Just a guess, because I was concerned about how the o-ring sealed when I had mine open. I made some measurements, I'll see if I can find them.
Also, there was a service bulletin about the bore depth, and some engines were produced with 2 o-rings per end of the thru-bolt.
http://www.pcarworkshop.com/images/f/f8/1010_1991_610791geolab.pdf