First, knocking USUALLY comes from a bad bearing. A hole in the piston will cause fumes in the crankcase and will easilly be noticed by taking the oil fill cap off while the car is running, and looking to see if there are fumes coming out. OR you can pull the dipstick and smell it for unburned gasoline in the crankcase. It really depends on what you want to do with the car as a whole. If you are going to keep the car for a while, pay for either a quality rebuild, or a recondidtioned motor. If you plan on getting rid of the car in the near future..get yourself a junkyard motor or repair the one you have. Most reputible yards will offer at least a 90 day warranty. You can tell if something is wrong within 90 days. Currently in that situation with a F**D Mustang in my driveway. My friend broke the crankshaft. I had to convince him that it was cheaper to just rebuild the bottom end than buy a whole motor. Would have paid $800 for a junkyard motor, plus $350 to put it in, ,plus a nothe $100 for incidentals. Now he is paying about $700 to have the motor pulled and a new lower end put in. Bottem line..how long you keeping the car??
John