Update: The C-5 was in production for many years; not always by the same sub-contractor, and it has been tweeked a bit, over the decades. It's been many years since it was made in the good old USA. The OD of the white version (mine) is darn close to 5.75", or about 146 mm. Parallax currently lists their 5.5" ID rings as fitting the C-5. Well, it doesn't fit all of them, and certainly not my version. Maybe the old orange tube version was 5.5" OD, but not the more recent white tube version. I HAD ordered the Orion 140mm tube rings, and cancelled once I actually measured MY C-5 tube OD. Parallax doesn't list 146mm, so I ordered the Orion 144mm ID rings instead, and if they are too tight, I will re-machine them a bit larger. I cancelled the order for the 140mm rings. There is sufficient material on the Orion rings to open them up by another millimeter in radius, easily. That is sufficiently small to permit it to be done using hand-tools on aluminum rings, in my "Woodshop" tradition. I have to hand it to Orion, cancelling orders is a painless piece-of-cake. Keeps me coming back to them. I appreciate their customer service, very much. So there is the latest chapter in adapting a 40-year menagerie of OTA's to a newfangled GoTo mount. Stay tuned.