Will these fit fasteners on my intake manifold and brake calipers?

The range 5/32" through 3/8" covers most metric and standard fasteners on modern engines. If your fasteners are smaller or larger than that spread, you'll need a different set or individual keys.

How is the flat-head design different from a regular hex key?

Flat-head keys contact both parallel sides of the fastener socket instead of just the corners. That means less wobble, better torque delivery, and less risk of rounding out the bolt—especially useful on soft aluminum or worn fasteners.

Can I use these on stripped or rounded fasteners?

Flat-head keys grip better than standard round keys, so they'll work on some rounded fasteners that a regular hex key would slip on. For severely damaged fasteners, you'll need a screw extractor or bolt removal tool.