Found a cracked head on a 6.7 Cummins that had been helicoiled 3 times before

I pulled this engine apart last Tuesday in Fresno and found 3 helicoils stacked in one cylinder head bolt hole from previous rebuilds. Has anyone else seen hack repairs like this holding up surprisingly well?

3 comments

3 Comments

zara_hill462mo ago

Told the guy who had our shop truck to just torque it to spec and send it" - that's exactly the kind of cowboy stuff that makes me wonder if these engines are tougher than we give them credit for. Nobody's talking about how the helicoils themselves can actually act like a thread locker over time if they're installed right, especially with the heat cycles in a diesel. I've seen a few 5.9s with helicoils that outlasted the rest of the block because the stainless steel doesn't fatigue like the aluminum does. But three in one hole though... that's more about luck than engineering at that point. The real question is whether the bolt was bottoming out or if the threads were just that chewed up from previous overtorquing. Wouldn't trust that on a high horsepower build, but for a stock truck it'll probably run another 100k miles just fine.

lucas_perez20d ago

The thing nobody's bringing up is that three helicoils stacked means the first one probably spun out and the guy just drilled deeper instead of fixing the hole right, which actually changes the clamp load on the head because you've got this weird column of stainless steel flexing different from the factory threads. I wonder if the torque wrench even clicked properly with all that extra material in there, or if the bolt was just binding on the way down and giving a false reading. That kind of stacked repair might hold vacuum fine but the real test is whether the head gasket stays sealed under full boost, since the clamping force is probably all over the place on that corner.

veramiller2mo ago

Told the guy who had our shop truck to just torque it to spec and send it, and it held fine for another 80k miles before we sold it.