Sorry for the late post but I am fairly new here and I am reading up on some of the older threads.
Last time we bored one to 4 inch I sonic tested 4 or 5 blocks before I found one that was worth boring that far... and then I put in a 3/4 grout job. It worked well for the guy on a oval track for quit a few seasons.
As a side note we now only leave a inch or so for water when we grout any type block... more strength and no cooling problems... street or track.
