Can i just replace Head Gasket on 1991 Buick Century?

No,you have to replace all gasket related and connected working parts too.Its not easy replacement.
To replace the head gaskets, you have to take a lot of other parts off, and you'll need to replace any gaskets for those items. Intake manifold, valve covers, possibly timing cover, exhaust pipe gasket or exhaust manifold gasket, intake manifold etc.To get the heads off, they sell what is called a "Valve Grind" gasket set that is what you need, it will have all the gaskets in it which you need.Valve Grind can be bought from auto parts store or from Car dealer.

This details will help.Thanks.

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