Brake Drum Measurement Tool: Micrometers and Calipers
A brake drum measurement tool reads the inside diameter of the drum, and this guide covers the gauges and calipers that do it, plus a pocket thickness gauge, with their stated ranges and resolutions.
On this page
The drum is off the hub and sitting on the bench, and the question is whether the bore is still round and true enough to run again. A brake drum measurement tool answers that question by reading the inside diameter of the drum, so the reading can be set against the discard limit in the vehicle manual before new shoes go in.
As an Amazon Associate I earn from qualifying purchases. GarageToolHQ earns a commission when you buy through links on our product pages, at no extra cost to you. Read our affiliate disclosure.
Below: the kinds of brake drum measurement tool on the listings, the ranges, scales and resolutions each one states, and how to take a reading that can be trusted.
What a brake drum measurement tool measures
A brake drum measurement tool measures the inside diameter of a brake drum, which is the surface the shoes press against when the brakes are applied. The drum wears where the shoes rub, so the bore grows larger with use and can wear out of round. A reading across the bore shows how far it has worn, and the gauge has to span the drum being measured, so check the range the listing states.
The ABN gauge covers 6.5 to 14.25 inches, which the listing also gives as 160 to 360 mm, while the iGaging caliper covers 2 to 18 inches, given as 50 to 450 mm. The HUAWOHOM set is a thickness gauge rather than a diameter gauge: it reads in 32nds and 16ths, with a stepped side for pads and a notched side for drums, and the listing says no wheel removal is needed to slide it through the slot. The Fowler and Central Tools listings state no measuring range, so their span cannot be checked from the listing alone.
Drum gauges, calipers and pocket gauges side by side
| Type | Range stated | Reading | Locking and zero |
|---|---|---|---|
| Electronic drum gauge (Fowler, Central Tools) | Not stated on either listing | Fowler: LCD display, inches, metric and fractions | Fowler: lock screw, covered rack, fine adjustment roller |
| Drum brake resetting gauge (ABN) | 6.5 to 14.25 in (160 to 360 mm) | Dual scale, 1/32 in and 0.88 mm increments | Knurled knob locks the sliding plates |
| Digital brake drum caliper (iGaging) | 2 to 18 in (50 to 450 mm), absolute range 16 in | Resolution 0.0005 in, .01 mm, fractions in 1/64 in increments | Floating zero sets zero anywhere |
| Pocket thickness gauge set (HUAWOHOM) | 32nds and 16ths dual scale | Laser engraved markings, stepped side for pads, notched side for drums | Not stated |
The table shows the split: diameter gauges state a range in inches and millimeters, while the pocket set states a scale in 32nds and 16ths. The Central Tools listing states the size and weight of its gauge, 23.5 x 7.0 x 2.0 inches and 3.0 pounds, but states no measuring range or resolution, so it cannot be matched to a drum from the listing alone. The Fowler listing states 4 measuring capabilities, inside, outside, depth and step, with a lock screw, a covered rack and a thumb-operated fine adjustment roller, and a wooden case in the box.
How to measure a brake drum: the checklist
- Check whether the drum has to come off. The iGaging caliper measures most heavy-duty drums while the wheels are still mounted on the vehicle, and the HUAWOHOM set slides through the slot with no wheel removal. A drum gauge like the Fowler reads the bore with the drum off the hub, as our guide to how to remove a drum brake covers.
- Set the zero. The iGaging caliper has a floating zero that sets zero anywhere, so set it before the reading. The Fowler gauge has a thumb-operated fine adjustment roller for one-handed use.
- Place the gauge across the bore. Open the gauge until it touches the bore on both sides, and keep the sliding plates parallel, as the ABN listing directs.
- Lock the reading. Use the knob to lock the tool, as the ABN listing directs, or the lock screw on the Fowler, which holds the sliding jaw in position, so the reading does not move while it is read.
