[mjf454]

Are there different grades of metric bolts? I need longer spindle bolts, and I want the right strength bolt.

[35th_Anniversary...]

Never heard of different kinds of taps. Typicall the bolt breaks rather than the thing it is screwed into. When you think about it the item that is tapped is larger (and stronger) than the bolt.

The majority of bolts do break at the first thread. Cutting threads into bolts adds a stress riser. By rolling threads onto bolts you are maing the bolt stronger by cold-working it AND you are moving the grain structure of the metal. Think of the metal as having layers. When you look at it sideways you are cutting VVVVV across the layers and are weakinging the structure. Rolling threads moves the material and keeps it continues.

As far as I know all internal threads (tapped) are cut.

Internal threads can be cut (using a die) or rolled (using a sophisticated forming machine). Roled threads are far superior to cut threads.