How to Measure a Timing Belt for Replacement
How to Measure a Timing Belt for Replacement
When a timing belt part number is worn, missing, or illegible, you can identify the correct replacement by measuring four things: pitch, tooth profile, width, and pitch length. This guide covers each measurement step with the tools and techniques that work for every timing belt profile family, whether trapezoidal (XL, L, H) or curvilinear (HTD, GT).
If you have any part of the original part number visible, start there. Even a partial number can help us cross-reference the correct belt. Call 888-203-2358 or send us a photo of the belt and we can usually identify it within the hour.
1 Measure the Pitch
Pitch is the distance from one tooth center to the next, measured along the pitch line (the belt's neutral axis, not the tooth tips). It is the single most important measurement because it determines which pulley the belt fits.
How to measure:
- Lay the belt flat with the teeth facing up.
- Place a caliper or ruler across 10 consecutive teeth, measuring from the center of the first tooth to the center of the 11th tooth (10 pitch spans).
- Divide the total measurement by 10. This gives you the pitch.
Why 10 teeth? Measuring across 10 teeth and dividing reduces measurement error. A single-tooth measurement can be off by enough to confuse similar pitches (like XL at 5.08mm vs 5M at 5.00mm).
Timing Belt Pitch Reference
Match your measured pitch to this table to identify the profile family.
| Measured Pitch | Profile | Family | Collection |
|---|---|---|---|
| 0.200" (5.08mm) | XL | Trapezoidal | XL Timing Belts |
| 0.375" (9.525mm) | L | Trapezoidal | L Timing Belts |
| 0.500" (12.7mm) | H | Trapezoidal | H Timing Belts |
| 3mm | 3M | HTD / GT | 3M Timing Belts |
| 5mm | 5M | HTD / GT | 5M Timing Belts |
| 8mm | 8M | HTD / GT | 8M Timing Belts |
| 14mm | 14M | HTD / GT | 14M Timing Belts |
Watch out for XL vs 5M. XL measures 5.08mm and 5M measures 5.00mm. The pitches are only 0.08mm apart but the tooth shapes are completely different (trapezoidal vs curvilinear). If your measurement lands near 5mm, check the tooth shape (Step 2) before ordering. See our Tooth Profiles Explained guide for a visual comparison.
2 Identify the Tooth Profile
Once you know the pitch, confirm the tooth shape. This tells you whether the belt is trapezoidal, HTD, or GT, which determines pulley compatibility.
How to identify:
- Trapezoidal (straight-sided): The teeth have flat sides that angle inward like a trapezoid. This is the original timing belt design. Profiles: MXL, XL, L, H, XH, XXH. Part numbers use an imperial format like 200XL037 or 225L050.
- HTD curvilinear (rounded): The teeth have a smooth, rounded shape like a half-circle. Profiles: 3M, 5M, 8M, 14M. Part numbers use a metric format like 535-5M-25.
- GT / GT2 / GT3 (modified curvilinear): Similar to HTD but with a slightly flatter tooth root for better engagement. Visually very similar to HTD. Part numbers start with the pitch: 8MGT-640-36.
If you cannot tell the difference between HTD and GT by looking at the teeth, check the part number format. If the number starts with a pitch followed by "MGT" (like 8MGT-640-36), it is a GT belt. If it starts with a three-digit length followed by pitch (like 560-8M-20), it is standard HTD.
Trapezoidal and curvilinear belts are never interchangeable. Even if the pitch is similar, the tooth shapes are different and require different pulleys. An XL belt cannot run on a 5M pulley. An L belt cannot run on an 8M pulley. Always match the belt profile to the pulley profile.
3 Measure the Width
Width is the simplest measurement. Lay the belt flat and measure straight across from edge to edge using a caliper or ruler.
Where to find it in the part number:
- Trapezoidal (imperial): The last three digits are the width in thousandths of an inch. 200XL037 = 0.375" = 3/8". 225L100 = 1.000" = 1".
- HTD / GT (metric): The last number after the second dash is the width in millimeters. 535-5M-25 = 25mm. 8MGT-640-36 = 36mm.
Common Timing Belt Widths by Profile
| Profile | Standard Widths | Most Common |
|---|---|---|
| XL | 1/4" (025), 3/8" (037), 3/4" (075) | 3/8" (037) |
| L | 1/2" (050), 3/4" (075), 1" (100), 1-1/2" (150) | 1/2" (050), 3/4" (075) |
| H | 3/4" (075), 1" (100), 1-1/2" (150), 2" (200), 3" (300) | 1" (100), 1-1/2" (150) |
| 3M | 6mm, 9mm, 15mm | 9mm |
| 5M | 9mm, 15mm, 20mm, 25mm | 15mm, 25mm |
| 8M | 12mm, 20mm, 30mm, 50mm, 85mm | 20mm, 30mm |
| 14M | 40mm, 55mm, 85mm, 115mm, 170mm | 55mm, 85mm |
4 Determine the Pitch Length
Pitch length is the total length of the belt measured along the pitch line (the line running through the center of the tensile cords, not the outer surface or the tooth tips).
