TPI in metric? And a few other leadscrew confusions for newbie

[IanParkin]

TPI stands for Teeth/Threads Per Inch so on a imperial lathe you would have a 4TPI leadscrew perhaps so meaning that one turn would advance the cut by 1/4 inch
Bolts are specified in tpi so an imperial bolt would be half inch by 12tpi

Metric bolts and leadscrews are measured by pitch so in similar amounts to above the lathe leadscrew would be 6mm pitch and the bolt would be 12mm x 2mm pitch

You can also have 2 start or 3...4 start leadscrews or bolts
this means on a 2 start that the pitch is doubled so the carriage will advance twice as far but there are 2 thread running concurrently..you can see the helix angle is steeper on a multistart thread

On your example i cant help you on the 8-4 bit