Timing chains don't actually stretch like a rubber band — the steel links themselves don't get longer. What people call "chain stretch" is pin and bushing wear. Each link pivots on a hardened pin riding inside a bushing, and every combustion event yanks the chain against those wear surfaces. Multiply by hundreds of millions of cycles, and microscopic material loss at each joint adds up to measurable elongation across the chain's total length.

Here's the math: a typical timing chain has around 100–140 links. If each pin/bushing joint wears just 0.001" (one thousandth of an inch), a 120-link chain grows by 0.120" — over an eighth of an inch of total elongation. On a chain wrapping cam and crank sprockets, that translates directly to retarded cam timing because the tensioner takes up slack on one side, letting the chain lag on the drive side.

Rule of thumb: 1° of cam retard per 0.030" of chain elongation on most inline engines. Modern engines with cam position sensors will throw P0016/P0017 codes (crank/cam correlation) when retard hits roughly 6–8°.

The classic real-world example is the BMW N20/N26 (2011–2017 four-cylinder turbo). BMW used an undersized chain and a plastic tensioner guide that wore rapidly. By 60,000–100,000 miles, chains had elongated enough to jump timing — often catastrophically, with valves meeting pistons. BMW eventually issued a revised chain kit with heavier pins and a redesigned guide. The Ford 3.5L EcoBoost and VW 2.0T EA888 engines have similar reputations.

What accelerates wear:

• Extended oil change intervals — dirty oil is abrasive slurry running through every pin joint
• Low oil level — the tensioner is hydraulic; low pressure means slack chain slap
• Cold starts with thick oil — chain runs dry for the first few seconds until galleries fill
• High-RPM operation — chain tension spikes exponentially with speed

Diagnosing stretch without teardown: pull the valve cover and look at the tensioner extension. Most tensioners have wear marks or steps on the plunger — if it's extended past the middle mark, the chain is near end-of-life. Better yet, log cam position error with a scan tool at idle; anything beyond 3–4° of steady-state deviation from commanded is a chain problem, not a phaser problem.

Silent chains (inverted-tooth) wear differently than roller chains — they distribute load across more contact area but are more sensitive to lubrication quality.