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Crimp connections hold wire harnesses together in everything from industrial control cabinets packed with terminal blocks to household appliances. They are also the least inspected part of most assemblies. A terminal that looks fine to the naked eye can hide a loose wire barrel or strands damaged during crimping, and that joint may heat up under load. That is why established harness shops give crimp inspection a fixed place in the production process.
The checks below cover visual inspection at the bench, crimp height measurement, and pull testing. Use them for incoming inspection of bought harnesses or for auditing your own crimping process. They give you a repeatable way to separate good crimps from defective ones. We also describe how we run these checks in our own shop.
Why Crimp Quality Matters
A crimp is a mechanical connection, not a fusion of materials. The terminal barrel deforms around the conductor and holds the wire in place. If that deformation is wrong, the connection can pass a quick bench test today and fail months later under vibration, heat, or repeated flexing.
The failure modes are well known. A loose crimp raises electrical resistance at the joint, which generates heat under load and eventually damages insulation or the terminal itself. An over-crimped joint can crack strands or the barrel, and resistance climbs the same way. Strands folded back or left out of the barrel reduce the effective cross section of the connection.
Most crimp defects are invisible until the connection is stressed, so inspection is the only practical defense. UL 486A and 486B both include dimensional checks and pull test requirements for this reason. Verify the crimp before it goes into service.
What a Good Crimp Looks Like
Before you measure anything, know what a correct crimp looks like. On an open barrel terminal, a good crimp is symmetrical, with the wire barrel formed cleanly around the conductor. The strands sit fully inside the barrel with nothing sticking out, and the insulation barrel grips the jacket without cutting into it.
Look at the seam. On most open barrel terminals it should be closed or nearly closed, with no sharp edges. The transition between the wire barrel and the insulation barrel should be smooth, and the wire should emerge from the front with all strands parallel and unbroken.
The conductor should be visible at the front of the wire barrel, a condition often called the bell mouth or funnel entry. A slight flare is normal and helps the wire enter. A missing bell mouth usually means the wire was pushed in too far or the tool setting was wrong. When the crimp does not match the reference photos in your terminal datasheet, stop and investigate.
Crimp Height and Tools
Crimp height is the most useful dimensional check for a crimped connection. It is the measured thickness of the wire barrel after crimping, taken with the terminal held in a specific orientation. Because the barrel is compressed to a controlled dimension, crimp height shows how tightly the terminal grips the conductor.
The measuring tool is a micrometer with a special anvil designed for crimp height, sometimes called a crimp height gauge. A flat anvil micrometer gives inconsistent readings on a rounded barrel, so shops use the dedicated tool.
Target values depend on the terminal and wire size, so check the datasheet. There is no universal crimp height number that works for every terminal, wire gauge, and plating combination. The manufacturer publishes the specification, and your job is to hit it within the stated tolerance. Verify the setting at setup, record the measurements, and recheck periodically as the tooling wears. It is the single most useful check in a crimp inspection program.
The Visual Inspection Checklist
Visual inspection is fast and cheap, and it catches most common defects before they reach a tester. For production runs, 100 percent visual inspection of every crimp is a realistic and common practice, especially on critical wires. Here is the crimp inspection checklist we use in our shop.
| Inspection Point | What to Look For | Reject If |
| Wire barrel formation | Symmetrical, closed or nearly closed seam | Asymmetrical, open seam, burrs, or sharp edges |
| Strand condition | All strands contained, parallel, and unbroken | Strands cut, nicked, folded back, or sticking out |
| Insulation barrel | Grips the jacket evenly without crushing it | Insulation cut, crushed, or not captured |
| Insulation clearance | Wire jacket ends cleanly at the barrel mouth | Insulation caught inside the wire barrel |
| Bell mouth | Slight flare at the front of the wire barrel | No flare, or the barrel distorted |
| Wire position | Conductor visible at the barrel front | Wire too short or past the barrel |
| Crimp height | Within the datasheet tolerance | Above or below the specified range |
| Terminal condition | No cracks, plating intact | Cracks, discoloration, or damaged plating |
Pull Testing and Sample Plans
Pull testing is the destructive check that proves the crimp actually holds. The test pulls the wire from the terminal in a straight line and records the force at which the wire breaks or pulls free. A good crimp fails by the wire breaking, not by the terminal letting go. When the terminal releases the wire at low force, the crimp was defective even if it looked fine.
Pull tests are destructive, so they run on samples rather than on every part. The common practice is a sampling plan tied to production lots: pull samples at the start of a run, at intervals during the run, and after any tooling change or maintenance. Sample size and frequency should follow your quality system or customer requirements, and results should be recorded so you can spot trends. Some shops measure crimp height on every pull test sample before pulling it, giving two data points on the same part.
Common Crimp Defects to Reject
Knowing what to reject is half the job. Here are the defects we see most often, and why each one matters.
Insulation in the wire barrel. When the jacket gets crimped into the wire barrel, the terminal grips insulation instead of copper. The connection may hold for a while, but resistance is high and the joint overheats under load.
Damaged or missing strands. A strand folded back outside the barrel, or nicked by a misaligned tool, reduces the conductor cross section. It is a classic cause of intermittent failures in the field.
Over-crimping. Compressing the barrel too far can crack the terminal or sever strands inside. The crimp looks tight, so it is easy to miss without a crimp height measurement.
Under-crimping. The barrel is not compressed enough, so the wire can slide out under tension. Pull testing is the check that catches it.
Whiskers. Fine wire strands poking out through the seam can short against adjacent terminals in a dense connector housing. Reject any crimp with whiskers, and check your tooling alignment.
How We Inspect Crimps in Our Harness Shop
At LANZ, crimp quality is built into the process instead of being checked only at the end. Every crimping station in our wire harness shop uses the terminal manufacturer’s recommended tooling and die sets, and operators verify the setup against the datasheet before the first part is made. Crimp height is measured on setup samples and recorded for every job.
For production, our operators do 100 percent visual inspection of each crimped connection as the harness is assembled. Our QC team also pulls samples from each lot for crimp height measurement and pull testing, with results logged so any problem can be traced to the exact run and machine. Terminal crimping is a process that drifts, so the checks continue through the run rather than happening once.
This is why we ask customers to share their crimp specifications and any special requirements up front. When we build wire harnesses for control panels, appliances, or power distribution, we want the inspection criteria agreed before production starts. A final audit is the wrong place to discover them. That conversation saves everyone time.
Partner With LANZ for Crimped Harnesses You Can Trust
When you order wire harnesses from LANZ, every crimp is inspected before the harnesses leave our shop, and we can provide crimp inspection documentation on request. We build IEC power cords, cable assemblies, and custom wire harnesses, and terminal crimping is measured and verified on every job.
Frequently Asked Questions
How often should crimp height be checked?
Crimp height should be verified at setup, whenever the tooling changes, and at planned intervals during the run. The exact frequency depends on your quality system and customer requirements.
What pull test force is acceptable?
There is no single number that applies to every terminal. The acceptable pull force depends on the terminal design, wire gauge, and crimp specifications; the terminal manufacturer or the relevant standard provides the value. The general expectation is that the wire breaks before the crimp releases.
Can a bad crimp pass a continuity test?
Yes, and that is exactly why visual checks and pull testing exist. Continuity only proves that current can flow; it does not prove the connection will hold under load, vibration, or thermal cycling. A crimp with half the strands missing can pass a continuity test and still fail in service.
Send us your drawings or terminal list, and we will confirm the crimp specifications before we cut a single wire. Visit lanzmfg.com or contact our team to discuss your next harness project.
