Looking to replace BB on mass-produced Raleigh M20
 

I have one of those mass produced Raleigh M20 mtn bike. I want to upgrade the BB and crank. What do I need to do to accomplish this.

Obvious answer: a better crank and bottom bracket. :)

A few more details would be useful. What does it have now? What do you want to accomplish with the upgrade? What specialty tools and mechanical expertise and experience do you have?

Right now it has the same run of the mill crank that it sold with. There is no brand name on it. Whatever it was that Raleigh used on their lower end bikes. I have the tools and expertise. I'm talkin thread pitch. I suppose I should just take the BB off and take it to the LBS and have them figure it out for me. I'm doing this for a friend who is new to commuting. My advice to him was to get a more decent frame and build it up, gradually.

The bottom bracket threading is almost certainly English. The bb shell width is either 68 or 73 mm so any appropriate cartridge bottom bracket will work. Other than shell width, the crank you choose will define the needed bb spindle configuration and length.

Correct, the threading will most likely (99% positive) be English, which is 1.37" x 24 threads per inch. Also, you'll probably need to match the spindle length to maintain the proper chainline. You can measure the spindle length (end to end) or a number like "113" (113mm) may be stamped or labelled on the part. For example, a 68x113 bottom bracket is 68mm shell width x 113mm spindle lenght. Then make sure the crank and bottom bracket interface is compatible (square taper or ISIS or Octalink).

is likely to be 122mm or even longer

but 118mm should fit

you might have to adjust the front mech for height and reach to suit the new axle and cranks
count how many teeth are on your chainrings.
some mechs are for 42 tooth big ring and dont work too well with a 48