There is a lot of difference in ply. For instance, I've found everything from 5 ply birch to pine, balsa, pine, 3 ply in 1/8" thick ply. I use 1/8" in a lot of my planes for mounting my 46 size engines right to the lugs or double it up for firewalls. After I've drilled the holes for mounts or engine lugs I always wick in some thin CA in the hole and both sides to fill some air space in the wood grain. This makes the wood denser and less likely to crush when tightening nuts, bolts or blind-nuts.