Hydraulic cylinder seal kits are used to seal the gap between different parts of the hydraulic cylinder. The function of hydraulic cylinder seal kits is to prevent oil leakage, to assure the working pressure of the hydraulic system, to maintain its power.
Boom Cylinder seal kit is, as the name suggests, fitted into the boom cylinder.
There are regularly three cylinders for the excavator, boom cylinder, arm cylinder, bucket cylinder.
Arm cylinder is the top one, and it's usually be called as top cylinder;
The bucket cylinder is the one that connects and controls the bucket.
Boom cylinder is the one (or the two) near the cabin. Usually, there is one piece boom cylinder for the small excavator under 10-tun; the bigger excavator's two pieces boom cylinders. But there is an exception too when the driver will remove one piece for a special purpose, say at some particular working place, it is convenient to work by one cylinder. Soft remind: all refit job should be operated by an experienced technician.

Before installation, you must to clean the counterbore first. The counterbore is in the head and the outside diameter of the wiper seal's metal shell with tipical cleaner. Clean the counterbore and the metal shell until they not get a clean white towel changed color.

Note:
There coated with a corrosion inhibitor for the metal shell of the wiper seal. So need o remove the inhibitor to make sure the wiper seal could bond to the cylinder head properly.
After cleaning, do not touch the counterbore or the metal shell because the oil from your fingers may prevent a good bond. Holding the the seal by the lip only.

The ring seal will be larger than the bore. It is decided to maintain an interference fit at assembly. So please don't trim the seal. Anything change of the ring seal will affect the sealing performance of the rig seal.
Before installation, it is better always inspect the piston and fix or clean it for sharp edges, nicks, gouges, or dir. They all will affect the sealing ability of the ring seal.
Then follow the correct operating progress and specifiions to do it.