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Date:         Mon, 19 Jan 2004 12:01:44 EST
Reply-To:     THX0001@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         George Goff <THX0001@AOL.COM>
Subject:      Re: Exaust / Pilot Drill Sizes / Stud Removal
Comments: To: dhaynes@OPTONLINE.NET
Content-Type: text/plain; charset="US-ASCII"

In a message dated 1/16/04 9:46:57 PM, dhaynes@OPTONLINE.NET writes:

<< A trick! . . . Start with a 1/16" bit. Make sure it is sharp. The OEM studs are only grade 5.8. The bit will usually follow center. Then work your way up to the size needed for the 8mm tap or the Heli-Coil. >>

In this situation, 1/16" is a poorly chosen size for a pilot drill. Using a 1/16" drill in a handheld drill motor is risky under any circumstance let alone under a Vanagon. Such a small drill is easily broken by a number of causes and snapping off a drill in a broken stud adds a new dimension to the problem. The drilling pressure can readily deform (bow) a small drill causing it to err in its intended path. Also, while I suppose they exist, I have never seen a right angle drill motor with sufficient speed to properly spin such a small drill.

The quality of the pilot hole drilled in an attempt to relieve a tapped hole of a broken fastener is all important because any subsequent drilling will follow the path established by the pilot hole. A pilot drill must be sized so that it is slightly larger in diameter than the web thickness of the next drill which will be used. In drilling an 8mm Vanagon exhaust stud, a 1/8" drill is acceptable and robust enough for the task. Ideally, it is best to start the pilot hole with a center drill.

In all this talk of drilling out studs, there is one method which has not been mentioned which saves the original tapping: drill out only the body of the stud. It takes precise work, but if the drill used is sized correctly, it is possible to waste the root diameter of the fastener leaving only the thread behind. The remnant of the thread is then picked out and pulled from the hole. It comes out much the same as removing an unwanted HeliCoil insert. Done properly, the oiginal tapped threads are unharmed and perfectly useable.

There is another possibility in sparing a stripped stud hole. The drilling for the stud extends beyond the tapping. If there are some good threads remaining below the stripped portion it is possible to run a bottoming tap to the extent of the drilling. As long as the total length of the newly formed threads combined with the length of the remaining good threads is equal to or greater than one and a half times the diameter of the fastener, the amount of thread engagement is perfectly adequate. Of course, then only a stud can ever used in this hole.

A word of caution. At different times I have done all that I described above, but I know myself. These approaches are not for someone who does not have the patience to pecker around with tedious tasks.

George


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