I have a BMX bike with some decent cranks on it laying around. They would be a improvement over what I have on my single speed currently.
They would be similar to this ~~>https://www.jensonusa.com/store/produ...s.aspx?sc=FRGL

I do not know much about BMX mechanics and size but I know that bottom brackets come in euro and american sizes. The bike I would take these cranks off of has a american shell. So would I just take the cranks and spindle and find a euro shell? or??

BMX, kids and huffy type department store gaspipe bikes use a large diameter, unthreaded bottom bracket shell. To confuse things a bit further, I think there are two slighlty different sizes of shells used. Design is principly to accomidate (cheap and heavy) one piece cranks. Better quality BMX cranks are three piece, but with large diameter unthreaded (press in) bottom bracket. There are adapters that allow the use of a standard threaded road bottom bracket in the unthreaded shell, I think this is what BMX riders think of as a "euro" conversion. THis mostly done so that it is possible to use square taper road cranks on a BMX frame. I dont think you are likely to ever find a bottom bracket that will allow the use of the splined BMX spindle into a french threaded road frame or a french threaded BB with splined spindle to accomidate the BMX cranks.

To correct myself, aparently some BMX bikes do come with standard english threaded bottom bracket shells for use with 3-piece cranks. (once you abondon using 1 piece cranks there is less of a reason to use the large diameter unthreaded shell design). I am pretty sure however that "euro" is BMX speak for english threading, I really doubt that you will ever find any newer splined BMX specific bottom bracket parts for a french, swiss, or italian threaded BMX bike since those threadings largely went extenct before the advent of the newer splined BMX cranks.

My bottom Bracket threading is english. My U-08 might be a 80", I have had the bike for about 2 years now and I am still not sure. Maybe it was re-tapped or something.....

So you are saying it could work? right? Just be a matter of getting my chain line right.

If you have English threading on both the bike and cranks, then do all your chainline adjusting at the rear wheel. You may be good to go.