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Maximum Length of Helicoil

 
Old 01-17-2019, 11:07 PM
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Joel 67
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Default Maximum Length of Helicoil

Not a Corvette question per se but I need to repair a stripped M14x1.5 bolt hole with a Heli-coil or similar thread insert. The length of the threaded part of the bolt is 40mm (a bit longer than 1.5"). Does anyone make a thread insert that is that long? Seems like most a less than 1"
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Old 01-17-2019, 11:11 PM
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HeliCoils come in lengths as a function of the diameter of the thread (D). In standard (inch) sizes, standard HeliCoil lengths are 1D, 1.5D, 2D, 2.5D and 3D. In metric, the standard sizes are 1D and 2D (your M14 is available in 2 lengths: 14 and 21 mm). If the parent material (the hole that the bolt is going into) is the same as the bolt, the bolt achieves full strength with a threaded grip length equal to the diameter of the bolt. Any threaded insert longer than the diameter of the bolt becomes overkill. 2X the diameter is considered the max needed for any application. Having a 40mm long thread for a 14 mm bolt is pointless, so no such insert is made.

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Last edited by lars; 01-18-2019 at 02:52 AM.
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Old 01-18-2019, 12:09 AM
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Originally Posted by lars View Post
HeliCoils come in lengths as a function of the diameter of the thread (D). In standard (inch) sizes, standard HeliCoil leghts are 1D, 1.5D, 2D, 2.5D and 3D. In metric, the standard sizes are 1D and 2D (your M14 is available in 2 lengths: 14 and 21 mm). If the parent material (the hole that the bolt is going into) is the same as the bolt, the bolt achieves full strength with a threaded grip length equal to the diameter of the bolt. Any threaded insert longer than the diameter of the bolt becomes overkill. 2X the diameter is considered the max needed for any application. Having a 40mm long thread for a 14 mm bolt is pointless, so no such insert is made.

Lars
Exactly, length of thread engagement at 1 D is the max strength achievable for an 80000 psi tensile strength bolt.
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Old 01-18-2019, 12:10 AM
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Originally Posted by lars View Post
HeliCoils come in lengths as a function of the diameter of the thread (D). In standard (inch) sizes, standard HeliCoil leghts are 1D, 1.5D, 2D, 2.5D and 3D. In metric, the standard sizes are 1D and 2D (your M14 is available in 2 lengths: 14 and 21 mm). If the parent material (the hole that the bolt is going into) is the same as the bolt, the bolt achieves full strength with a threaded grip length equal to the diameter of the bolt. Any threaded insert longer than the diameter of the bolt becomes overkill. 2X the diameter is considered the max needed for any application. Having a 40mm long thread for a 14 mm bolt is pointless, so no such insert is made.

Lars
Super helpful, thanks Lars! Giving me some piece of mind! To make things clearer to me, I hope you will indulge some additional questions.
This application would be for a brake caliper bolt. The original called for 104 ft/lb of torque. I can get a 28mm Time-Sert. If I used this, can I still torque to the recommended value safely?

Also, the caliper bolts are AL and the steering knuckle is AL. THe Time-Sert insert is presumably steel. Will I have galling issues with this? If so, should I use anti-sieze or is that a bad idea on something like a caliper bolt? Am I better off with blue Loctite?
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Old 01-18-2019, 12:57 AM
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In aluminum blocks, and other applications, 1 or 2d just won't cut it. They make extended length Helicoils for many threads in your case 14x1.5x42. The question is how bad do you want them. I only see them available in packages of 50. I have used extended length Helicoils in many repairs. LS and Cadillac 4100 head bolts are 2 such cases. If you used a standard Helicoil to repair those i seriously doubt they would hold torque specs. They do come with extra length inserts but they are a different size. In some cases I have put 2 in the hole for extra length but make sure if you can't use a full length in the second Helicoil you screw in shorten the first before you install it. The Helicoil doesn't have to be one continuous piece.


