Heli-Coils

Heli-Coil^® Installation

What is a Heli-Coil threaded insert?

Heli-Coil® inserts are precision formed screw thread coils of stainless steel wire having a diamond shaped cross section and a good surface finish. When they are installed into Heli-Coil® tapped holes, they provide a permanent conventional 60deg internal screw thread that accommodates any standard bolt or screw. Because the Heli-Coil insert is larger in diameter than the tapped hole the insert when it is in place locks tight. To achieve the installation, the insert reduces in diameter while being screwed into place and expands as soon as the pressure is released on the installation tool. The product was invented in the late 1930's to provide a solution for the repair of aircraft engines. The stainless steel insert can withstand high temperatures and so is useful for repairing sparkplug holes.

There are two types:- Free-Running and Screw-Lock. The standard being the free-running thread is the type we would use to repair motorcycle threads. Available in various sizes in BSC, BSF, BSW, UNF, UNC and in metric coarse and fine threads. Lengths of inserts are referred to in diameters, therefore a 1.5D 1/4"-26 insert is 3/8" long. I have constructed a table at the bottom for British Classic Bike Threads.

Properly installed, a Heli-Coil insert will bring the hole back to its original thread size and has the added benefit that it gives a thread that is 5 times stronger than the original tapped hole in the parent material.

An unenviable task is to drill out a broken stud. It may be possible to drill a hole and use an easi-out to remove the old stud, but if it is ceased in place you will find it difficult to remove in this way. An alternative is to drill a series of concentric holes, each larger than the previous one to slowly remove the stud and leave a hole the right size to suit a Heli-coil and a new stud. The first step for either task is to prepare the surface for drilling. The surface must be smooth so the drill will go in straight. Saw off the old stud and file flat, as close to the case as possible, but be careful not to remove any metal from the case. Next center-punch the stud and then with a small drill, approx half the size of the stud, drill a pilot hole. Remember to cover up openings to engine cases if you are not stripping the engine down. If it is possible to put the case on a Press-Drill, you will get a much better result. It is possible to do it without, but extra care must be used. Usually there is a small gap bellow the stud, so gently drill until you break through. Measure the depth of similar hole on the case and use this as a guide. I wrap some electrical tape around the drill to mark the depth.
	Enlarge the hole using the recommended sized drill for the coil.
	Typical automotive Heli-Coil installation set. Comprises: Heli-Coil inserts, tap and installation tool.
	Apply lubricant to the tap and "cut" new threads into the hole. The lubricant not only helps the tap cut, but also helps with the removal of swarf which sticks to the tap flutes.
	Now the hole has new threads ready to accept the Heli-Coil insert.
	The insert is threaded onto the end of the installation tool. Note how the inserts "tang" is hooked against one of the raised lands of the tool end. Also note the notch opposite the tang to allow easy removal of the tang after installation.
	The insertion tool with insert is threaded into the hole. It is wound into the hole until the top coil is 1/4-1/2 turn into the tapped thread. (ie. Just below the surface)
	The insert is threaded into the hole until the end (red circle) is just below flush. Note the position of the tang (red arrow). Now insert a punch smaller than the hole and place the flat end of the punch on the bend of the tang (blue arrow) and hit it with a blow, shearing off the tang. Job done, and the thread is better than original.
Tips. When you install the new stud or bolt, you should only apply grease to the joint. DO NOT put on any thread lock as this will reduce the strength of the joint and can cause damage when removing the item. To removing wire thread inserts ideally you should use an ‘blade’extractor or sometimes an easy out type will do. You have to do the reverse of how you put them in, i.e. reduce the last coil and unwind properly. By just pulling out the thread you will damage the tapped hole and using a wire thread insert may not be possible anymore.
Credit to ATV Connection & WTI Fasteners for technical tips. UK Home Installer:- Kevin Moss UK Supplier:- UniThread & WTI Fasteners Heli-Coil^® is a registered trademark of the Emhart Corporation.

Available Heli-Coil Sizes for British Motorcycles
UNF	BSF	BSC (CYCLE)
LENGTHS 1D, 1.5D, 2D, 2.5D & 3D	LENGTHS 1D, 1.5D, 2D, 2.5D & 3D	LENGTHS 1D, 1.5D & 2D
# 4 x 48	3/16 x 32	1/4 X 26
# 6x 40	1/4 x 26	5/16 X 26
# 8 x 36	5/16 x 22	3/8 X 26
# 10 x 32	3/8 x 20	7/16 X 26
1/4 x 28	7/16 x 18	1/2 X 26
5/16 x 24	1/2 x 16
3/8 x 24	9/16 x 16
7/16 x 20	5/8 x 14
1/2 x 20	3/4 x 12
9/16 x 18	7/8 x 11
5/8 x 18	1 x 10
3/4 x 16
7/8 x 14
1 x 12
Inserts are supplied in various lengths. Typically a 1.5D means 1.5 x Diameter of the insert So a 1.5D for a 1/4" 26 insert x 1.5 = 3/8" long. Inserts may be stacked or cut short for different lengths.