NS40C Low Compression

[Zbhsmith]

The top cylinder has 30lbs of compression and the bottom one has 120 lbs. What would cause this other than burnt piston or ring failure. What I need to look for when I open it up. How hard is it to change a piston and rings if the bore is fine?

 

[JB]

Re: NS40C Low Compression

Lots of things can cause that, Zbh. Most common is a failed head gasket. If that is it you could have a warped head. Check head and block surfaces carefully before just slapping in a new gasket.

Also possible are broken rings, broken/burned piston, grooved/scored cylinder wall (usually goes with broken rings). Fixing those is a bit more complicated, so I will leave it to Tohatsu Guru to address them.

 

[Sea Rider]

Re: NS40C Low Compression

Start by re torquing head gasket to factory settings, if ok, check gasket condition by removing it from head, also check for warped cylinder head if suffered a high overheat. If ok, probably scored cylinder walls not perfect round, piston damage, stuck or worn piston rings. If severely scored, probably will need to rebore for the 0.5 mm piston kit, change all gaskets, check crankshaft upper & lower ball bearings & piston pin needle bearings for play/wear. Get a service manual to have some idea or send crank case to a machine shop to measure inside cylinder wear, for the work to be done right.

Happy Boating