Crafsman Chainsaw C944.419520 18 inch bar need chain replacement

[Jive Turkey]

I need a new chain I got an 18 inch chain it is too small. the bar is 71-36598 an 18inch/46cm 318 S62. The bar I have is an Orogon 7877 EW 200---A195 3/8 Bar, it is a 20 inch bar, but measures 18inch from the guarde cover to the end. I have no idea what chain I need to buy. The book and chainsaw say a 71-3619 chain and is an 18 inch chain. The bar removed is 20inch???? I'm not adding things up here. What is the chain length I'm looking for????