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Spiro

PSM SPIRO® inserts are engineered to meet the challenges posed by hard, brittle, or thermosetting plastics. Traditional knurled or press-in inserts can generate too much radial stress in these materials, risking cracks or failure. The SPIRO® design addresses this by using a sharp, precision knurl pattern that cuts gently into the substrate, reducing stress and allowing thinner boss walls without compromising pull-out strength or torque resistance.

Advantages

Product Features

Sharp precision knurl pattern enables clean cutting into hard, brittle plastics, keeping radial stress low and reducing cracking risk.
Low bursting stress allows designers to use thinner walls around the boss.
High torque resistance provides strong threaded performance under twisting loads.
Self-aligning geometry helps the insert settle squarely during installation, aiding consistent seating and alignment.
Press-in installation requires only a controlled squeeze action, with no need for complex tooling or high energy.

Selection of Spiro Insert or Stud

ISO Metric

Unified

Specifying an Insert & Stud

Boss Design Specification

Hole Preparation

Holes for Spiro inserts should be molded to remove the danger of drill induced stresses. The taper on a molded hole should be 1° inclusive and the hole diameter recommended should apply at the point reached by the bottom of the insert. The top of the hole should not be chamfered or counterbored and care must be taken to avoid bell mouthing. Hole diameter tolerance: -0.00 +0.10mm.

Installation

The insert must be installed using a squeeze action press, NEVER a hammer blow. The insert must be allowed to rotate in the direction of the knurl during installation. This is best achieved by the use of a punch having either a polished face or a thrust bearing. The insert must be kept axially square during installation, as any tilting will induce side loads on the boss wall. The recommended hole size must not be increased beyond the top tolerance limit since oversize holes reduce or remove the self-aligning effects, producing side loads and consequent risk of boss cracking.

Wall Thickness

A general guide to minimum wall thickness is given in the data table but this will vary depending upon the nature of the plastic. Where thinner walls are required these can often be accommodated, but consultation with the P.S.M Technology Centre or local sales office and pre-production testing is strongly advised.

Performance Data

The complexity of materials and variations in service conditions make it impossible to detail fastener performance for specific applications. The chart below gives a general guide and show the relative performance of the inserts in the range.

Enhance Your Assembly with Our Precision Spiro Inserts

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Frequently Asked Questions About Spiro Inserts

What are Spiro Inserts used for?

Spiro inserts (SP, HSR, SPTS, SPHS) are threaded brass inserts designed for hard, brittle thermoset plastics. They give thermoset components strong, reusable metal threads that the plastic itself can’t reliably hold, which is essential in parts that get assembled and disassembled repeatedly.

What's the difference between SP, HSR, SPTS, and SPHS Spiro Inserts?

SP is the standard insert version. HSR is the flanged (“headed”) version, which adds a head/flange for surface bearing and load distribution. SPTS is the stud version (external thread for mating into a captured nut or fixture), and SPHS is the flanged stud variant. All four share the same knurl design and material, differing mainly in geometry and mounting style.

Why use a molded hole instead of a drilled hole for Spiro Inserts?

Drilling introduces stress concentrations in the plastic that increase the risk of cracking. PSM recommends molding the hole directly so the boss is free of drill-induced stress before the insert ever goes in.

What hole tolerance do Spiro Inserts require?

The recommended hole should have a slight taper — no more than 1° inclusive — measured at the depth the bottom of the insert reaches, with a diameter tolerance of -0.00/+0.10mm. The top of the hole should be left un-chamfered and uncounterbored to avoid bell-mouthing.

What makes Spiro Inserts suitable for thermoset plastics specifically?

Spiro’s knurl pattern is cut sharp and precise enough to bite into hard, brittle material without the high radial stress that causes thermosets to crack — letting designers use thinner boss walls than most competing insert designs allow.