I am trying to replace my chain on my saw 455 rancher 18". the problem is that the metal band inside the clutch cover seems to not be fitting properly over the wheel causing the cahin to not freely turn. how do i get the cover back on so that metal ring is in the right position?

Question makes no sense. To remove or attach the clutch cover, the chain brake must be released. Period.
Failing that, you're going to wrestle, and lose, with the brake band. For whatever other bits you want to muck with, download an IPL (illustrated parts list) and see how the bits interrelate.

The chainbrake has activated, you need to load the brake before it will go back on, the easiest way to do this is to remove the hand guard from the machine , engage it into the brake mechanism and pull firmly back to load it, you can also remove the clutch and sprocket, the brake will then go back onto the saw, and use the hand guard as normal to load the brake, to remove the clutch jar it off against the compression of the saw, it is a left hand thread, there is a cut a way in the clutch centre to insert a screw driver and strike with a suitable hammer.

The 50/51 and 55 always had the same bar mount, the bar was part no. 508912164 for 15" 508912172 for 18" and 508912180 for the 20", the oil is not fed through the bar adjuster hole.

The reason the cover will not go back on is because the brake has activated, the band is now smaller than the drum, clamp the brake in a vice, be careful not to damage the cover, remove the hand guard from the saw, now use this to engage into the brake mechanism and pull back to load the brake, as for the running issue, i have seen some fuel tank breather problems, when the saw starts to die, just quickly slacken off the filler cap, if it picks up again it is the breather, the breather is situated under the starter cove to the side of the filler cap, replace the the white breather.

Start by removing the chain brake/side cover, make sure the brake is off before removing, remove the guide bar and chain, the clutch centre has two cut a ways designed to take a screw driver blade, the clutch is just spun onto the crank with a left hand thread, insert the screw driver into one of the cut a ways parallel with the machine, and strike with a suitable hammer, this will eventually jar the clutch off against the compression of the saw, it may well take a good few strikes, but it will go. There is a workshop tool availabe, with a 19mm nut on the top, and two legs to fit into the clutch centre, so as to fit an impact wrench and jar it off , part no. 502541601

This may not be the answer you are looking for but I would get a small file and file the ring just enough to make it fit. I really don't understand why it got too tight in the first place but you stated that it only needs a fraction wider to work. Use a small file or drill to open the hole to the size you need and start cutting wood again. The Rancher is a great chain saw.

not sure if the 455 rancher is the exact same as my rancher but when the oil feed on my saw seems slow i remove the chain and bar and clean the chain grove. pay special attention to the end that is near the motor there are a couple of places that allow the oil to flow from the saw body to the chain groove and then subsequently to the chain

Make sure your inertial brake is not engages before taking the cover off the saw. If you brake is engaged and then you take the cover off the side, sometimes you can get it "close enough" and use the forward brake guard to reset it (like normal) but more often than not you will need to use a screw driver to manual reset the brake on the cover before reinstalling. This can be difficult as you no longer have the added lever of the front handle...

You have to pull it back as if it were still on the saw until it clicks. I may be hard, but that is what you have to do.

the rancher uses a plastic oiler gear to a metal gear.the plastic gear always looses in the battle.pull the clutch off and clutch drum.under the drum you will find the plastic gear.replace it and all should be well again