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Here is a typical stripped out bolthole in an engine block. The bolt felt spongy when being torqued then finally let go taking the threads in the block with it. The block can be easily repaired with a thread insert kit.
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The thread insert kit we used for this repair was made by HeliCoil and is available at most automotive stores. It contains an oversized tap, a supply of thread inserts and a tool for screwing the inserts into the bolthole.
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The first step for installing thread inserts is to drill the bolthole oversized for tapping. The drill size is listed on the package. Here a drill block is used to keep the newly drilled hole straight (perpendicular to the block surface).
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The same drill block is used to guide the tap. Be sure to keep the tap coated with cutting oil and to continually back the tap up to clear the metal chips from the cutting surfaces.
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Screw the thread insert all the way onto the installation tool. The tang on the bottom of the thread insert should be engaged on the end of the tool. This tang is notched so that it can be broken off for longer bolts to pass through the insert.
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Coat the thread insert with a thread-locking compound and screw it into the newly tapped threads until it is flush with the sealing surface. We actually screwed ours in slightly further because the deck needs to be resurfaced to repair the fracture around this bolthole.
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The finished thread repair is actually stronger than the original threads. The head bolt will be steel against steel (instead of steel against cast iron) and the thread surface area that the new insert resides in is larger than the original tapped bolthole.
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We intentionally broke off this head bolt to simulate your worst nightmare. Removing a broken bolt is much more difficult than restoring damaged threads, but with the proper tools and lots of patience it can be done.
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The first step is to drill the center of the broken bolt to accept the appropriate size bolt extractor. Care must be taken to drill exactly in the center of the bolt so as not to damage the threads in the block.
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Broken bolt extractors come in many sizes and shapes. They are tapped into the hole in the middle of the bolt and the flutes dig in and grip the bolt the more it's turned. Do not break one of these off inside a bolt, as they are hardened and almost impossible to drill out.
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Our broken head bolt backed right out. You might not be so lucky. Try penetrating fluid to loosen rust and corrosion. Heating around the bolthole area with a torch sometimes helps. Be patient and try several applications of penetrating fluid and heat.
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Here's the trophy bolt. As you can see, the hole we drilled wasn't exactly centered in the bolt but the extractor still did its job.
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Any restorer needs to have a full set of taps and dies. They're not that expensive and are something you'll use for years to come. Thread chasers such as these available from ARP can be purchased one at a time as needed.
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This is a graphic example of why you should use a thread chaser instead of a tap or die to clean threads. These two ARP head bolts actually have rolled threads. Running a normal die over the bolt on the right removed a lot of metal, weakening the strength of the bolt.
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