Some of Honda's earlier manuals made this a little clearer, and is the "purist's way" of measuring the wear, still. It went about like this:
1. Remove the chain and wash in solvent until clean in all the pitches (that means "links").
2. Lay chain flat and straight, push all links together tightly. Measure the length.
3. Hold one end of chain solidly and pull [50 lbs] to full length. Measure length and subtract distance.
4. Distance must not exceed 0.4" (1 cm). More equals worn chain.
That's about as Jinglish as I remember it...
The 'stretch' rating for chains actually is appointed by the sprocket manufacturers. The amount of extra pitch length that a sprocket tooth can accommodate also determines how far stretched the chain can be before it impacts the leading edges of the teeth, causing wear to both and loss of power. This also causes shock that tosses the lube out of the chain. For these bikes with JIS or ISO-built sprockets, stretch must be NO MORE than 1.5% of total length. On the 750K1-K6 bikes, this is simple as they are [stock] 100-link chains, so 1.5 links is the 0.938" limit. Keep in mind: this method includes BOTH the pressed-together + pulled apart distance, so divide that by 2 to get the figure found in the later manuals, which neither included the wash-down nor the push-it-together-first part, so it only measures the nominal-to-stretch length. This is 1/2 of the 0.938", or about 0.47". By 1971, Honda was getting very conservative with their chain-stretch limits because so many 750s had broken cases from overly-worn chains that they were nicknamed "the $1695 chain breaker" bike, after their sales price.
