I had that happen to me. My car had been sitting for quite some time. I got the car running and after about 200 miles it developed a rod knock. I took the bottom end apart and found that No 4 rod had too much clearance. I rectified it and put it back together. about 300 miles later it developed another rod knock. I was disappointed to say the least but I was determined to find out what was going on. After a lot of looking and some swearing I finally found the problem. While it had been sitting water had run past the spark plug into the cylinder and down the rod and had rusted a flat spot in the crank Luckily i found it before I lost my mind! Anyway, after that I decided to do a complete engine rebuild. I had to bore it .060 over to clean up No 4 cylinder, had the main caps rebabitted and the rods redone, put that sucker back together and it was on of the best engines and quite powerful for a car of that era I ever had.