Question: Can I modify the shape of Wing Spar Joiner to provide a better fit in my particular wing?

Answer: Yes with cautions. See Below

Better Answer: The Wing Spar Joiner is a very important component. Looks simple, is simple... but the most vital piece in the entire wing.

Yes you can modify the shape of the wing spar joiner if you really have to provided that you strictly pay attention to the following cautions:

1) Do you really have to? Understand what you are doing and why.

2) By how much? Sanding a corner that binds or reducing the thickness a couple of thousands of an inch at each end to assist with insertion or trimming the overall length by 1/8" or so is one thing... but doing anything more than that requires careful consideration. If you require big changes, make sure that something else is not wrong.

3) Remember LOTS of 30 Minute Epoxy must be used when gluing the wing spar joiner and wing roots together. LOTS, fill up the entire cavity and cover all surfaces and the wing roots thoroughly. Never use 5 minute or 15 minute Epoxy for joining wings... 30 Minute Epoxy only. Lots of it!

4) Never score or mar the surface of the wing spar joiner with a knife, scribe, vice grip teeth, plier teeth or meat tenderizer. Each time you indent the surface of the wing spar joiner you create a risk of a stress fracture later. If you want to roughen up the surface a bit, use 80 grit sand paper and blow away the dust before glueing into place.

5) If you want to make the vertical dimension of your wing spar joiner thicker (i.e. in the dimension that lies from the top to the bottom of the wing) you can use 1-16-3/16 thick balsa or other strip wood if you prefer to eliminate any gap in the fit. Any gap in the fit is normally found only on trainers where we use a wing spar joiner for several different models each with a different degree of dihedral. By making the wing spar joiner a bit narrow, we can vary the dihedral a few degrees between models.  Adding strip material to fill the gap is fine as long as you can flush fit the wing roots of your wing with the spar joiner inserted AND as long as you can still insert plenty of 30 Minute Epoxy into the joiner cavities in the wing.