I was trying to change the oil in my sons' 125cc chinese atv (Leopard) and what i thought was the bolt for the oil was actually holding back a spring and long piston looking piece of metal with two holes running one way and one hole running the other.

What is this and how do i put it back in? Is this something to do with shifting?

 

from what i read this is the timing chain spring that fell out, however i can't find out how to put it back in. Does anyone know?

 

Yes, timing chain tensioner.
I'd stick the"piston looking piece" in, then the spring and bolt it up.
See how it feels going together.

 

I did the exact same thing except I was not smart enough to come here before I stuck it back together and I put the spring in first and now I can't get the spring out any help would be greatly appreciated