twin alpha drives.i change the drive fluid every 20 hours i have found the last couple of times small particles on the magnetic lower plug.is this normal or do i rip them down? they seam to run and shift fine.

I find a "dusting" of fine particles every time I drain the drive oil.
I figure as long as there aren't any large particles I'm ok.
Besides, the particles you are seeing are from the lower gears. The oil only has one small passage to flow through to fill the upper gearcase and does not circulate throughout the drive.

If all you're seeing a fine dust I wouldn't worry. If you start seeing chunks, then it's time to tear it down.