ok so take a look at a microfiche. look at the 'steering' section. the bolts that go in the top of the handlebar clamp (#14 on some). that should be p/n: 91317-08030-00
from what i understand the second bunch of numbers means its an 8 mm(bolt diameter) and 30 mm long.
If you have a local Fastenal store they have a bolt gauge they sell or you can get one from them online.
There are several gauges at Nuts and Bolts dot com.