Edwin, The 1/8" ply spar is plenty, but you need to understand that it stands the full height of the wing thickness. Much like those little ply joiner pieces they give you but all the way through the center section and much farther out into the wing. All the wing cracking occurs right up near the front and this is right where that original spar section goes through. See my highlighted photo of cracks on my wing. Put large lightening holes in the spar as it goes through the center section to save some weight. No hole at or near the intersection between the center and outer wing panels. Bond your spars, wing sections, and joiners with either Gorilla Glue or Elmers Pro-Bond Urethane adhesives. They are superior to epoxy on these foam/balsa/ply structures and the adhesive will foam up inside to fill voids and the lightening holes that you put in the ply. It is like riveting it to the foam. Cool stuff. Leo