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To maximise the strength of the bolt in steel you need to go in 1 1/2 times the diameter , m8 = 12mm. The bolt will snap before the thread pulls out
In ally it's 2x the diameter

But on a rail...

re bolting stuff. Generally bolt dia less 1 to 2 x pitch. So for M8x1mm pitch thread, 6 - 7mm is typical, though for light loads 4 - 5mm might be adequate. A simple rule is to look at the thickness...

Jpg or gif or png all work and display in the web page. Pdf is good for downloadable documents but doesn't display in the page.