Need Heli Coil help
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Junior User
Joined: Apr 2001
Posts: 96
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Need Heli Coil help
Okay, what I need to do is to repair a stripped out 5/16x18 hole. I got the inserts...5/16x18...but I need to know what size to drill/tap the holes. The kits I see have a 21/64 bit, and what looks to be a 5/16x18 tap. That bit size is bigger than the tap. What am I missing here? What sizes do I need? Thanks.
Fleet Owner
Joined: Aug 2010
Posts: 21,253
Likes: 1,656
From: Reynoldsburg, Ohio
This is what you need:
Upside to spending a few $$$ is the bit and tap are reusable for next time that thread profile needs to be repaired. Many times it doesn't hurt to use LocTite #271 assuming the repair doesn't see 400* or more over a long term.
HTH
Upside to spending a few $$$ is the bit and tap are reusable for next time that thread profile needs to be repaired. Many times it doesn't hurt to use LocTite #271 assuming the repair doesn't see 400* or more over a long term.
HTH
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Junior User
Joined: Apr 2001
Posts: 96
Likes: 0
Thanks guys. That explains a lot! Using a standard tap I could get the insert in there no problem. However the bolt wouldn't go in very far before it got tight and obviously something was wrong....and now thanks to ya'll I know. Thanks again!
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 39
From: EXTREME southwest CT
What "standard sized tap" did you use on the 21/64" hole???
Actually I just had to buy a 5/16-18 kit on Wednesday because someone "borrowed" my kit and never returned it.
Keensert's and Timeserts use commonly available tap size but require a much oversized hole that sometimes won't fit in the object being repaired.
Another option (sometimes) is to go to the next closest size in Metric.
Unfortunately 5/16-18 and 8x1.25 are VERY darn close.
So close -in fact- that you can usually get either bolt started in the other hole.
10mm (.394") does work for 3/8 (.375) though.
Actually I just had to buy a 5/16-18 kit on Wednesday because someone "borrowed" my kit and never returned it.
Keensert's and Timeserts use commonly available tap size but require a much oversized hole that sometimes won't fit in the object being repaired.
Another option (sometimes) is to go to the next closest size in Metric.
Unfortunately 5/16-18 and 8x1.25 are VERY darn close.
So close -in fact- that you can usually get either bolt started in the other hole.
10mm (.394") does work for 3/8 (.375) though.
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Logistics Pro
Joined: Aug 2005
Posts: 3,884
Likes: 180
From: Morgan Hill, CA
Just FYI, the tap you need for a helicoil is an "STI" tap, or standard thread insert. It's oversized to allow for the helicoil material. When properly done, it's stronger than the threads were originally since it's a larger sized thread in the base material and the insert is a hardened stainless steel.
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