2:1 Internal & Spur Gear

[Dan Llewellyn2]

This is not exactly EFI related - but my local sources have
been unable to help me.

I am trying to build an experimental transmission.  It uses
an internal gear (teeth on the inside) and a spur gear (teeth
on the outside) that rotates inside the first gear.  My only
requirement is that the internal gear needs to have exactly
twice the teeth of the spur gear (and mesh, of course).

I went some automatic transmission shops, but all the sets
of internal and planetary gears won't work because the planets
are too small.  I was hoping internal and planets from different
sets might work,  but haven't found a match.  I figured that
buying some used automatic transmission gears would be
the cheapest way to get some quality gears.

Any ideas?

TIA

Dan L