This is a big job and if you want to do it get a manual on it and do it right. You will need torque wrench etc. I don't know about thermogasket. There is a chance the head is cracked but it has to be checked when off.
Replacing the head gasket on any vehicle is a long and complicated process, however if you wish to see basic step-by-step instructions go to the related question on the right side of the page "How do you replace a head gasket?"
.044 inch , according to fordparts.com
A large vacuum leak can do that.
4:11 gear ratio
The fuse box is located in the footwell of the drivers side by the accellorator pedal. They simply pop out
1-5-3-6-2-4
no ECM's in a 1981
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Chevy didn't make a 1981 Celebrity
From the factory only one six cylinder was a option. 258 AMC inline six!
There is no computer.
The oil sending unit is located on the 1981 Pontiac Bonneville 265 engine is located on the Starter Disable Kit.