I've got a 1990 F-150. Are the fasteners supposed to be standard (inch) or metric (mm)? Or is 1990 one of those switching over years.

I'm not sure if they are supposed to be metric or if all the nuts are just so rusted that only metric fit well. I ended up using 15mm wrench on the exhaust manifold to exhaust bolts -- the inch size wouldn't seem to fit right.

 

Probably a mixture of both all around the vehicle.

 

I have a 91 and it's a combination of SAE and metric

 

They're mixed. Fasteners on the frame & engine are usually standard, but where parts have been updated (like exhaust manifolds with ports for EFI components) use metric fasteners. Most body fasteners are metric except on some holdover parts like the ventilation system. You just have to guess & try.