Chain Stretch

My previous post referred to a replacement chain that I had to fit to my Giant Defy2 road bike. The wear on the chain crept up on me really and when I measured the chain it had stretched by 0.75% If you havent come across the phenomenon of chain stretch before, this might not seem like very much, but most experienced cyclists wouldnt let the chain go above 0.5% stretch.
You measure the length of the chain using a tool that slots in between ten or so links.of the chain. Depending on the length of those links, the tool slots further into the chain link and shows the total % of chain wear. The term "stretch" is a little misleading. What actually happens is that the chain pins wear and allow the links to be a little further apart.
This has a dramatic effect on how the chain runs around the sprockets on your bike and the most affected is the smallest sprocket on the rear of the bike.
This picture shows the 12 tooth sprocket that I have just removed from my road bike. You can clearly see that the tooth shape has changed with a distinctly different angle on one side of the tooth to the other. There also appears to be a ridge on some of the teeth where the deformed metal has literally been pushed out of the way.
This led to the chain slipping when I selected that gear. Other gears were un affected but the 12 tooth sprocket was almost impossible to use as any pressure caused the chain to ride up the worn slope and slip.
There is only one option and that is to replace the worn sprocket.
I could have simply replaced the one sprocket (most bike shops sell them for a few pounds each). I decided to use this as an excuse to enhance my gear range. The Defy 2 comes with a 9 speed cassette ranging from 25 teeth to 12 teeth. I decided to go for 28t-11t.
I ordered one from Chain Reaction Cycles but in my hurry I ordered a 10 speed cassette. Half an hour later I was checking over the order and spotted the mistake but it was too late to change the order as it was a Sunday. I rang them on Monday morning only to be told that the order had already been despatched. Chain Reaction offer a 365 returns policy (Ive since found out that all online sales have to offer this by law) but the service is fantastic! There is a return label included in the box, you fill in the form and stick it to the box then take it to a Collect+ depot - a shop half a mile from where I live. This service is free!
Anyway, the 9 speed cassette arrived on Tuesday and I am really pleased with the extended gear range.
Top tip - check your chain wear every 500 miles and dont let it get past 0.5%