Gates V-Belt Size Chart FAQ — Plus Dayton AX21, Micro Stepper Motors, and LM8LUU Bearings

I've been the person who orders drive belts and small motion components for our shop since 2017. In that time I've personally made—and written down—thirteen genuinely expensive mistakes, something like $4,000 in parts that didn't fit, had to be returned, or became expensive shelf decoration. These days I keep a checklist the whole team uses, and the questions below are the ones that land in our inbox most weeks. If you're here for one answer, jump straight to it:

• What's the difference between a Gates V-belt and a timing belt?
• How do I read a Gates V-belt size chart?
• What does "AX21" mean on a cogged V-belt like the Dayton AX21?
• Can I cross-reference Dayton or Aisin part numbers to Gates?
• What belt do I need for a micro stepper motor?
• What size is an LM8LUU linear bearing?
• Should I measure the old belt or trust the number printed on it?

What's the difference between a Gates V-belt and a timing belt?

The short version: V-belts grip, timing belts lock in.

A V-belt—classical A, B, C, D, E sections, wedge 3V/5V/8V, or metric SPZ/SPA/SPB/SPC—sits in a sheave groove and transfers power through friction. It can slip under a shock load. That sounds like a weakness, but on some machines it's a mechanical fuse: the belt slips before a gearbox or motor mount lets go.

A timing belt has teeth that mesh with the pulley. No slip. Shaft positions are locked together, which is what you need when something has to stay synchronized—engine timing, a 3D printer motion stage, or a CNC axis. Gates PowerGrip is the name you'll see most often on timing belts; Gates V-belts cover the friction-drive side of the family.

Don't learn the difference the way I did. In January 2022 I used a V-belt on a small automation rig because it was "the right length" in the drawer. The stage drifted every time the motor changed speed. Right length, wrong product family.

How do I read a Gates V-belt size chart?

Gates V-belts by size follow a pattern that's easier than people think. The letter is the cross-section: A, B, C, D, E for classical belts; 3V, 5V, 8V for wedge belts; SPZ, SPA, SPB, SPC for metric narrow-section belts. The number is the effective length in inches for classical belts, and in tenths of inches for wedge belts, so a 3V475 is 47.5 inches long.

An "X" in the name means the belt is cogged—notched on the underside so it can wrap smaller sheaves and runs cooler doing it. An AX21 is an A-section, cogged, 21-inch effective-length belt. A BX21 is a wider and thicker section at the same length, and it won't fit an A-section sheave no matter how hard you push.

Here's where I've seen people go wrong, including me once: they measure the old belt with a cloth tape, then pick whatever number looks closest on the chart. The industry standard—and the Gates V-belt size chart uses it—is effective length, measured around the sheave grooves at a set tension, not the outside circumference of a belt hanging on a hook. Use the chart, not the tape.

What does "AX21" mean on a cogged V-belt like the Dayton AX21?

"AX21" decodes as: A-section, cogged (that's the X), 21-inch effective length. The A-section is the classical V-belt profile at a nominal top width of 1/2 inch and a height of 5/16 inch. The X is not a brand suffix or a "heavy duty" tag—the underside of the belt is notched, which lets it wrap tight-radius sheaves and shed heat.

A Dayton AX21 is an aftermarket example of exactly this spec. The Gates equivalent is an A-section cogged belt at 21 inches effective length—in the Gates catalog, that's the Tri-Power line. Same fit, same function. Just don't order the solid A21 by mistake if the original was cogged. A solid belt will fit the same sheave, but on a tight-radius drive it runs hotter and fails earlier.

Can I cross-reference Dayton or Aisin part numbers to Gates?

Yes—and that's a standard part of purchasing, not a sketchy workaround. Gates publishes cross-reference data for most aftermarket belt and kit numbers, and the cross-reference tool on the website does the lookup in a couple of minutes.

