Heavy-Duty Brake Drum Sizes Explained: 16.5 x 7 vs 16.5 x 8-5/8 and How to Measure
November 9, 2026 · RPS Distributors
Heavy-duty brake drums look alike, but a drum that is the wrong size or mounting type simply will not fit. Getting it right the first time keeps trucks and trailers moving. This guide explains the sizes, what the numbers mean and what to check before you order.
What the size numbers mean
A drum size such as 16.5 x 7 gives two measurements in inches:
- 16.5 is the inside diameter of the braking surface.
- 7 is the width of the braking surface, which must match the width of your brake shoes.
Common heavy-duty sizes
| Size | Where it is usually found |
|---|---|
| 16.5 x 7 | The most common size on Class 8 drive axles and trailer axles |
| 16.5 x 8-5/8 (8.62) | Wider shoe, used on heavier-duty drive and trailer applications |
| 16.5 x 5 / 16.5 x 6 | Steer axles on many trucks |
| 15 x 4 | Steer axles on some older trucks |
Always confirm against the axle and brake shoe you have. The same truck can use different sizes on the steer, drive and trailer axles.
Mounting type: outboard vs inboard
Outboard-mounted drums sit over the hub and can be removed without pulling the hub. Inboard-mounted drums are mounted behind the hub flange, so the hub must come off to change the drum. The two are not interchangeable, so check which one your axle uses.
Pilot type and bolt pattern
Drums are either hub-piloted or stud-piloted, and must match the hub and wheels. Check the bolt-hole count, the bolt circle and the pilot diameter against the old drum or the hub specifications.
Balanced drums and weight
Balanced drums are machined or weighted to cut vibration, which helps ride quality and reduces uneven wear. Heavier drums hold more heat during long downhill braking. Lightweight drums save weight for payload-sensitive fleets.
When to replace a brake drum
- The inside diameter has worn past the maximum diameter cast or stamped on the drum.
- Cracks that run to the open edge of the drum, or heavy heat checking.
- A drum that is out of round or shows deep scoring.
- Blue discoloration from overheating.
Replace drums in pairs on the same axle, and fit new shoes when you replace drums so the braking surfaces seat together.
Frequently asked questions
How do I measure a brake drum?
Measure the inside diameter of the braking surface with a drum gauge, and the width of the braking surface. Compare the diameter with the maximum diameter marked on the drum.
Can I use a 16.5 x 8-5/8 drum instead of a 16.5 x 7?
Only if the brake shoes and the rest of the brake assembly are also made for 8-5/8. The drum width must match the shoe width.
Should brake drums be replaced in pairs?
Yes. Replacing both drums on an axle keeps braking balanced side to side.
Brake drums we carry
Shop the Alliance 16.5 x 7 Balanced Brake Drum, the Gunite 16.5 x 7 Outboard Brake Drum and the 16.5 x 7 Brake Drum 151.6726BATR, or browse our Air Brake & Wheel parts.
Related guides: Truck Air Brake Basics · Winter Air Brake Maintenance
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