The numerical suffix of KATO, NAS, MS, or NA, part numbers for helical coil inserts is the designation of the insert length. The insert lengths are designated in Nominal Diameter increments. Let’s look at the following examples:
The reason the length is designated as a multiple of the Nominal Diameter, and not a simple measurement, is due to the fluctuating dimensions of Screw Thread Inserts from the free state to the installed state. Screw Thread Inserts in the free state have a larger outer diameter and a shorter overall length than in the installed state. When the insert’s outer diameter is compressed into the threaded hole, the insert naturally elongates. As a result, the only constant is the Basic size designation.
The process for confirming the identity of an insert is a simple one. The only two dimensions needed are the Free Outer Diameter and the number of Free Coils. If using a caliper to measure the Free Outer Diameter, do not measure at the locking coil or constrict the insert as either can lead to a false reading.
Next, compare these values to the Insert Specification page. The Free Outer Diameter will need to fall within the range specified. The number of Free Coils will need to match to one of the lengths listed (±.25 coil).
To identify the number of Free Coils of any Tangless insert, count the number of coils in the free state from one notch to the other notch. When identifying a unified size tanged insert, start counting 90° from the tang. When identifying a metric size tanged insert, start counting from the tang break off notch.
The origin for the length of the insert is based on the absolute minimum amount of material needed. If, and this is a big IF, there is no countersink, a through hole, full threads throughout, and the insert is installed to only 0.25P (minimum based on NASM33537) below the surface, the Nominal Length of the insert is the absolute minimum thickness of the parent material required. If the Nominal Length of the insert is, for example 0.250, the minimum material thickness needed is 0.250 inches. This allows the installed insert to be 0.25P below the surface, while giving 0.25P support on the bottom thread. This is why the Nominal Length is a calculated value and cannot be measured. It is the Installed Length plus 0.5P.
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