I've been wondering....

How does thread pitch affect strength, torque, tendency to loosen with vibration etc?
If same thickness bolt, same material but different thread pitches, how does it affect application.
I notice that the engine crank case halves are held together with coarser pitch (high vibration environment) than generally used in airframe.
Thanks.
Johan

Mel, I always figured they were course pitch because when Lycoming designed the engines, fine pitch hadn’t been invented yet.....

No general conclusion there

Both of those fasteners are threaded bolts with nuts, known hardness and controlled thread fits. Vibration "tolerance" involves the clamped material, (steel-aluminum), washers (contact areas), stretch length of the bolt, and loading case (shear, tension combo).

Bolts can be loaded for shear or tension. Example is an RV wing attachment or a connecting rod. A connecting rod must have sufficient tension for tension loads and ensure the tension of the fastener creates a rod cap contact stress that it will not slide from side to side. The connecting rod bolts are a grade 8+ bolt.

The spar bolts are not intended to prevent shear of the parts under all conditions.
They are grade 5.

The only conclusion I can draw is that if the clamped parts separate or slide with loading they can come loose, but not based on thread pitch.

Way back

Fine pitch threads react to a torque value closer to the intended purpose of properly preloading the male fastener. They are also less susceptible to loosening than a course thread bolt.

Course threads are stronger in theory; a bet I would have lost. Despite fine pitch having more threads engaged per length, the course threads have more contact/load area engaged due to the thread geometry.

This memory goes back to Mechanical design class in 1985. No differentiation was made for aerospace grade fasteners if there is such a difference. Someone will surely correct me if needed.

Nothing beats a good old cross thread!

Threads into soft metal are generally courser.