A Dayton AX21 maps to the Gates A-section cogged belt at 21 inches. An Aisin timing belt kit maps to a Gates PowerGrip kit, with the tensioner included where the original kit had one. And to be clear: this isn't a "Dayton is bad, Gates is good" article. Dayton and Aisin make solid parts. Cross-referencing is about fleet consistency, lead times, and which distributor has your account.

But treat the cross-reference as a starting point, not a promise. I have mixed feelings about these tools. On one hand, they cut our lookup time from fifteen minutes with a paper catalog to about two. On the other, I've been burned twice by assuming they're current—once with a belt that was 1/8 inch off the expected length, once with a timing kit that didn't include a gasket the original had. The rule now: cross-reference to narrow it down, then confirm the section, the effective length, and whether the original was cogged. It took me a few years and some returned freight to make that a habit, but it's cut our wrong-order rate to almost zero.

What belt do I need for a micro stepper motor?

For a micro stepper motor—NEMA 8, NEMA 11, NEMA 14, or the very common NEMA 17—the answer is usually a timing belt, not a V-belt. Small motion-control builds like 3D printers, desktop CNCs, and pick-and-place rigs run on Gates PowerGrip GT2 or GT3 belts. The common sizes are 2 mm or 3 mm pitch, 6 mm or 10 mm wide.

The motor shaft on a NEMA 17 is typically 5 mm, so the toothed pulley needs a 5 mm bore with a set screw that actually bites. And the belt pitch has to match the pulley's tooth profile exactly. GT2 teeth and GT3 pulleys do not play nice together—the profiles are different, and you'll hear it before you see it. I'm not 100% sure why the hobby world standardized on 2 mm-pitch GT2 belts for so long, but it did.

The mistake I made in 2020 was grabbing "a belt that's close to the length" instead of reading the pitch and tooth count. Small timing belts don't have the forgiveness V-belts have. A few millimeters too long and it skips teeth; too short and you're riding the tension limit. Measure the center distance, or count teeth and use the belt length calculator, then order to the millimeter.

What size is an LM8LUU linear bearing?

LM8LUU is a metric linear ball bearing, and the size is simple once you know how to read the name:

• Bore (shaft diameter): 8 mm
• Outside diameter: 15 mm
• Length: 45 mm

The "8" is the shaft size—it rides on an 8 mm hardened shaft. "UU" means it's a metric ball-type linear bushing. The "L" means long: the standard LM8UU is only 24 mm long. The extra 21 mm of length is the whole point in a carriage that gets any cantilever load; it stops the bearing from rocking and the carriage from wobbling.

Why does this belong in a belt article? Because LM8LUU bearings live in the same machines that use GT2 belts and micro stepper motors—the belt-driven axes of 3D printers and small CNC routers. If your drawing says LM8LUU, don't "save" by ordering LM8UU. I made that substitution once, trying to be efficient, and the carriage shook enough to ruin print quality. We swapped in the long bearings and the same machine held tolerance all day. Our checklist now says it best: L is for length, not luxury.

Should I measure the old belt or trust the number printed on it?

This is the question people don't think to ask, and in my opinion it's the one that's cost me the most money.

Trust the number printed on the old belt if you can still read it. Don't trust a cloth tape measure wrapped around the outside of that same belt. A V-belt wears in service: the top edge rounds, the belt sits deeper in the sheave, and the effective length shifts. The number printed on the side is the effective length it was built to—that's your ground truth.

If the number is worn off, measure the pulley diameters and center distance, then use the size chart to get the effective length. A cloth tape is fine as a rough idea, but a rough idea is how I once ordered a batch of AX21s that were each about half an inch too long to seat properly. The belts went back, we lost a week, and the lesson stuck: codes over measurements.

One more rule belongs right next to it: if the drive uses two or more V-belts as a matched set—say, two drives with four belts each—replace the whole set together. A new belt running next to a stretched old one means the new belt takes the load and the old ones just ride along. That was the September 2022 incident that pushed my personal mistake budget past $4,000. The checklist now says it in bold: matched drives get matched sets, always.