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Replacement 8mm Tube-ring screws

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Hi guys,

I've been wanting to mount my small refractor onto the top of my 150P via the screw that comes with the scope on the tube rings. It works quite well but the single screw that came with my scope has snapped. I've tried to buy some replacements but the thread pitch is different and so they don't fit, you can just about see the difference in the picture.

Can anyone tell me where I can buy the correct screws or if there is a better way to mount a small refractor onto my scope?

Thanks guys,



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I'll not comment on how to mount the small refractor to the main OTA (I assume you have some reason), but it looks like how you did it is overloading the screw.

The 'default' screw size these days is metric, which is what you will get if you go to a local shop and buy a screw pack, but from a good hardware store or ebay, you can buy all sorts of different threads: BSW, UNF, BSF, UNC, BA ( and maybe more). If you find yourself a thread chart and a micrometer you can sometimes deduce what thread you are trying to replace.

I do have Sky-watcher type tube rings and I think I might have used BSW screws in them to fit a grab bar.

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Hi @Cosmic Geoff, thanks for advice.

Before I source screws I just want to check if I'm going about this the right way as your comment suggests I might not be. Essentially I want to mount my small refractor onto my main OTA to act as a guide scope by screwing it onto the tube rings. Would you say there was a better way to do this?

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I believe those screws are an imperial size and have a 1/4 inch thread because they are intended to mount cameras on. The more precise size is known as 1/4-20 TPI I believe.


Edited by John

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