When I built the first aileron, I got a pretty significant twist in the spar when I riveted the counterbalance pipe on. I was able to get the twist out by holding the aileron down onto a flat surface really well throughout the whole drilling/riveting process.
On the second aileron I decided that I would try to clamp the spar down to a flat surface as I drilled and riveted the counterbalance pipe. It worked really well. Here is a
Picture.