Excellent chain wear tool
"Shimano make the point in the maintenance manuals on their website that often their chains are changed when there is still useful life left in them. My 105 groupset includes a Shimano chain and it is fine but showing over 0. 75% wear on conventional gauges. Shimano recommend using their CN42 tool and there are good videos on their website showing how to use it properly. Plenty of life left in my chain so no need to change it! So a useful purchase for me!"