But bolts and screws are as important as anything in a machine! Sure a con-rod screw may be a little simpler to spec than the con-rod itself since they are, as you suggest, built to a standard, but there's a lot that goes into it.

Lucky for us engineers they're usually cheap, readily available, and strong fasteners available (Grade 8s are mass produced and wicked strong). I don't know the struggles of a Volvo engine designer sitting in Gothenburg in the 1980s, why exactly they couldn't spec an off-the-shelf screw, but perhaps in their world it is just that.

I work in a role where I get to design bolted joints fairly often. Bolts and screws are fun! (Nuts not so much) Making sure they meet the customer specs, finding the balance of size, material strength and number of, making sure they fit and can be installed (and not loosen), material interactions, corrosion issues - It's a good time! Even more fortunate for me, it's low-volume and cost usually isn't a driving factor. In one particular application, backed into a corner requiring high strength and corrosion resistance, I saw a five-figure price tag for just the material to make a custom screw. A lot of mother-may-I went into that one.

Even further off-topic, look into "Superbolts". They're pretty wild.

-Will
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XC60 / Odyssey