I've measured drive chains like that, but not sure if it's accurate with multiple row chains like the primary. In any event, less than 1/2 inch arch doesn't seem that bad.

The only directive I remember on primary chains in to change it when you're unable to get correct adjustment any longer. I doubt I'd push it that far, but if you had plenty of adjustment left when you took it apart that chain is probably fine.

What do the teeth on your comp. sprocket and clutch hub look like? If they are getting pointed or beginning to develop a curve they need to be changed. They should have about 1/16th inch shoulder across the tip, maybe a little more. New sprockets - new chain, regardless of condition.

 

From the Diamond Chain Company, I hope you can make sense of it, LOL

How can the wear in a roller chain be accurately measured and when is the chain considered worn out?

Roller chain can be measured relatively accurately by measuring the tight span of the drive, when the drive is OFF, by measuring from center of one pin to the center of another. The more pitches (pins) contained within the measurement increase the accuracy. If the measured value exceeds the nominal by more than the allowable percentage the chain should be replaced. The maximum allowable wear elongation is approximately 3% for most industrial applications, based upon sprocket design. The allowable chain wear in percent can be calculated using the relationship: 200/N, where N is the number of teeth in the large sprocket. The relationship is often useful since the normal maximum allowable chain wear elongation of 3% is valid only up to 67 teeth in the large sprocket. In drives having fixed center distances, chains running in parallel or where smooth operation is required, the amount of wear should be limited to approximately 1.5%.

 

Someone told me once, wait until it runs out of adjustment. Dunno.