https://www.globalindustrial.com/p/s...-of-50-inserts

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Old 01-18-2019, 02:58 AM
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Originally Posted by Joel 67 View Post
I can get a 28mm Time-Sert. If I used this, can I still torque to the recommended value safely? Also, the caliper bolts are AL and the steering knuckle is AL. THe Time-Sert insert is presumably steel. Will I have galling issues with this? If so, should I use anti-sieze or is that a bad idea on something like a caliper bolt? Am I better off with blue Loctite?
I am not familiar with the "Time-Sert" insert - it is not used in the aerospace business, that I am aware of. Galling issues normally occur with stainless-on-stainless applications. Aluminum on SS or carbon steel does not normally have galling issues. However, any threaded fastener should be lubricated for proper installation and torque - this is normal assembly practice as mandated by any of the governing MIL-SPEC documents for hardware assembly. Full torque values can be obtained at the diameter-to-length values noted in my previous post.

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Old 01-18-2019, 08:09 AM
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Originally Posted by lars View Post
HeliCoils come in lengths as a function of the diameter of the thread (D). In standard (inch) sizes, standard HeliCoil lengths are 1D, 1.5D, 2D, 2.5D and 3D. In metric, the standard sizes are 1D and 2D (your M14 is available in 2 lengths: 14 and 21 mm). If the parent material (the hole that the bolt is going into) is the same as the bolt, the bolt achieves full strength with a threaded grip length equal to the diameter of the bolt. Any threaded insert longer than the diameter of the bolt becomes overkill. 2X the diameter is considered the max needed for any application. Having a 40mm long thread for a 14 mm bolt is pointless, so no such insert is made.

Lars
Originally Posted by Robert61 View Post
In aluminum blocks, and other applications, 1 or 2d just won't cut it. They make extended length Helicoils for many threads in your case 14x1.5x42. The question is how bad do you want them. I only see them available in packages of 50. I have used extended length Helicoils in many repairs. LS and Cadillac 4100 head bolts are 2 such cases. If you used a standard Helicoil to repair those i seriously doubt they would hold torque specs. They do come with extra length inserts but they are a different size. In some cases I have put 2 in the hole for extra length but make sure if you can't use a full length in the second Helicoil you screw in shorten the first before you install it. The Helicoil doesn't have to be one continuous piece.


https://www.globalindustrial.com/p/s...-of-50-inserts
The torque spec for this is 104 ft/lbs. Not sure what a 28MM insert is rated to however. Anyone know?
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Old 01-18-2019, 11:04 AM
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The Time Sert or Helicoil will handle the torque. However your failure is not the result of the thread repair. Aluminum doesn't have the same strength obviously as steel. Your threads failed because the parent material let go. Im sure the engineers that designed the brackets calculated the proper torque. Since yours failed and I have no idea if they failed because the aluminum sheared or if you had some type of thread damage, i.e cross threaded or a few threads messed up. I wouldn't go back to 104 lbs. it's your car and you do as you decide. I would probably stop at 85 flbs. this is only my opinion you make your on decision here. I just finished rebuilding my LS1 and the torquing procedure is crazy for them. It calls for an initial 22 flbs followed by two additional steps of 90*. I don't go the full 90* on the second step because it gets very tight and I'm not stripping out my block. I hate torque to angle bolts. I posted a pic of head studs and a standard Helicoil. They are both for a small block Chevy, one for iron blocks the other for aluminum. You can see they extended the length of the threads double for the aluminum. If you were to install either the standard stud or Helicoil in an aluminum block the threads would fail they might reach torque a time or two but they would fail. I worked in aircraft maintenance for 18 years both Helicoil and Time serts are approved for thread repair. In aircraft maintenance you don't just say oh I'll fix it this way, you have to follow manufacturers instructions. When you strip a thread in aircraft it almost always has a Helicoil in it already. The normal procedure is to use what is called a Twinsert. This is a Helicoil that you install in the repaired original hole and then install the original size Helicoil back in it. This allows you to use the original size fastener, they are very picky about using the correct fasteners. You are allowed in some cases to go to a second or third size fastener.





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