Sthil ms 251 no spark

Did you try cranking with the kill switch disconnected?
 
What did you gap the coil to flywheel at?
 
Is the spark plug gapped at .020"?
 
Also, as most Stihl machines don't have a strong spark to start with,
remove the plug (and wire). 
Put a screw driver in the cap, YOU, hold the steel part of the screwdriver
and GROUND YOURSELF to the steel block(same hand). Pull slowly, then faster to feel
for spark.  
 
I suspect you probably have spark, there's just not much. 
 
.