You'll need a minimum of 3 3/8" bolts due to bolt bearing area failure.

The 3"x1/8" bar you are bolting through is A36 steel, which has an ultimate strength of 58,000 psi. The formula for bolt bearing capacity is:

Bolt diameter*material thickness*ultimate strength*.75

3/8"x1/8"x58,000x.75=2,039lb capacity per bolt.

So to equal the 6,000lb capacity of your winch (because you want the winch to break before your bolts tear out) you need a minimum of 3, and they don't have to be grade 8. It doesn't matter how strong the bolts are if the base metal tears way before the bolts approach their full strength.

It appears that chart shows the breaking strength, not the safe load. Also, the values for shear are for single shear plane failure.

Keep in mind that the maximum strength will result when shear or tension acts through the bolt. Any eccentricity causes a prying action that increases the stress on the bolt.

Consider all the limit states when designing a connection.

See http://www.bgstructuralengineering.c...BGSCM00306.htm for a little more detail.