How to measure for a circlip
The ring you pulled out is the least reliable thing to measure. Measure the feature it sits in, and the size falls out of the standard.
Why not just measure the old ring?
A used ring has been expanded or compressed at least twice, and may have taken load in between. Its free diameter has almost certainly moved. More importantly, its outside diameter is not the number the part is sold against — external rings are listed by shaft size, internal rings by bore size, so a calliper reading across an old ring maps onto nothing in the catalogue.
The old ring is still useful as evidence: it tells you the type, roughly the thickness, and whether the assembly used a plain or lugged ring. Use it to confirm what you measured, not to replace measuring.
What the standard gives you free
For a given nominal size, DIN 471, DIN 472 and DIN 6799 each fix the groove diameter, the groove width and the ring thickness. That is the point of a standard: measure one dimension accurately and the rest are determined.
So the job is not to measure everything. It is to measure the one governing dimension well, then check a second dimension as a sanity test that you have identified the right standard.
Measuring for a replacement circlip
Work on a clean, dry assembly with the retained component removed if you can. Five minutes with a vernier beats a return.
What you need
- Vernier or digital callipers
- A clean rag and degreaser
- The old ring, if you have it
- The assembly drawing, if one exists
Identify which type you are replacing
Look at where the groove is. A groove around the outside of a shaft means an external ring to DIN 471. A groove inside a housing or bore means an internal ring to DIN 472. A groove on a shaft you cannot reach the end of, retained by a clip that was pushed on from the side, means an E-clip to DIN 6799.
Clean the groove
Degrease and wipe the groove and the surrounding surface. Old grease, paint and corrosion products will each add a few tenths of a millimetre to a calliper reading, which is enough to put you into the next size up.
Measure the governing diameter
For an external ring, measure the shaft diameter next to the groove, not across the groove. For an internal ring, measure the bore diameter next to the groove. For an E-clip, measure across the bottom of the groove itself. Take three readings at different angles and use the mean — shafts wear oval.
Measure the groove width
Measure the width of the groove along the axis of the shaft or bore. This is your cross-check: the groove width fixed by the standard for the nominal size you just measured should match what you have in front of you. If it does not, you have either measured the wrong surface or the assembly does not use a standard ring.
Check the ring thickness
If you have the old ring, measure its thickness across the flat. It should sit a little under the groove width so the ring can seat without binding. A thickness that is nowhere near the standard value is a sign of a non-standard or imperial part.
Confirm against the standard before ordering
Take the nominal size you measured and check the groove diameter, groove width and thickness for that size against the applicable standard or your drawing. Order only once all three agree. Where they do not, send the measurements to a trade desk rather than guessing.
Common questions
Do I measure the shaft or the groove?
It depends on the standard. For DIN 471 external circlips, measure the shaft diameter beside the groove. For DIN 472 internal circlips, measure the bore diameter beside the groove. For DIN 6799 E-clips, measure across the bottom of the groove itself, because that is the dimension the clip is listed against.
What if the groove is worn or damaged?
Measure an undamaged section of the same groove if there is one, and cross-check the shaft or bore diameter, which wears far less. A groove that has widened enough to let a correctly sized ring move axially needs the component repaired or replaced — fitting a thicker ring to take up the slack is not a repair.
Can I identify a circlip from the old ring alone?
Partly. The old ring tells you the type and roughly the thickness, but not the size to order against, because rings are listed by the shaft or bore they fit rather than by their own diameter. Use it to confirm the type and then measure the assembly.
What tools do I need?
Vernier or digital callipers and something to clean the groove with. A micrometer helps for ring thickness but is not essential. Guessing from a steel rule is what produces the wrong size.
My measurement falls between two sizes — what now?
That normally means the surface is worn, dirty, or you are measuring an imperial part against a metric range. Re-clean and re-measure first. If it still falls between sizes, send the shaft or bore diameter, the groove width and the old ring thickness to a trade desk, which is enough to identify an imperial or obsolete reference.
Other guides
DIN 471, DIN 472 and DIN 6799 explained
Three standards cover almost every metric retaining ring you will meet. Picking the wrong one is usually a matter of measuring the wrong surface, not of choosing the wrong part.
Fitting and removing circlips safely
Most circlip failures are fitting failures. A ring that was over-expanded on the way in will not hold, however right the part number was.
Circlip materials and finishes
Carbon spring steel does almost every job. The cases where it does not are predictable, and they are worth knowing before the assembly is closed up.
Need it identified?
Send the dimensions you were able to measure, along with the quantity you need. Imperial sizes, obsolete references and parts identified from a drawing or a sample are handled through the trade desk.
Send a trade enquiry