A bolt does not come back to the same condition when it is undone. It has been stretched, heat cycled and exposed to whatever the joint has seen, and each of those leaves something behind. The question is not whether it looks serviceable but whether it still does the job it was chosen for.
The job is to hold a joint at a controlled clamp load. A fastener that has lost section, gained a stress riser or lost its thread form can still feel tight with a wrench and still be nowhere near the load the joint needs.
Fasteners in exhaust and charge-air service are usually tightened into their elastic range, which means they stretch a little and stay stretched. That is normal and repeatable up to a point. Past that point the shank yields, the bolt becomes longer than it was, and it will never hold the same load again.
You cannot measure this by eye and you should not try. What you can do is judge history: a bolt that has been tightened many times, or was once tightened by feel until it stopped moving, is a candidate for replacement regardless of how it looks.
Stainless fasteners in a hot, wet, salt-exposed place do not rust away evenly. They pit, and a pit in a thread flank removes the surface the nut bears on. The nut then binds before it reaches load, which feels like tightness and is actually friction.
Look at the first few threads that were inside the nut and at the shank under the head. Bright metal with a uniform finish is a good sign. Dark pitting, a chalky deposit or a thread that feels rough when the nut is run on by hand means the fastener has already lost the surface it needs.
Reuse is normal on low-load hardware: brackets, heatshield fixings, cosmetic covers. It is also normal on joints designed to be serviced repeatedly, where the fastener is generously sized and the clamp load is not critical. The distinction is what the joint depends on.
It stops being acceptable where the fastener sets the clamp load on a sealing joint, or where it holds something heavy against vibration. On a stainless steel exhaust system that means flange bolts and hanger hardware get more scrutiny than a heatshield screw.
Run the nut down the full length of the thread by hand. Any point where it drags or clicks is a defect, not dirt. Look for necking, a bright ring where the shank meets the thread, or a head that has been rounded. Check the shank for galling, which shows as torn, smeared metal.
Then check the female thread as well, because a good bolt into a damaged nut will fail the same way. A thread chaser cleans a slightly damaged thread; it does not restore a stretched bolt, and the two jobs should not be confused.
Replace any fastener that has yielded, pitted, galled or lost its head shape, and replace anything on a critical sealing joint where the history is unknown. Reuse clean, straight hardware on brackets, covers and small mounting parts where the load is modest.
If you are unsure, the price of one new bolt is trivial against the price of a joint that lets go on the road, so the rule should lean toward replacement wherever the part matters. Suspicion is enough reason.
