How common is a broken main bearing bolt?

[Waorani]

I was recently given a Model A engine and have started tearing it down. It was obvious externally that one of the front main bearing cap bolts was broken and sticking up. Bolt head was in the oil pan. Crank still turned freely and cap isn't broke nor any other obvious damage. I don't know what bolt quality was like back in the day but it sure seems like it would take a lot to break one. Is this common or the result of a gorilla with a wrench?


The head is B marked which I've learned is a high compression or "police" head. But I wouldn't think that would be a factor.

 

[Flathead]

I vote for the gorilla, those bolts are pretty rugged. People will often over tighten them trying to get that cotter pin in.

 

[nkaminar]

I am also going to vote that the bolt was over torqued. The break looks like a clean break from too much torque and not something that fatigued over time. It could be that the mechanic was trying to line up cotter key holes in the nut above after the torque had been reached. Remember that Ford did not use torque wrenches on the Model A, just wrenches of a certain length. Maybe the worker that put this engine together had Wheaties for breakfast.

An interesting thing related to this is how the main bolts and head bolts are torqued on huge truck engines. The bolts are brought to a specified torque and then marked. They are then turned another 90 degrees using a huge breaker bar and a mechanic with huge arms. I guess this stretches the bolts a curtain amount.

 

[Flathead]

I wouldn't recommend that on an A.

 

[Jim Brierley]

Very rare! I broke one once while racing with an OHV head on it, and it was competitive with the flathead V8's and one GMC in a jalopy. I think the babbitt went first and the crank was bouncing around in there. It was a center main bolt.