Gotta like the gear heads on this site. Thanks for the posts. I'll make sure the shear point location is not threaded.

My engineer friend says this:

This is what I found.

SAE KSI Metric KSI
Grade 5 120 8.8 120
Grade 8 150 10.9 150
Grade 9 180 12.9 180

The cross sectional area is slightly different. The 5/16 bold is .78%
smaller making it that % weaker. I'm not a structural engineer, but my guess
is it would be okay.

My friend (a mechanical Engineer) says the shear area of the bolt at the join should be unthreaded.
Peter S

Good question. Since the application is strictly shear related I suggest find more data. I'll also aska friend who's got some data books.

PS

Anybody out there tried using 5/16" diameter grade 8 bolts for the outsides of the rear half shafts (where they bolt to the spindles). A 5/16" diameter bolt fits great, they are cheap and readily available. Came with 8MM 10.9 grade bolts. My internet searching shows grade 8 and 10.9 to have practically identical tension load strength. No luck finding good shear strength data.