Three ways to determine pitch length:
- Read the part number. For trapezoidal belts, the digits before the profile letter are the pitch length in tenths of an inch: 225L050 = 22.5". For HTD/GT belts, the first or second number is the pitch length in millimeters: 535-5M-25 = 535mm, or 8MGT-640-36 = 640mm.
- Count teeth and multiply. Count the total number of teeth on the belt and multiply by the pitch. For example, 60 teeth x 0.375" (L pitch) = 22.5" pitch length. Or 107 teeth x 5mm (5M pitch) = 535mm pitch length.
- Wrap and measure. If the belt is intact, wrap a flexible tape or string around the outside of the belt (over the teeth) and measure the total circumference. This gives you the outside circumference, which is slightly longer than the pitch length. Subtract approximately one tooth height to estimate pitch length, or simply send us the measurement and tooth count and we will confirm the part number.
Counting teeth is the most reliable method when the part number is missing. Count every tooth on the belt, then multiply by the pitch. This gives the exact pitch length with no guesswork. If you are not sure of the pitch, send us the tooth count plus the 10-tooth span measurement and we will identify the belt.
Timing Belt Part Number Quick Decoder
If you can read part of the original part number, use this reference to decode it.
| Format | Example | How to Read |
|---|---|---|
| Trapezoidal | 200XL037 | 200 ÷ 10 = 20.0" pitch length, XL profile, 037 = 0.375" (3/8") wide |
| Trapezoidal | 225L050 | 225 ÷ 10 = 22.5" pitch length, L profile, 050 = 0.500" (1/2") wide |
| Trapezoidal | 510H100 | 510 ÷ 10 = 51.0" pitch length, H profile, 100 = 1.000" (1") wide |
| HTD | 535-5M-25 | 535mm pitch length, 5M (5mm) pitch, 25mm wide |
| HTD | 800-8M-20 | 800mm pitch length, 8M (8mm) pitch, 20mm wide |
| GT3 | 8MGT-640-36 | 8mm GT3 pitch, 640mm pitch length, 36mm wide |
| Poly Chain | 14MGT-1190-37 | 14mm GT pitch, 1190mm pitch length, 37mm wide |
What You Need to Tell Us
If you cannot fully identify the belt yourself, provide as much of the following as you can and we will confirm the correct replacement:
- Any visible part number (even partial)
- Photo of the belt teeth with a ruler or coin for scale
- 10-tooth span measurement (the single most useful measurement)
- Total tooth count
- Width measurement
- Equipment make and model (we can often look up the belt by machine)
- Pulley markings (pulleys often have the pitch stamped on them)
Send this information to our contact form, email sales@texasbelting.com, or call 888-203-2358. We cross-reference part numbers across Gates, Continental, Bando, Diesel Belting, and Megadyne and can match any manufacturer's number.
Frequently Asked Questions
What tools do I need to measure a timing belt?
A caliper (digital or dial) is ideal for accurate pitch and width measurements. A steel ruler works if a caliper is not available. For pitch length, a flexible measuring tape or a piece of string and a ruler will work. No special tools are required.
How do I tell the difference between a trapezoidal and HTD timing belt?
Look at the tooth shape. Trapezoidal teeth have flat, angled sides like a trapezoid. HTD teeth are rounded like a half-circle. If the teeth are rounded, the belt is HTD or GT curvilinear. If the teeth have straight sides, the belt is trapezoidal (XL, L, H, XH, or XXH). See our Tooth Profiles Explained guide for visual examples.
My belt measures about 5mm pitch. Is it XL or 5M?
Check the tooth shape. If the teeth are straight-sided (trapezoidal), it is an XL belt (5.08mm pitch). If the teeth are rounded (curvilinear), it is a 5M HTD belt (5.00mm pitch). They require different pulleys and are not interchangeable. Also check the part number format: XL uses 200XL037 format, 5M uses 535-5M-25 format.
Can I measure pitch length by wrapping a tape around the belt?
You can get a close estimate that way, but the outside circumference is slightly longer than the pitch length because you are measuring over the tooth tips rather than along the pitch line. The most accurate method is to count all the teeth and multiply by the pitch. If you wrap a tape, subtract approximately one tooth height from the measurement to approximate the pitch length.
What if I can only read part of the part number?
Send us whatever you can read. Even a partial number helps narrow down the belt. For example, if you can see "5M-25" you know it is a 5mm HTD belt, 25mm wide. We can cross-reference the rest from there. Call 888-203-2358 or use our contact form.
Can I identify a timing belt from the pulley?
Yes. Pulleys often have the pitch stamped or engraved on them (e.g., "8M," "14M," "XL," "L"). The pulley marking tells you the pitch and profile directly. Measure the belt width to confirm the remaining spec, and count belt teeth to get the pitch length.
Can Texas Belting identify my belt from a photo?
Yes. Send us a clear photo of the belt teeth with a ruler, coin, or other reference object in the frame for scale. Include a photo of any visible markings or part numbers. In most cases we can identify the belt and quote a replacement within the hour. Email sales@texasbelting.com or use our contact form.