. Use current service information for the actual vehicle, axle, hub and installed brakes. No universal test threshold or replacement authorization is supplied here.
- ConMet, Service Manual for Steer, Drive, and Trailer Brake Drums, Part 10081803, printed January 2018. Identification pages 2-3 and replacement-selection page 11/Figure 22. Drum identification only; its service limits are not transferred to other brakes.
- ConMet, Stepped vs. Standard Pilot for Steer Brake Drums, technology bulletin 10119067A, November 2023. Steer-drum pilot geometry only; no quoted pilot size is a universal fitment specification.
- Bendix, Air Disc Brake Pad & Rotor Replacement Parts, BW8138 Rev.003, September 2023. Public catalog field definitions and differing configurations; no part-number cross-reference table or current availability claim reproduced.
- Bendix, Air Disc Pad Replacement Kit, S-1441 Rev.002, March 2017. Identification page 2, pad-selection distinctions pages 3-4. Historical Bendix kit guidance, not an installation procedure or permission to substitute pads on other brakes.
Send the confirmed component request
Use the fitment verification form with the vehicle, axle, hub and brake identities, complete markings, installed position, diagnostic finding and accessible photographs. Part fitment, package contents and availability require confirmation.
Related guide: Air-Brake Valves and Chambers: Identify the Circuit and Component.
