Hi," if the compression ratio will decrease or increase with this installation?"
Welcome to the Benelli Forum.
I have already done what you thinking about to do ,and I must tell that I feel good about it. I am restoring my Benelli 750 sei ,76 model and it is major restoration project.
But I go with the technical info now:
Honda 550 four bore is 58.5 X 50.6 and compression ratio is 9.0 : 1
Benelli 750 sei bore is 56 X 50.6 and compression ratio is 9.8 : 1
So to my understanding and therefore the answer to your question is : it will decrease.
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Benelli 750 sei being a Honda 500 cc Four carbon copy engine to some extend with addition of two cylinders gives clue and one may guessed right that some parts will be compatible for both engines.Pistons are the parts that look the same and the other part is oil sump of Honda650 which holds half a liter of oil more.The good reason for using deeper oil sump part is to eliminates the overheating problem that can damage badly Benelli 750 sei engine.On the subject of gasket my mechanic recons that the original Benelli 750 head engine gasket will do the job and as for other question I have no answer at this stage.