C8 - Finder screw size

[dweller25]

I would like to change the standard 6x30 finder bracket on my C8 to a Synta type shoe but need replacement screws.

I thought the screws were UNC 8-32 but I bought some of these and they do not fit.

Does anyone know what the size of these screws are please ???

[perfrej]

David,

I just checked on my new EdgeHD 9.25. It is not M4, and if you say it is not UNC (doesn't look like it), it just has to be 8/32 UNF.

Just a guess

/p

[dweller25]

Thanks /p, it's definitely not metric, it could be UNF 8-32. It would be great if someone could confirm that .....

[Mr Spock]

I don't know the size, but they are Imperial rather than metric. I had a similar problem attaching my ADM mini dovetail bar - in my case I didn't have Imperial hex keys.

[AstroTim]

Realise this is an old thread but didn’t want to start another for the same topic.

I am having the same problem as @dweller25. I measured the existing screw as 32tpi and 0.154” (3.92mm) major diameter. I ordered some 8-32 UNC Allen head screws but they don’t fit (one turn and they bind solid). The screws I ordered have a major diameter of 0.161” (4.09mm) and therefore a fraction wider than the existing screws. The nominal major diameter for 8 UNC is 0.164”.

I’m not sure if it’s just a tolerance issue or whether they are a different screw thread entirely. They aren’t M4 as that would be 0.7 pitch (about 36tpi). They also aren’t 6-32 UNC as I have some of those and the diameter is too small. I found another thread on here that suggested 8-24 UNC but that isn’t a UNC size as far as I can tell, and I’m fairly sure the existing screw is 32tpi. I also can’t see 8-32 UNF, as mentioned above, as a standard UNF size (32tpi would be a 10-32 for UNF) As far as I can tell, an 8-32UNF would be identical to an 8-32UNC anyway.

I’m a reluctant to try and force the new screw in, even though it would probably just cut out a wider thread with the OTA being aluminium.

Has anyone had any success finding a replacement screw for the finder bracket? Is it a case of ordering several 8-32UNC from different suppliers until you find one that fits? Would the type of screw make a difference (the ones I ordered were high tensile steel cap heads)?

I can’t believe how much time I’ve spent simply trying to fit a finder bracket, or how much I’ve learned about screw threads in the process! The scope is around 3 years old so assuming it’s manufactured in China, rather than US like the older ones, if that makes a difference.

Regards,

Tim

[Alkaid]

Hmm...I replaced the bracket on my C8, I bought the screws from eBay ages ago...I might be able to track down what I bought in my order history...bear with...

[Cosmic Geoff]

See here:

And here: https://www.cloudynights.com/topic/598018-celestron-edgehd-screw-sizes/

Don't force the screws. If you get the right thread they will turn easily and not jam partway down.  My C8 has Imperial screws but I believe some may be metric.

Edited August 20, 2020 by Cosmic Geoff

[Alkaid]

On 19/08/2020 at 19:14, Alkaid said:

Hmm...I replaced the bracket on my C8, I bought the screws from eBay ages ago...I might be able to track down what I bought in my order history...bear with...

Checked my purchase history last night, unfortunately it takes me back to the page where you then select the screw size from a drop down box....no idea what I actually ordered....I’ll check my spares draw later as the plastic bags that the screws arrived in may be labelled (and hopefully I didn’t use all the contents and then chuck the bag).  Not being very helpful am I...apologies.

[AstroTim]

Thanks for the replies, and apologies for the delayed response, I didn’t get notified of the replies.

I’ve read the referenced threads and the consensus across those and most threads I’ve seen to date seems to be 8-32 UNC, which is what I tried but don’t fit.

Looking at the cloudy nights thread, the suggestion is that the rear holes have been drilled and tapped slightly undersized from the standard (although presumably with screws to match) and that you need to force the replacement 8-32’s in. Still not sure I like the sound of that.

I’ll probably try a few different packs and types of 8-32 UNC screws and see if any are a better fit.

 

Tim

 

[AstroTim]

Looking again through the various thread specifications and I think it might actually be a 5/32 BSW (Whitworth) thread. Just ordered a pack so will update once they arrive.

[AstroTim]

Just got hold of the BSW screws and they fit the finder bracket holes perfectly.

So, to confirm for any future viewers of this thread, the correct size screw for a circa 2018 C8 rear finder bracket hole is:

 

5/32 diameter (32 TPI) British Standard Whitworth (BSW) thread

 

Tim