Hall Depth Gauge

[Admiral]

Have any of you flathead experts heard of this tool before? It's called the Hall Depth Gauge. My dad's buddy gave this to me awhile back since he knew I was into old cars; I think it was his father's. After reading the instructions several times I'm still not certain how to use it properly. Any pointers? Looks like it could be super helpful for setting valve clearances with solid lifters.