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Thread Repair 101 (Stainless Helicoil Inserts)

Posted by enigmas

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Thread Repair 101 (Stainless Helicoil Inserts) #1
enigmas Vince Stok Melbourne, Australia	Posted: June 20, 2012 02:38AM Registered: 3 years ago Posts: 391

Hi Guys...This is something I put together for the Rover P5 Forum, but you may find this link useful, especially if you've sheared a bolt or stud in aluminium.

http://roverp5.proboards.com/index.cgi?action=display&board=rover12&thread=6281&page=1

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Re: Thread Repair 101 (Stainless Helicoil Inserts) #2
Gofanu Fletcher Millmore Titusville PA, USA	Posted: June 29, 2012 02:43AM Registered: 6 years ago Posts: 610

Vince-
An issue:
When drilling threaded holes OS as for Helicoil, there is a potentially serious problem of the drill feeding quite viciously into the hole, as both the threads and the drill flutes drag the drill into the hole. The problem can easily cause the drill to break in the hole, or go off sideways and break the side of thin wall bosses, and even cause arm and hand injuries if you have a decent drill. Problem is worse on cast parts. I have very nearly broken my wrist on a 1/2" hole in cast iron when my excellent 3/8" drill got a better grip than I had - it hurt a lot! Perhaps it was annoyed about that stepped drill bit I had. Note the problem still happens if you are using a drill press. I had 7 more holes to do in the tractor final drive housing that had been run with all the bolts loose for maybe years, so I figured out how to do it painlessly and with safety to man and beast. Then I though about what an idiot I was for all the nerve wracking drilling I had done in my life - I now have a fair collection of flat edged drill bits.
A cure:
Grind about .060" of the leading cutting edge of the flutes to be parallel to the drill axis. This changes the drill from a self feeding monster to a well behaved scraper, and your life will be the better for it. In technical terms, and analogous to a lathe tool, the top rake is zero.

Details:
The insert does not go "flush with the surface", especially if the hole is chamfered, by design or event. Rather, the outer end of the insert must be fully engaged in a thread in the part; this normally means one full thread below the surface. Otherwise, the insert is prone to partially backing out next time you remove the bolt, most annoying!

If your inserting tool is threaded, as all mine are, you do not "give a firm wack" as nothing will happen, except maybe damaging tool and insert. Instead, you use a plain punch of an OD to match the minor diameter of the insert to break the tang.

Should you wish to add this to your Rover post, be me guest!

FRM

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Re: Thread Repair 101 (Stainless Helicoil Inserts) #3
enigmas Vince Stok Melbourne, Australia	Posted: June 29, 2012 05:25AM Registered: 3 years ago Posts: 391

Thanks for the reply Fletcher. The insert I installed is a couple of threads below the surface not flush with it.

The .060" off the flute to the leading edge of the drill bit is a good tip.

I take my time when I do this and use progressively larger bits as the job proceeds but didn't show every detail as the post with images can get out of hand. The aim was to illustrate that this task (often mysticized) is within the scope of most amateur mechanics if they take care and use patience.

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