Sunday, February 14, 2016

How to Restore a Stripped Bolt Hole with a 
Helical Coil-Thread Insert
1955, 1956, 1957 Classic Chevy Tri Five
  -Alan Arnell

If you work on any piece of machinery turning wrenches you will strip a bolt or bolt hole.  If you strip a bolt you can just go buy a new bolt.  Strip a nut, buy a new nut as a replacement.  But, if you strip out the bolt hole for one of the water pump holding bolts you might want to take the Lord’s Name in vain.  If you believe in the Judeo/Christian tradition don’t break the Third Commandant, because installing a helical coil-thread insert in not really that hard and the insert works as well or maybe better than a threaded bolt hole.

Screw thread-Inserts, also known as a threaded bushing, is a threaded plug that is inserted into an object to replace a stripped threaded hole with the same size and thread count as the original threaded hole.  Coils may also be used to provide a durable threaded hole in a soft material and/or place a thread on a material too thin to accept it,
If you have a stripped bolt hole, the hole is drilled out slightly larger to a prescribed diameter.  A tap is used to place in a different pitch than the standard thread you are fixing.  A coil is screwed in the offended stripped hole with the different pitch threads.  The coil uses the installed threads to hold onto and the coil provides the correct thread for the bolt you need.
There are several types of insert kits.  Black and Decker makes Helicoil system and there is Perma coil.  The only difference between the two is the supplied installation tool.  The Perma-Coil kit has a one-piece steel insert tool.  The helicoil kit includes a diamond tipped plastic insert tool.  Typically perma-coil are less expensive but you have to supply you own tap handle or use an open-end wrench to use the installation tool.  There is also Time-Serts and Katonet Inserts.  See links below.

Coil Insert How To:
Note:  One size does not fit all.  You have to know and buy the correct bolt size and thread pitch of insert you wish to use for a repair.

For example: I will list steps for a  ⅜ inch stripped bolt hole.

  1. If you broke the bolt off in the hole, try to remove it with vice grips or a punch and a hammer.  If you are unable to remove the broken bolt you will have to drill the bolt out.  
  2. Grind broken bolt flat on top.
  3. Center punch the broken bolt.
  4. Drill the broken bolt out.  Depending on the size of the bolt you may have to use a pilot hole before drilling out the full girth of the broken bolt.
  5. For a ⅜ inch bolt hole the final size and correct size hole diameter is 25/64 inch drill.
  6. The coil kit has a tap supplied.  Run the tap to the bottom of the hole.  You must use spray lubricant several times while cutting threads with the tap.  You have to turn the tap a half a turn in, then back a half turn out, then a full turn in and back a half a turn out, et cetera, until the tap bottoms out.  Using lubricant and the back and forth turning of the tap cleans the metal shavings out of the way and keeps you from breaking off the tap in the hole.  If the tap breaks in the hole there is no drilling the tap out, because it is made with the same material as the drill bit.  The broken tap may be there forever!
  7. Place the coil over the end of the installation tool with the tang at the bottom.
  8. Using the installation tool screw the coil into the hole.
  9. With the insert installed, break off the tang at the bottom of the threads so it does not interfere with the bolt.  A standard punch will make the required results of removing the tang. There is an available tool called a back tang-removal tool,  The tool makes it easier to remove the tang but but is not necessary to have to break the tang.  

Note:  If you have a stripped bolt hole that you are worried that the coil will not supply the needed  holding thread strength, such as an exhaust manifold bolt, you may install two tandem coils in the hole for more needed strength.

Note:  Not all auto parts stores have thread repair coils.  I would call before I went to a parts store.  Below are vender suppliers of thread repair coils.


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