Hi All,
I may have a definitive answer on this subject. I have a Classic Orange C8 from the early 70's as identified by CTS (Celestron Technical Support). I have encountered the same cell thread issue and contacted CTS.
Hopefully, the following information may assist those individuals with older C8's to avoid frustration and possible thread damage if attempting to deepen the threads for a good fit due to the real root cause of this issue for the earliest C8 models.
When CTS told me, in an earlier email, that the visual back threads have remained the same for 50 years, I challenged them due to the issue we have all encountered here. I even sent them a link to this CN thread for additional evidence. Upon further investigation, CTS replied with the following email this morning:
Good morning Bill,
Well, if Your two-inch diagonal will not properly fit the thread on the back end of the old C-8 you acquired, this allows me to further refine the date of manufacture. You have one of the very earliest Celestron 8s, I am envious of your find!
The thread pitch on the earliest C-8s manufactured from 1970 to 1973, and possibly 1974, was a little coarser than the now industry-standard 2-inch 24TPI "SCT" thread.
We no longer have the specs on this, but it was something like 2"-18 or 2"-20, so current accessories will cross-thread. A solution would be to have a machine shop measure the thread on the tail of the scope and create a short converter adapter from the old thread to 2"-24 in order to adapt current accessories.
I thought my own 1975 C-8 was one of the older ones in use, but you now have me beat by a year or more.
Good luck!
Celestron Technical Support
I did carefully measure my threads and they are indeed 2"-20. Personally, I will investigate having a short adapter ring made out of aluminum to allow me to use my 2-inch eyepiece adapter and other SCT optical accessories I already own for use with my 2 other and more modern SCTs.
I trust this revelation helps anyone encountering this situation by providing a workable solution and avoid additional frustration.