Low compression, why?

[samaritan]

After new piston, cylinder, rings and seals, compression is only 115, and it wont start, its a Stihl ms 251c . The piston and cylinder look fine, and so do the rings. I added oil to the cylinder and compression went to 125, still wont start.

 

[Patrick62]

that should be enough, it is something simple.
Spark?
Fuel?
We can assume it has air...

 

[Clint Eatswood]

Had a similar issue recently and some member reminded me to check piston ring end gap. That was my problem.