I am looking to buy an 81 Yamaha 650 Special but there is a problem with the engine in that is knocking i have not personally heard the knocking noise because the owner said he killed the engine when it first started making the noise. The way this happened is he accidentally pulled out infront of a car and missed 2nd gear to hurry up and get out of the way and over-revved the engine then that is when it started. He seems to think that he bent a rod but i think that he might have stretched it. i would like to get a second opinion because i am not a professional at troubleshooting motorcycle problems. whatever the problem is i still would like to purchase the bike and he is going to cut me a deal on it. I would also appreciate someone explaining to me what would the repair entail and about what you would think it would cost to have someone fix it or what it would cost if i were to do it. that is if the problem is what we think it is.