Pelican Technical Article: EDM Broken Bolt and Stud Removal Wayne R. Dempsey
Time: As long as it takes
Tab: $100-$300
Talent:
Tools:
EDM type removal machine
Applicable Models:
911 (1965-89) 930 Turbo (1975-89)
Parts Required:
New studs
Hot Tip:
Some tasks should be left to a professional
Performance Gain:
Getting your engine back on the road
Complementary Modification:
Clean and/or bead blast your engine parts
This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
Check out some other sample projects from the book:
Most of us in our lives have been stuck in a situation that they wish they hadn’t been. It’s the same feeling that you get when you’re sitting on the side of the road waiting for the police officer to get off his motorcycle after he pulled you over. You keep thinking, “Maybe I wasn’t speeding, maybe my taillight is out or something.” That’s the same feeling that you get when you break off a stud in your very expensive Porsche 911 engine case. Or transmission case, or the rear calipers that aren’t made any more and took six months to find! Never fear, there is a solution to all these problems that most people haven’t even heard of.
My own personal dilemma occurred when I was removing the engineering marvels called Dilavar studs from my 3.0L engine case. See Pelican Technical Article: 911 Engine Teardown, engine teardown for a more detailed description of this laborious process which involved several specialized tools including a blowtorch! Regardless of how it happened, I was soon left with a broken stud in my engine case. Repeated attempts to grip it with a pair of vise grips proved fruitless. An attempt at welding a nut to the Dilavar studs also resulted in failure and a string of shouted obscenities.
All was not well in my world when I phoned Alex Wong of Precision Tech for advice. He promptly sympathized, and then proceeded to tell me that he had been in similar dire straits many times previous. His solution was to take the case to Tap-Ex, a company that I hadn’t really heard of before, but would come to be forever engrained in my mind.
Tap-Ex is owned by John Blackwell, who is probably regarded as one of the country’s premier experts at removing broken studs from just about anything. The tools he has in his arsenal are based around a set of machines called electrical discharge machines or EDM. These machines work by passing a large electrical current through metal, literally zapping away bits of material until nothing remains. The process is utterly fascinating to watch, as it’s something that you don’t normally see.
The beauty of John’s process is that it is only destructive to the bolt or stud being removed. A small electrode is used to literally burn away the stud, making it increasingly easier to remove with an extractor tool. With my 3.0L aluminum case, the Dilavar stud had broken off on one of the cylinders, and was very difficult to reach with normal tools. I could have opted for the traditional approach of having my machine shop drill out the hole, but if something went wrong, then the case might be permanently damaged. In addition, the Dilavar studs don’t make the best candidates for drilling.
The first step is to mount the case flat on the tool bed. It’s important to have a straight line down the hole for the machine to burn away the stud. A hollow copper tube that is smaller in diameter than the stud was used as the electrode in this case. The electrode burned away the middle part of the stud, leaving a core, and the outer threads. Because the electrode is disintegrated along with the stud, it’s quite difficult to figure out when the electrode has reached the bottom of the case. When operating the machine, John needs to be attentive for the one to two second delay that occurs as the tool pauses at the end of the hole.
The process begins literally with sparks flying. As the electrical current burns away the middle of the stud, sparks and small bits of metal are thrown out of the case. The coolant that is sprayed on the whole process serves a dual purpose: to cool the area and also to remove the little bits of metal that are burned off. John covered the case with a clear plastic sheet so that we could observe what was happening without getting hit by any flying debris or coolant.
When the electrode reaches the bottom of the case and the machine is stopped, the center core of the stud can easily be picked out of the hole using a pair of needle-nose pliers. At this point, only the thin outer edge of the stud remains in the case, and it can be easily removed with a simple extractor tool.
While a lifesaver, this process did have one major drawback. It was impossible to keep the coolant out of the inside of the case. Since the coolant was used to wash away all the small bits of metal from the process, the entire inside of the case became contaminated with a fine grit that you could feel with your fingers. That, of course, meant that the entire bottom end of the motor needed to be rebuilt: something I had hoped to avoid.
When I went back to pick up the case, John had a whole stack of cylinder heads from a manufacturer that he was working on. He routinely takes on jobs that are shipped from all around the country. If you have a tough job that needs the help of such a process, then John is your man. The tab for the process was about $160 for both the broken Dilavar stud and a small sheet metal screw that had been broken off as well in my engine case.
Contacts:
Tap-Ex
310-323-3834
1940 W. Rosecrans Ave.
Gardena, CA 90249
Figure 1
Your worst nightmare! This head stud has been broken off in the case. This particular stud was a Dilavar stud, which cannot be easily welded to, and has a real tendency to break. The 911 exhaust studs located in the cylinder heads in particular tend to shatter and break because they have been tempered repeatedly by the heat from the engine’s exhaust system.
As with any precision machining process, the setup process must be performed very carefully. The engine block must be placed exactly square to the disintegration tool, otherwise the electrode might wear away the inside threads of the hole instead of the broken stud.
The electrode is a hollow copper tube that is positioned carefully with respect to the broken stud. The electrode must be positioned square to the case, otherwise it might damage the threads as it wanders off course.
The engine block is covered with plastic in order to keep the coolant and lubricant from spraying around the shop. The coolant serves a dual purpose: to cool down the area that is being disintegrated, and also to carry away bits of metal from the area. In this case, we attempted to cover up the case so that the fine particles of metal wouldn’t find their way into the recesses of the engine. To put it mildly, we weren’t successful, and the engine case had to come apart to be cleaned.
Sparks fly from the stud as it is slowly disintegrated. As the machine automatically drives the electrode deeper into the case, the operator must be careful to watch for the point where the electrode hits the bottom of the stud. The machine will pause for a brief moment when the electrode is not contacting any metal, and then start eating into the case when the electrode hits the bottom of the hole.
The hollow electrode burns away the middle of the stud, leaving both the center core, and the outer threads in the case. When the electrode reaches the bottom of the case, the center core can be easily removed.
The outer threads that remain are now significantly weaker, and can be simply removed using a beveled tool inserted into the inner section of the threads. The hollowed out stud is then easily backed out of the case with an extractor tool.
The usefulness of this process is clearly shown by this photo that John had in his collection. Anyone who has worked extensively on older 911s knows that the machine is set up to remove an old broken exhaust stud. A job that would have been nearly impossible to do without removing and tearing down the engine is now completed in under an hour. $200 versus a complete engine rebuild? Kind of puts things in perspective.
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