Thanks BillGrissom! I took it to Ace and they said it was m10 x 1.5 ~5" grade 8 would be best. They didn't have that size and I ended up at Autozone, where I actually decided that 3/8-16 (grade 8) 5" was the closest fit. I eventually got someone to look it up for me and that's also what they had in their system for the '84 300td alternator bolts. I eventually got it back in and now I see that you're right - nothing special, it just holds the alternator in place.