240D Alternator Bolt Size/ part #?

[wilburtual]

1981 240D

I gone and lost my alternator mounting bolt.. (don't ask..)
It's not the fancy adjusting one, its the plain old bolt that is nearest the ground, that holds the alternator to the engine and lets it swivel..
anyone know the size/length of that bolt? is it something I can just get at the hardware store?

thanks!!