Crank ARM
 

I have a nikishi custom sport. I busted the thread on the crank arm (the one with the sprocket) were the peddle screws in (dont ask, my dumb ass laizy mistake)

looking at the back of the peddle the only ovious marks i can find are 170, I am assuming its 170 MM crank arm set. Will any crank arm set work as its 170 MM,

Or can i use one from a 10 speed, I gone on ebay and i find some on there but they say they are for a 10 speed, i have a 12

you can potentially have the threads replaced on that crank arm. Don't remember my Nishikis too well, I don't know what crank that bike had.

The crank doesn't care if the bike is a 10 speed or 12 speed. What matters is the bottom bracket interface.

yea i was gonna tap the threads myself, unfortunately i have guestimated that the its a 15 or 14 mm fine thread..I know its not a 16mm...if anyone knows that would be aswome

It should be 9/16th diameter, not sure the thread pitch or type.

You should be able to take it to your LBS and have them install a Helicoil for you.

Pedal thread on a Nishiki has got to be 9/16-20, and if your LBS can't install a helicoil insert (tap hole out larger, install insert), then start going someplace else. The pedals do thread in opposite directions, so make sure they do it the correct way.