5/16 on my 3100 not sure if you call it a true bolt as you cant put a wrench on the end as it is round so it sits down in the axle or your shims. the tension of the springs hold it tight while you tighten the nut.

1950 3100

You never get out of life alive -- so enjoy it!

Youtube.com/watch?v=sGTQp21P_Uo

http://i1321.photobucket.com/albums/u558/smoloney08/truck-01238_zps0c317d61.jpg

http://s1321.photobucket.com/user/smoloney08/slideshow/