Originally Posted by bossman429
Not really following you on the first one...From what i read the only advantage fine thread has is tensile strength (pulling the bolt apart literally from opposite ends). Wouldn't Shear strength depend on the material and thickness (example 1/2" grade 8 vs 3/8 grade 5) I understand if you were putting the shear load at the thread then coarse thread could be less (deeper thread cuts) but a proper sized bolt will have the shank at any shear point, not the threaded portion.
When you clamp two things like this the idea is to tighten the bolts to proof. The way the clamping works, as you load the joint there is no change in the load on the bolts. In this case the bolts stay in pure tension and the shear load is only between the clamped parts. A fine thread bolt will provide better and more reliable tension due to the way the bolt tightens.
The engineers that designed the joint choose fine thread for this reason. Normal bolts are metric coarse on a Defender. They use UNF when there is a valid engineering reason.