Tube rings and dovetail coupling

[Martthebass]

Sorry for the daft post as I should know this but, what bolt sizes do I need to fix together my tube rings and skywatcher/vixen dovetail? The rings came with my WO 81GT - but no info on the fixing sizes. Many thanks in advance.

Edited September 14, 2019 by Martthebass

[Martthebass]

Well I’ve worked out that the holes in the tube rings are M6.....however the threaded holes 🕳 in the dovetail are smaller than M6. Oh well think again.

[Astro-Geek]

I think you might find they are 1/4 inch UNC  so they would stop an m6 bolt from screwing in because they're a different thread, even though almost identical in diameter.

It's in order to enable Vixen bars to be fixed to a standard photo tripod which are all fitted with 1/4 inch 20tpi UNC threads  (or 3/8 UNC).

 

[Martthebass]

Thanks AG, does that mean I need a different dovetail?

[Astro-Geek]

I'm not familiar with that particular design of tube ring, but all of the ones that I've used had tapped holes, so the two clearance (non tapped) holes at the ends of the vixen bar are normally screwed up into the rings, rather than the rings being screwed down into the tapped holes in the vixen bar.