“What I Learned Today” With Jeff Smith — Blind Bolt Hole Blunders

This snafu found its way to us through a back door. A friend who shall remain anonymous commissioned someone who claimed he was an “engine builder” to assemble a Dart Sportsman iron block 383 small-block Chevy — a blind bolt hole design. The process seemed to be going well until the builder torqued the head bolts. He claimed “something didn’t feel right” with full torque on the first head bolt in between the two center cylinders. They discovered the head bolts were too long because the Dart block uses blind head bolt holes that are not drilled into the water jackets like a production block.

The result was catastrophic. When the bolt bottomed out in the blind hole, it cracked the cylinder wall in both adjacent cylinders. This demanded not only repairing the cracked head bolt hole but also new sleeves in two cylinders. The cascade effect took over which meant all eight cylinders had to be re-honed because the sleeves affected the neighboring cylinders. The cost to repair eventually came out to be almost half the cost of the new block. All of this could have been easily avoided.

Most heads bolts are designed specifically for the cylinder head application but even when the correct head is selected, it’s critical to check the overall depth of the holes on any block that has blind head bolt holes. The advantage to this style of block is that there is no chance for corrosion in the head bolt threads and also that there can be no opportunity for coolant to intrude into the engine. All of these issues relate to LS engines as well, because they also use blind head bolt holes

It’s difficult to see in this photo but the crack from the head bolt hole extends into both cylinders. This required sleeves for two cylinders plus repairing the damaged head bolt hole.

It’s critical to ensure that the head bolts do not bottom out in the block on blind head bolt holes because this places all the stress on the top thread in the block, as evidenced by the damage to the block where it cracked at the deck surface. With head studs, it’s important to make sure the studs are fully seated in the block. ARP places a hex in the end of the stud for an Allen wrench to help seat them, but they should never be torqued in place — finger tight is acceptable. You will know the studs are fully seated when the threaded portion of the stud will be below the deck surface. If a portion of the thread extends beyond the deck, this will cause a stress riser around the top thread in the block that will cause problems for the fastener.

Also with blind head bolt holes, make sure there is no liquid like water or oil at the bottom of the hole. When a stud or bolt is threaded in place, this can cause a hydraulic situation that will also instantly crack the block. As a final thought, make sure the chamfered portion of the washer is placed next to the head of the bolt. The chamfer is designed to match the radius of the transition from the bolt shank to the bolt head.

Here is the block after the head bolt hole has been repaired and the block has been sleeved. The machinist then had to deck the block again and completely re-hone the block in order to return all the cylinders to a usable condition.

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About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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