The piston slapping is on a 240 that my niece is driving. (freshman in college) This past summer she ran the car into the red (hot) a couple times until her father replaced the thermostat. I also found the car with only 2 quarts of oil in it about a year or so ago. Its a 1990 with just over 100,000 miles. (sad) Had no rust or dents until my sister put a dent in one door and my niece has now got red and yellow paint on the other plus a tail light I replaced and a front head light mount with turn signal cover cracked. So the car has been suffering from neglect. But before I go a head and change the antifreeze/hoses/cap/rotor ect I need to know how to deal with this piston slap.

IN the Bentley it described the low pitch slap as three different things pertaining to the piston, rings ect... I did a compression test tonight and found cylinder #3 was 20 psi lower than the others. When using a hose I can hear the knocking/slap sound coming from that area as well. Can the one cylinder be rebuilt without too much pain. Can I do it? I still need to read up on it, just thought I'd get some advice first.

The cylinders read 140psi after hearing the engine make 5 rounds. The #3 was at 120. Compression isn't too bad but before letting my niece drive the car too much more I need to figure out what to watch out for in way of more damage or how should it even be driven til a rebuild? Thanks.