This is awesome, both full of great little characterization details and really hot. Tony doing hands-on mechanic's work in goggles never stops being sexy (thank you, Adi Granov, for showing us all this truth).

almost innocent, almost just a massage, but traveling steadily downward, and if anyone else touched Tony this way, Steve might have to hit them.

I love the not-at-all subtle possessiveness in this, which lets you know how intimate Steve finds/intends this gesture to be, innocent-looking or not.

the shaking as though joints and threads are about to snap and crumble

And this, describing Tony as if he's a piece of machinery, is somehow really fitting.

they're face to face, forehead to forehead after Steve peels off the goggles. "Hey, Shellhead," he greets blue eyes, blinking in adjustment.

Tony smirks, "Hey, Cap."

I think I died of the dorky adorable here. Died. This is absolutely the perfect ending.