Chain wear gauges: how they work

What a gauge measures

Chain doesn't stretch, it wears. Every pin-and-bush joint opens up a little clearance as it wears, and across dozens of joints those clearances add up to a chain that is measurably longer than its pitch count says it should be. That growth is elongation, and the replacement rule of thumb is around 2%, with 1.5% the sensible planning trigger (our wear guide covers the thresholds in full). Every wear gauge, whatever its shape, is a way of comparing the chain's actual length over some number of pitches with that nominal length. The gauge just does the arithmetic in metal.

Three ways to check

• Steel rule or vernier. Measure pin-centre to pin-centre over 10–12 pitches under light tension, divide by the nominal, read the percentage. Works on any chain, gives a real number, needs a moment's arithmetic. Best when you want to trend wear over time.
• Sliding (percentage) gauge. A two-part gauge that hooks two rollers a set number of pitches apart and reads elongation directly off a scale. Quick and quantitative, but pitch-specific.
• Go/no-go plate. A flat gauge with fixed probe pairs for each pitch, set at the replacement point. No scale, no reading: if it seats, replace. The fastest for routine walk-round checks across many drives, and the type explained below.

Inside a go/no-go gauge

The trick is simple. For each pitch the gauge serves, two probes are spaced at n × pitch × 1.015 That is, n pitches of chain plus 1.5%. On new chain the rollers are 1.5% too close together for both probes to drop between them at the same time: one seats, the other rides on a roller. As the chain wears the roller spacing grows, and at 1.5% elongation both probes seat fully. So the whole test is: press the gauge onto the taut chain, and if it seats, replace the chain.

The number of pitches spanned is chosen so the gauge stays a hand-size and so 1.5% amounts to a few millimetres, enough to see and feel. Worked spans for the common pitches:

Two details decide whether a plate gauge is trustworthy. The probe centre spacing is the measurement, so it needs to be held to a tight tolerance: a tenth of a millimetre over a 180 mm span is the difference between 1.5% and 1.44%. And the probe tips must be narrow enough to reach the roller centreline in the smallest chain the gauge serves, otherwise they bottom on the rollers and never seat at all, which reads as "chain fine" forever.

Using one properly

1. Taut side only. On the slack side the joints are not pulled to their worn limit and the gauge under-reads. On a drive, check the tight span; off the machine, hang or tension the chain.
2. Several places round the chain. Wear is rarely even; the worst section is the one that counts. Three or four positions is a fair check.
3. Right pitch, right probes. A 1/2" gauge on 5/8" chain will neither seat nor tell you anything useful. Identify the chain first if you are unsure; the identification guide takes a minute.
4. Look at the sprockets while you're there. Hooked teeth or a polished band high on the tooth face mean the sprocket has worn to the chain, and fitting new chain to it wastes the new chain.

When a gauge is the wrong tool

A gauge set at 1.5% is right for general industrial drives. It is too generous for drives with very large sprockets: the limit there is 200 ÷ N percent (N = teeth on the largest sprocket), which drops below 1.5% once N passes about 133. For those, and for reversing, precision or high-shock drives where the machine maker gives a lower figure, measure with a rule and calculate. A gauge also tells you nothing about why the chain wore. If it seats long before the chain's expected life, the fault finder is the next stop, because the replacement will go the same way unless the cause is fixed.

Common questions