If you mean part #13, yes you have to have that. Without it the chain will cut right into the swingarm. I had to replace that part on mine a few months ago. I was doing some other work on it, and while working on it that part just broke up into pieces and fell off. My swingarm is from a 1994 model, and it just dried out and rotted over the years. Yes it's expensive, but that is the least of your problems. You have to completely remove the swing arm, shock, and linkage assembly to replace it. And when I got everything apart, I found a bunch of other rubber and plastic parts that needed to be replaced. I spent 2 weeks and over $300 in parts replacing pretty much all the rear suspension bushings, spacers, and grease seals. The good thing is, now the rear suspension will outlast me. Besides the engine, the rear suspension is the hardest part of the bike to work on, mostly because of that complicated linkage. The first Yamaha single shock design just had the shock connected to the frame on one end and swingarm on the other, with no linkage. I had a 1979 DT250 like that and it worked fine.