Broken/Stripped Bolts

Back in the day of cast iron heads and engine blocks, stripped bolts were not a super common concern. Now with all aluminum engines in an all-out quest for weight reduction, we’re seeing more and more stripped and broken head bolts. A couple of common occurrences that I’ve seen recently come from a 2.4 Toyota engine and a 4.2 Chevy engine.

The Toyota 2AZ engine tends to present with a few bolts on the back side of the engine stripped prior to disassembly. Most of the time, it’s discovered as a coolant leak.Toyota released TSB 0015-11 with information regarding the thread repair.

Many repair facilities recommend a rebuilt or good used engine, but you can purchase a TIME-SERT thread repair kit which I believe Toyota uses to repair the affected engines.

It’s important to note that the two bolt holes on either end of the block (left and right as the engine sits in the engine bay) cannot be repaired because of the water jackets and the block must be replaced if they’re damaged.

The Chevy 4.2 Vortec (and the smaller 4 and 5 cylinder versions 2.8, 2.9, 3.5, 3.7) head bolts tend to break when removing for head or valve service. These bolts, like many other bolts used in modern vehicles, are TTY (Torque To Yield) meaning that they permanently stretch when properly torqued to provide more even clamping force over traditional torquing methods. GM released TSB 05-06-01-026A to help with removal, but as with the Toyota 2AZ many shops replace the engine instead of fooling around with removing and drilling bolts, etc. There’s a lot of potential for damage and mistakes with that kind of repair and another engine is good insurance.

These bolts came out of a 2004 Trailblazer EXT 4.2. Luckily we had already replaced the engine and I was just playing around when this happened. If I’d tried to replace the head gasket in the vehicle, I would have been up a creek without a paddle.

Bottom line: it’s a good idea to do some research before you tear into any engine and look for problems like this. Happy wrenching!

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