5L to B Belt Conversion: Length Table + Safe Crossover Rules
A 5L belt and a B-section belt share the same 5/8" top width. That's where the "they're interchangeable" myth starts, and it's why half the maintenance techs who swap one for the other end up replacing the new belt six months later wondering what went wrong.
They're not interchangeable. They're sometimes substitutable, under specific conditions. Here's the actual breakdown.
What 5L and B belts have in common
Both use a 5/8" nominal top width. Both sit in the same sheave groove angle. In isolation, a 5L belt will physically fit a B-section pulley and vice versa.
That's the entire overlap.
Where they differ, and why it matters
| Specification | 5L (FHP) | B-Section (Industrial) |
|---|---|---|
| Top width | 5/8" | 5/8" |
| Construction | Lighter cord, thinner rubber | Heavy cord, full reinforcement |
| Power capacity | Light/fractional HP | Full industrial HP |
| Min pulley diameter | ~2.5" | ~3.8" |
| Operating temperature | Lower range | Higher range |
| Duty cycle | Intermittent / light commercial | Continuous duty |
Power capacity is the difference most maintenance teams notice first. 5L belts are Fractional Horsepower construction — lighter cord, thinner rubber compounds, less reinforcement. Put a 5L on a drive designed for a B and you'll see premature stretch, slippage, and failure well before expected service life.
Minimum pulley diameter is the difference most people miss. B-section belts need pulleys of at least ~3.8 inches for reliable service. 5L belts can run on pulleys as small as 2.5 inches because the lighter construction flexes more freely around tight bends. Go the other direction — a B belt on a small pulley designed for a 5L — and the belt will overheat, crack, and fail early. This is the exact scenario Dayco's engineering team warns against in their industrial belt guidance.
Temperature and duty cycle matter for outdoor or continuously-running equipment. 5L belts are designed for residential and light commercial duty cycles, which means they cool between runs. B-section belts handle higher operating temperatures and continuous loads.
When you can substitute (and when you can't)
The substitution works in one direction only: 5L → B, under specific conditions.
| Scenario | Substitute? |
|---|---|
| B-section drive running at 30% of rated load → 5L belt | ✓ Usually fine |
| B-section drive running near rated load → 5L belt | ✗ Belt fails in weeks |
| 5L (FHP) drive → B belt | ✗ Almost never works (small pulleys cook the belt) |
| Outdoor equipment, temperature swings | ✗ Stay with rated spec |
| Safety-critical applications | ✗ Stay with rated spec |
Use the spec the equipment was designed for. It exists for a reason.
The length conversion
5L belts are numbered by outside circumference in tenths of an inch. A 5L320 is 32.0 inches outside.
B belts are numbered by inside circumference in inches. A B29 is 29 inches inside, roughly 32 inches outside.
The conversion formula:
Subtract 3 from (5L number ÷ 10) to get the approximate B number.
A 5L320 → roughly B29. A 5L460 → roughly B43. A 5L680 → roughly B65.
This is approximate. Always confirm the outside circumference matches within about 1/4 inch. A belt that's too short won't tension properly. A belt that's too long will slip.
Common 5L sizes and their B equivalents
| 5L Size | Outside Circumference | Approximate B Equivalent |
|---|---|---|
| 5L320 | 32.0" | B29 |
| 5L360 | 36.0" | B33 |
| 5L420 | 42.0" | B39 |
| 5L460 | 46.0" | B43 |
| 5L510 | 51.0" | B48 |
| 5L560 | 56.0" | B53 |
| 5L620 | 62.0" | B59 |
| 5L680 | 68.0" | B65 |
| 5L720 | 72.0" | B69 |
Always measure the belt you're replacing. Part number stickers degrade. The sticker on a belt that's been running for three years might not match what's actually on the drive.
What to actually do
If you need a direct replacement, get a 5L in the same length.
If you're considering a B-section upgrade for higher capacity, verify three things first:
- The pulleys are large enough for B-section construction (minimum 3.8 inches)
- The drive's horsepower rating justifies the heavier belt
- The outside circumference of the B belt matches your existing 5L within 1/4 inch
If any of those three fails, stay with the 5L.
For 4L to A-section conversions, the same logic applies with different dimensions. Our guide on 4L to A-section belt conversion covers those details.
Cross-reference specific 5L sizes
Partmatch has verified cross-references for 5L belts across Gates, Continental, Dayco, Bando, and Optibelt. If you need equivalents for a specific 5L number, search directly: 5L320, 5L360, 5L460, or search any 5L size on the main search page.