Hey YZ owners, Today I was taking off my swingarm and forks/etc (in preparation for my new springs and gold valves which get here on Thurs)...and after awhile I came inside to take a break! Well I got on TTalk and went over to the 250f forum and saw a post titled "CAUTION-Your Chain Rub Block May Be Wearing Your Swing Arm!" after reading this post (posted below), I decided to go check it out on my bike! Well after taking off the chain slider (held on my 4 allen bolts) I couldnt believe how much metal was gone on my swingarm! All you guys need to go into the garage and check your swingarm right now...because if you dont you most likely will end up with a huge hole in your swingarm! I think I am going to fix this with some pieces of old tires tubes or weather strpping! I also think this is the cause of "chain slap" under low speed riding? I am really glad I checked this because Im sure a new swingarm costs alot of $! Just thought I would let you all know! Later, Garrett Symptom: The loud clacking sound that comes from the chain hitting the rub block is more than just irritating, it may be abrading the aluminum from the top of the swing arm (at least on my WR). Cause: The swing arm rub block retaining bolts that go through little inverted top hats are suppose to locate and secure the rub block. However, the design allows the top hat to seat against the swing arm BEFORE a significant clamp load is applied to the rub block. This, along with bolt spacing that is rather wide, allows the rub block to slide laterally as well as bounce on the swing arm each time the chain smacks it. The rub block is actually wearing away the swing arm material at the contact surface. To inspect your bike, remove the two bolts on the upper side and lift the rub block. The contact area on the swing arm should be perfectly smooth. Mine was pitted and worn down slightly. My Solution: 1) Modify the top hat shape to provide higher clamp loads. The brim of the inverted top hat needs to contact the rub block before the top hat bottoms against the swing arm. Rolling the brim towards the top of the hat, which is toward the rub block as installed, will ensure that it contacts the rub block and maintains clamp load as the bolts are tightened. Place the top hat brim against a 9/16-inch socket with the hat portion sticking out of the socket. Then, using light pressure from a vise (or hammer?), gently roll the brim towards the top of the hat. How much? Enough to let the brim dig into the rub block and hold as you seat the hat against the swing arm. I'd guess about 2mm. 2) Provide an isolation material between the rub block and the swing arm to cushion the impacts and help secure the rub block. I thought about using a thin rubber membrane (bicycle inner tube) but wanted something that would also help secure the rub block. I had some "Super Yellow" weather strip adhesive on hand and decided to use it instead. First, I filed off the rub block's sharp edge where it contacts the swing arm. I was only concerned with the top of the swing arm. I then applied a thick layer of weather strip adhesive to the top of the swing arm and to the mating surface of the rub block. By applying a thick layer I gained some impact isolation as well as adhesion. After tacking up for a few minutes the parts were mated and pulled apart several times, and then secured with the modified top hats. A two-mile check ride shows that the rub block is much more secure and the chain clack noise is reduced. I'm not sure of long term durability, only time will tell - if any one has another idea let me know. To do the job I had to remove the lower chain roller and c/s sprocket, and then roll the chain off the rear sprocket. I then wired the chain up for room to work. A long hex key is required for the lower front bolt - I had a long 5/32-inch that fit the 4mm just fine.