May 2013 Machining Tip



Toughen Up Your Parts with Helical Thread Inserts

Helical thread inserts serve two primary functions. One is the replacement of stripped or damaged female threads to allow the use of original size bolts, sparkplugs, etc. The other is the creation of robust threads in parts that might be vulnerable to stripping if they were directly threaded. The helical thread insert itself is a tightly wound coil of diamond shaped steel wire; the shape of the wire forms threads on the inside and outside of the coil. The inside thread matches the thread you want to replace, while the outside thread holds the coil in place in its new home.

Figure 1: Coil insert

Figure 1: Coil insert


A typical replacement application occurs when a spark plug has been cross threaded in an engine’s cylinder head. To replace the damaged threads instead of having to replace the head, a mechanic will:


  • Select an insert with an inside thread matching that of the sparkplug.
  • Drill out the damaged threads in the cylinder head.
  • Use a tap to thread the enlarged hole to match the outside thread of the insert.
  • Screw the insert into place with a special tool, leaving a new inside thread identical to the damaged one.

In manufacturing applications, helical thread inserts are used to create a strong steel thread in a softer material like aluminium, brass, or plastic. This accomplishes two goals. First, while the insert itself is threaded into the softer material just as a bolt would be, it has a significantly larger diameter thread. This spreads the force exerted on that thread over a larger area, reducing or eliminating the likelihood of failure. At the same time, insertion of the bolt can actually expand the coil insert outward, increasing its grip on the surrounding boss.


Second, a steel thread insert supports disassembly and reassembly better than a softer material can. A steel screw or bolt can be torqued tightly each time it is inserted in a steel insert without damaging threads. Softer material like plastic or aluminium may deform when steel screws are inserted and torqued, so they may not hold as well on subsequent insertions as they did the first time. Designers should be particularly careful, in non-permanent connections, to avoid the use of self-tapping screws. These are meant for one-time assembly and by cutting their own threads assure a tight fit. If they are reinstalled into existing threads they cannot achieve the same tight fit. In fact, because they are designed to cut threads they present a greater risk of cross-threading upon reinstallation. For all these reasons, helical thread inserts are especially useful for non-permanent connections—removable covers for example—in machined plastic or soft metal.


Firstcut does not install coil inserts, but we can thread machined holes to accommodate customer-installed coil inserts in any Firstcut material.


You do not have to specify a coil insert or thread; all you have to do is present a model to FirstQuote with a machinable hole and FirstQuote will respond with threading options for that hole which may include:


  • Threads for bolt(s) that will directly fit that hole.
  • Threads for bolt(s) sized to fit coil inserts that fit that hole. Obviously, bolts sized to fit a coil insert will be smaller than bolts sized to directly fit the same hole.

All holes, including those designed for coil inserts, must be parallel to any of three cardinal axes.

Figure 2: Threaded holes example from FirstQuote interactive quote.

Figure 2: Threaded holes example from FirstQuote interactive quote.


If you have a specific bolt size you want to use and want to create a hole that will accommodate the appropriate coil insert, consult Table 2 on the Firstcut Threaded Holes web page. Find the UNC, UNF, or metric bolt you want to use in the left column and model a pilot hole whose diameter falls between the specified minimum and maximum diameters for that thread. FirstQuote will then offer you the option of having that hole threaded for the appropriate coil insert.


Pilot holes for coil inserts follow the same requirements as holes to be directly threaded, which can be found on the Firstcut Threaded Holes web page. For assistance, contact the Proto Labs Customer Service Engineering team at +44 (0) 1952 683047