What are metric fittings?
The designation M5, M6, M8 etc. refer to the thread size, rather than the across flats size, for the fastner. This is the normal way of describing fastners, so Screwfix would seem to be using the correct description. The length given is from the underside of the head for all types other than countersunk, which are given as the overall length from the top of the screw. If you want the thread to go right up to the underside of the head you want a setscrew, as bolts only have set length of thread for a given thread size & will have an un- threaded section on longer items. Socket head cap screws (Allen screws) come in the latter category.
To be pedantic, the designation is incomplete & for metric coarse thread fastners should be M5x0.8, M6x1.0, M8x1.25 etc, where the second number is the thread pitch. There is a metric fine thread series (M8x1.0, for example, or M18x1.5 thread) but, as the coarse series predominates, lack of the pitch value can be taken to infer coarse pitch. Knowing the pitch is handy if you need to drill & tap a hole, as the tapping drill for a metric thread is the nominal diameter minus the pitch - 4.2mm for M5, 5mm for M6, 6.75mm for M8(though usually 6.8mm is used here - easier availability for little difference in thread strength.) The European standard sizes for the bolt heads are 8mm A/F for M5, 10mm A/F for M10, 13mm A/F for M8 and 17mm A/F for M10. For socket head cap screws the socket size is 4mm A/F for M5, 5mm A/F for M6, 6mm A/F for M8 & 8mm A/F for M10. Socket countersunk & button head screws (and grub screws, I think)use the next smaller size (3mm A/F for M5, 4mm A/F for M6 etc).
You may also notice that some bolts have a number stamped on the head - 8.8, 10.8 - these refer to the tensile strength of the bolt & the higher the number the higher the tensile strength. Some bolts have to be high tensile (brake caliper & suspension mounting bolts spring to mind)& these should not be replaced with a lower rating. Having said that, stainless steel bolts are not available in high tensile ratings & plenty of people fit them in the areas mentioned above & don't have seem to have problems - though frame & suspension bolts should be kept as factory items.
Are there any general torque recommendations available?
kg-m/lbs-ft kg-m/lbs-ft
SCREW pan 5mm 0.35~0.5/ 2.5~3.6 Bolt/Nut (hex) 10mm 3.0~4.0 /21.7~28.9
SCREW pan 6mm 0.7~1.1/ 5.1~8.0 Bolt/Nut (hex) 12mm 5.0~6.0/ 36.2~43.4
Bolt/Nut (hex) 5mm 0.45~0.6 /3.2~4.4 Bolt (flange) 6mm 1.0~1.4 /7.2~10.1
Bolt/Nut (hex) 6mm 0.8~1.2 /5.8~8.7 Bolt (flange) 8mm 2.4~3.0/ 17.2~21.7
Bolt/Nut (hex) 8mm 1.8~2.5/ 10.1~18.1 Bolt (flange) 10mm 3.0~4.0 /21.7~28.9
