What parameters should I consider when choosing a crankset? I have to change the crankset on the quite old bicycle. I need to order it online so what parameters should I consider when choosing the right chainset? It's used 7 gear bike and I don't know what type of crankset it has.
In general, the basic considerations are:


Number of chainrings and chainring bolt center diameter (BCD) which dictates the chainring sizes that can be fitted.
Bottom bracket type: cranks are generally advertised as being compatible with a particular type, of which there are many.
Crank arm length: measured from center of crank spindle to center of pedal spindle, typically 170, 172.5 or 175mm.
Chainline: how far away from the center line of the bike the chainrings are, this needs to be compatible with the width of the rear hub and position of the cassette.
Rear cassette speeds: 9, 10 and 11 speed drivetrains use progressively narrower chains which means the chainrings can be closer together.


In reality it's not quite as complicated because a crankset intended for a particular style of bike will have the appropriate combination of BCD, choice of bottom bracket types and chainline.

Older bikes used a threaded bottom bracket shell with either a loose or cassette type bearing and spindle. There are a couple of different types of interfaces between the spindle and crank arms. The bottom bracket bearings on an older bike may be worn out so replacing the bearings should be considered - this also gives you some more crank choices as the there are a few types that work with threaded shells.

If you add photos of the existing crank and bottom bracket from both sides we can give you more specific guidance.