I am in the process of having my 1954 Mercury 256 Y-Block engine rebuilt. The standard size cylinder bores were good so the mechanic lightly honed the walls and I did not have them bored.  One of the pistons had a small piece of the bottom skirt casting broken off but no damage was done to the cylinder wall.  My mechanic recommended that I replace all eight pistons which bought from Summit.  The machinist who did the work on the cylinder heads and installed the new rod bushings noted that the replacement pistons are 63 grams heavier than the OEM pistons.  He recommended balancing the entire engine claiming that the increased weight of the new pistons is likely to make the engine run with a vibration.  I have already put over  $2K in this engine and I don't really want to go much higher if I can avoid it.  This engine is going in a car that will be a low-stressed "Sunday Driver".  Is the use of these slightly heavier replacement pistons likely to result in an engine vibration or is the machine shop just trying to get more work for themselves?    Thanks, Alan S. Freeman