How to Remove a Broken Bolt from a Thermostat Housing

by TJ Hinton
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Thermostat housing bolts are particularly vulnerable to corrosion and seizure due to their proximity to the coolant and the engine water jacket. Any weeping of coolant at the housing gasket will corrode the bolt, weakening it while locking it in place. Remove the broken bolt requires working room sufficient to get lined up on it with a drill motor and bit, so you must remove whatever components necessary on your engine to achieve this working room, such as the radiator upper cowling, radiator hose and possibly the radiator itself.

Step 1

Insert a shop rag into the opening of the thermostat housing to prevent metal shavings from entering the engine. Index the center of the broken bolt, using a deadblow hammer and punch. Ensure that you keep the punch centered and do not allow it to slip. Lubricate the bolt with penetrating oil and give it time to soak in.

Step 2

Chuck a 1/16-inch, titanium-nitride-coated drill bit in a drill motor. Mark the approximate length of the bolt on the bit with a piece of tape to act as a reference point to prevent overdrilling. Drill out the center of the bolt, making sure that you stay centered and lined up with the bolt. Use cutting oil to cool and lubricate the bit while drilling to prevent seizure and ruining the temper of the bit.

Step 3

Enlarge the hole one drill-bit size at a time until the size is right for your extractor as per its manufacturer's instructions. If you're using an E-Z-type extractor, carefully drive the extractor in until it bites the bolt securely, then unscrew the bolt using an adjustable wrench or T-handle on the extractor. If you're using a screw-type extractor, screw the extractor into the bolt in a counterclockwise direction until it bites into the bolt and unscrews it. If you do not have an extractor set, then you must continue enlarging the hole, using progressively larger drill bits, until the threads can be disengaged from the hole. Dig the threads out using the drill bit or a dental pick.

Step 4

Chase the threads in the bolt hole, using the appropriately sized tap and T-handle, to reshape any damaged threads. Inspect the threads after chasing them for signs of having been worn past the point where they can effectively engage with the bolt threads. Severely worn bolt holes must have a thread-repair kit such as Timesert or Heli-Coil installed to restore the threads in the hole.

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