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Skywatchers do not need an extension tube for visual.
The "extension" you may be referring to (Rick) is actually just the eyepiece adaptor.
Admittedly, the 1.25 adapter does unscrew, to allow a T-Ring to be attached for photography, but it is not an additional extension tube as such.
I have found the clamp screws need to be only just touching the mirror surface, very lightly.
I don't know exactly with the 8" but I know on some sizes of SW Dobs there is an additional set of thumb screws in the side wall of the cradle. (they were on my 12" SW but not on either of my 10" SWs.) I think they try to make one mirror cell cradle size fit two mirror sizes. (while my 10 was snug, my 12 could also take a 14, hence the need for the extra thumb screws. I think)
I have found that mirror cell movement (in my case at least) had nothing to do with mirror clamp screws, but was caused by the locking screws (next to collimation screws on base) coming loose.
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