The reason for the GMC Piston Rings in the cylinder is to help contain the explosion so nearly all of the force is exerted against the piston's surface and not allowed to blow by on its edges. On each of the pistons in your GMC engine are three sets of piston rings performing their task of sealing the moving joint of the piston as it moves in the cylinder.
In each set of GMC piston rings are three rings. Two of them are the compression rings and one is the oil ring. The compression piston rings are the ones situated closest to the combustion chamber. They are made with a beveled edge that faces this chamber. When replacing the rings on your GMC truck engine make sure this bevel edge is in the correct position or the performance of your engine will never achieve what you had expected it to be.
Below the compression rings is the place where the oil piston ring is located. This is a three piece ring that's primary purpose is to prevent the oil that is lubricant the walls of the cylinder from entering the combustion chamber. It also prevents any oil from the crank case from splashing up in the chamber as well.
When the new set of piston rings is put into place on the piston they are slipped over it by opening them up. This is accomplished because there is a slit in each of the rings. Each of these slits must be offset from the other rings on then piston. This will prevent there from being a pathway of the explosion created in the chamber from leaking directly down into the crank case of your GMC engine. It will also help to transfer more of the explosive power being generated by the ignition of the air and fuel mixture to the piston so it can move in a more efficient manner. The set of GMC Piston Rings in each cylinder are what makes it possible for compression to be generated in the quantity that exists when the engine is running. The piston rings are what come in direct contact with the cylinder walls and the piston keeping them slightly separated while still sealing this junction so maximum power of your GMC engine can be produced on a continuous basis.
There are three different sections of GMC piston rings on each piston in your engine. The upper two are identical and are the compression rings. There are in place to seal the junction of the piston to the cylinder walls as the piston moves up and down inside of it. The lower piston ring is the oil rings and there are three of them working in unison with each other. This is how the piston is lubricated without excessive amounts of oil reaching the combustion chamber and fouling out the spark plug.
There is a slot on each of the piston rings in your GMC engine. These slots make it possible to put the rings in place on the piston. To help seal this junction of the piston in the cylinder, these slots must be offset from one another. If they are lined up or close to it, then there will be blow by every time an explosion in the cylinder takes palace reducing the amount of power that cylinder will produce.
Symptoms of worn piston rings inside of your GMC engine include low compression and the slight burning of oil. If the engine is burning a large amount of oil then the problem is probably the valve seals or a hole in one of the pistons.