OK guys - need some help.

I have a pair of 291-16K drivers (thanks Chris!) for which I have bought new GPA aluminum frams.

Unfortunately the 291s do not have the adjustable locater pins. Just a pair of small screws. So even though the new frams came with the spider shim, they are giving me a headache getting them centered. Without the pins, the shim is no good.

I can play a 1KHz tone and look at it on FFT (arta). As I shift the fram around, the harmonics change. Even tho I can get to a spot with low distortion, or a nice even fall-off, once I start to snug the screws the 3rd harmonic shoots up. I've tried and tried, but every time I tighten even a bit, the distortion shoots up.

Any hints or tips for doing this right. I know some of you guys have practice at this. What worked for you? Thanks!