To measure a snap ring, you need three key dimensions: the free diameter, the ring thickness, and the groove size it fits into. Which diameter you measure depends on whether the ring is internal (fits inside a bore) or external (fits around a shaft), and getting this wrong is the most common mistake people make when ordering replacements.
Internal vs. External: Which Diameter Matters
Snap rings come in two basic types, and each one is sized by a different diameter. For an external snap ring (the kind that clips around a shaft), the critical measurement is the inner diameter, meaning the opening in the center of the ring. For an internal snap ring (the kind that sits inside a housing or bore), you measure the outer diameter, meaning the widest point across the outside of the ring.
This measurement is called the “free diameter,” and it refers to the ring in its relaxed, uninstalled state. It’s the natural size of the ring when it’s just sitting on your workbench, not compressed or expanded to fit anything.
Measure Before Installation, Not After
Free diameter can only be measured accurately on a ring that has never been installed. Once an external ring’s lugs have been expanded or an internal ring’s lugs have been compressed during installation, the ring may not fully return to its original shape. Measuring a used ring will often give you a slightly false reading, which can lead to ordering the wrong size.
If you’re working with a used ring, measure the shaft or bore it came from instead. The nominal size of a snap ring matches the shaft or bore diameter directly. An external snap ring designed for a 12 mm shaft is simply called a 12 mm ring. So if you measure the shaft at 12 mm, you need a 12 mm external snap ring.
How to Take Each Measurement
Free Diameter
For standard snap rings (sometimes called C-clips or C-type retaining rings), use a caliper to measure across the widest or narrowest point of the ring, depending on type. Place the caliper jaws on opposite sides of the ring and read the measurement. One important exception: E-clips (the kind with three prongs) should not be measured with a caliper. The correct method for E-clips is a pin gauge that makes three-point contact with the ring’s prongs, since caliper jaws can’t reach the true inner diameter accurately.
Inverted snap rings (where the lugs face inward instead of outward) also require a different approach, because the lug positions interfere with a straightforward diameter reading. If you’re dealing with an inverted ring, check the manufacturer’s catalog for the specific measurement method for that series.
Ring Thickness
Use a caliper or micrometer to measure the thickness of the flat material the ring is stamped from. Place the jaws on the flat faces of the ring, not on the lug tips. Thickness values are precise, often specified to fractions of a millimeter with tight tolerances. A ring listed at 1.6 mm thick, for example, may actually be manufactured at 1.5 mm depending on the standard being followed, so checking the exact thickness matters when you’re matching a replacement.
Groove Dimensions
If you have access to the groove the ring sits in, measure three things: the groove diameter, groove width, and groove depth. The groove diameter is usually slightly larger than the ring’s free diameter for external rings, and slightly smaller for internal rings. This offset is what allows the ring to snap into the groove and stay seated. Use a caliper for the groove width (the distance between the two walls of the channel) and a depth gauge or the depth rod on your caliper for groove depth.
The Three Numbers You Need for a Replacement
When you’re trying to order a replacement snap ring without a part number, you need at minimum:
- Free diameter (or the shaft/bore diameter if the ring has been used)
- Ring thickness
- Groove dimensions (diameter, width, and depth)
Knowing whether the ring is internal or external is equally important, since two rings with identical diameters will have completely different shapes and functions depending on type. You should also note the material if possible. Snap rings come in various steels and alloys, each offering different strength and corrosion resistance. A stainless steel ring in a marine application can’t be swapped for plain carbon steel without risking early failure.
Matching to a Standard Size
Snap rings follow standard sizing charts where the nominal size corresponds directly to the shaft or bore diameter in whole millimeters (metric) or fractional inches (imperial). A 10 mm nominal ring fits a 10 mm shaft. A 20 mm nominal ring fits a 20 mm bore. This one-to-one relationship holds across most standard DIN and ANSI ring series, which makes identification straightforward once you know the shaft or bore size.
If your measurements fall between standard sizes, or if the ring has an unusual cross-section, you may be dealing with a proprietary or specialty ring. In that case, the manufacturer’s catalog for the equipment it came from is your best resource, since aftermarket suppliers won’t always carry non-standard sizes.