Failure mode - GM 8.1L / 6.0L Vortec Gasoline (HD 2001-2007, Express/Savana vans incl.)

Exhaust manifold bolt breakage (cold-start tick)

Reliability tier: aggregated - verify before ordering, see /disclosure/

Symptoms

loud tick at cold start that quiets as the engine warms - the signature symptom
exhaust smell / leak noise at the manifold, sometimes a visibly dropped flange
driver's-side rear bolts are the commonly reported first to go

Compare with the other causes of the same sign on this engine: ticking, knocking, rattle or whine (2 causes)

Root cause

The GMT800-era GM truck exhaust manifold bolts loosen and shear from heat cycles and corrosion - reported across both the 6.0L and the 8.1L HD gas engines and across the 1999-2007 truck era. Ignored leaks can warp the manifold and burn the gasket.

Diagnosis

locate the tick at the manifold (hose-listen on a cold engine) before condemning lifters or injectors
inspect for sheared/loose bolts at the flange, rear first
extract broken studs, chase threads; the community-standard upgrade is locking-style manifold bolts

Fixes

repair: extract broken studs, fit new (commonly upgraded) manifold bolts and gaskets ()

Prevalence

very-common