- Read the scale in the units the job needs. The ABN gauge reads in 1/32 inch and 0.88 mm increments, the iGaging caliper converts between inch, metric and fractions, and the Fowler converts between inches, metrics and fractions.
- Measure again across a different line of the bore. A bore can wear unevenly, so a reading across a different line shows whether the wear is even.
- Set the reading against the manual. Compare the reading with the discard limit in the vehicle manual, and with the shoe clearance the ABN gauge is made to set, before the drum goes back on.
Measuring the outside of the drum does not give the bore size. The Fowler gauge measures inside, outside, depth and step dimensions, so use the inside reading for the bore and keep it apart from the outside reading.
Use rated jack stands on solid ground, wear eye protection with power and air tools, keep fuel-burning equipment out of a closed garage, and follow the manual for every tool. Read our Safety Disclaimer.
Drum gauge or inside micrometer
An inside micrometer reads a bore too, and our best inside micrometer guide compares models for bore and engine work. The difference is the job: a drum gauge is built for the drum, with jaws or a frame that span the bore, while an inside micrometer is a general bore tool. The iGaging caliper spans 2 to 18 inches with bent jaws for wheel-on measuring, and the ABN gauge is made to set shoe clearance as well as read the drum. For drum work alone, the gauge that states the range the drum falls in is the gauge to reach for; for bore work across the shop, the micrometer guide is the place to compare.
Reading the scale: 32nds, 16ths and decimals
The HUAWOHOM set reads in 32nds and 16ths, with laser engraved markings that the listing says stay readable in dim garages. The ABN gauge marks 1/32 inch and 0.88 mm increments on a dual scale. The iGaging caliper resolves to 0.0005 inch, .01 mm and fractions in 1/64 inch increments, and converts between inch, metric and fractions. The Fowler converts between inches, metrics and fractions on an LCD display. Pick the scale that matches the units in the manual, and read the same scale each time so readings can be set side by side.
FAQ: garage tools
What is a brake drum measurement tool?
A brake drum measurement tool is a gauge, caliper or micrometer that reads the inside diameter of a brake drum, which is the surface the shoes press against. The reading shows how far the bore has worn, so it can be set against the discard limit in the vehicle manual.
How do I measure a brake drum with a drum gauge?
Place the gauge across the bore with the sliding plates parallel, lock the reading with the knob or lock screw, and read the scale. The ABN listing directs the same order: measure the drum, lock the tool, then place it next to the shoe to set the clearance.
What range should a brake drum gauge cover?
The gauge has to span the drum being measured. The ABN gauge covers 6.5 to 14.25 inches, given as 160 to 360 mm, and the iGaging caliper covers 2 to 18 inches, given as 50 to 450 mm. The Central Tools and Fowler listings state no range, so their span cannot be checked from the listing.
Can I measure a brake drum without removing the wheel?
The iGaging caliper is made to measure most heavy-duty drums while the wheels are still mounted on the vehicle, and the HUAWOHOM set slides through the slot with no wheel removal. A drum gauge like the Fowler reads the bore with the drum off the hub.
What warranty does a brake drum measurement tool carry?
It depends on the listing. The HUAWOHOM listing states a lifetime warranty in its bullets and a 2 year manufacturer term in its spec table, so the bullets and the spec table disagree on warranty. The Fowler listing states a 1 yr warranty on material and workmanship, and the Central Tools listing states no warranty term.
Is a pocket thickness gauge the same as a drum micrometer?
No. The HUAWOHOM set is a thickness gauge that reads pads and drum lining in 32nds and 16ths, with a stepped side for pads and a notched side for drums. A drum micrometer or caliper reads the bore diameter: the iGaging caliper covers 2 to 18 inches, and the Fowler gauge reads inside, outside, depth and step dimensions.
Related: how to remove a drum brake, the best inside micrometer guide, our wrench style measuring tool picks and the automotive multimeter guide.
