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.

 

[thompsoncustom]

Should start with that compression I'd check for spark first and if it passes that spray some fuel down the carb.

 

[muddy42]

As above, test the spark and fuel. Also why was the piston and cylinder replaced? Have you performed a pressure test? Has the original issue been fixed?

 

[samaritan]

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

my ring end gap is .014 for a 44mm piston ring. Was yours too wide?

 

[Clint Eatswood]

Mine was 0.020". I believe for that size saw you're looking for .006-.010". Somebody can correct me if I'm wrong.

 

[samaritan]

according to my google search, .006 is correct for min. gap Another source said .007-.010"

 

[sean donato]

115psi is pretty normal for a fresh not run in cylinder. I highly doubt it has to do with your ring end gap. Besides that there's no way to tighten it up without either replacing the cylinder or hoping a new ring is tighter.
Pressure and vac test the saw, check spark, fuel etc. It should run with the compression you have.

 

[grizz55chev]

I see no mention of your squish #, might have something to do with it?

 

[stihltech]

That Compression is not causing a no start. Recheck the basics.

 

[Bob Hedgecutter]

Did you ascertain why the saw needed a new top end in the first place?
Find and fix that reason and I am willing to bet the saw will fire up just fine on the compression you have.
New rings need to "learn" the new cylinder- sometimes this can take several tanks of fuel before they truly get to know each other and reach full compression values.