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Hi Nahtanha,
Yes, that was the conclusion I first jumped to when the broken casting fell to the floor, and also why I made a mental note of the torque on the second bolt.
This morning I did some experimenting in my shop. I have a small 3/8 drive torque wrench measuring 0-50 inch pounds, and a 1/2 drive with a range of 200- 1200 inch pounds. ( 16.6 - 100 foot lbs.)
I set up a clean 8 mm bolt in my vise with a spacer, a clean washer and a flare nut. I set it finger tight and applied the smaller torque wrench. The nut moved to 24 inch pounds or a little more but did not move any more despite the wrench continuing on it's path to it's limit. So for my purposes, I'm going to define 'snug' as 2-3 ft. lbs.
I marked the nut and bolt head with soapstone, and tried to simulate the torque I would have used to tighten the bolt on the u joint using the same 8" box end wrench. It moved the nut between 1/6 and 1/4 revolutions on multiple tries. I then set the larger torque wrench to 225 inch lbs. (18.75 ft lbs.), which moved the nut about 1/8 revolution further. Loosening the nut, it took as much effort as the original nut on the u-joint. To me it felt secure, tight enough and wasn't going to loosen.
I repeated the whole mess over, except that I increased the final torque to 25 ft. lbs. It felt over torqued to me, and I'm confident that I wouldn't have made it that tight.
My conclusion is that over torqueing is highly unlikely to be a cause of the failure.
Regards, Peter
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