Top cylinder lower compression phenomenon

[Beno]

Picked up another engine did a compression test and again the top cylinder is 25 PSi below the rest of the three..

This is the third engine like this.  Is this some normal thing that I was unaware of?  Perhaps the top cylinder is less likely to get the cooling water?   Or the top cylinder heats because heat tends to go up?

 

Top cyl closer to outboard heaven?

 

 

[labrador-guy]

Need more info Beno!   What are you working on?  Has the motor been setting a long time?  If two cylinders are 100 and one is 75 that not so bad.  If two are 75 and the other is 50 that’s not so good!   Compression usually changes once a motor comes up to temperature a couple times.  Motors that have been setting a long time compression don’t mean diddly!  JMHO

dale

[Beno]

Oh sorry I actually noticed this started long ago with a white 1961 Mercury 100 10hp .. top cylinder low compression and I tore it apart and it’s had a stuck ring.

Then a few years later I had an Evinrude super 25 1996??? and the top cylinder bad compression.   Tore it down and the head gasket was bad on the top cyl.

But lately it’s been a rash of 86-97 4cyl.Mercury Force 120s.  Three of them. All have the same issue when i get them in  its 123/150+/150+/150+ psi three motors in a row like this.. cant be a coincidence? Can it?

[Author]

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