Replacement primary cell screw for 130PDS?

[BrendanC]

Hi all,

I just had a bit of a mare with the external screws that hold the primary cell in place which I've just fixed. One of them had stripped but after about 30 minutes of panic, I finally got it out. Now, of course, I need a new screw. So, does anyone know please which are the right screws for the job? I'm talking the ones that secure the primary cell - see photo (complete with stripped screw that has since been dislodged!)

Actually, thinking about this some more, if anyone knows the size and pitch, I might invest in something more robust.

 

 

Thanks, Brendan

Edited August 16, 2023 by BrendanC

[Mandy D]

@BrendanC I've just checked on mine and it measures up as 4 mm with a thread pitch of 0.70, so it should be a standard M4.  Length is 6 mm and it is a pozi-pan head. If the new one does not screw in easily, then it is probably the wrong thread.

[BrendanC]

Excellent, thank you so much!

 

[Carbon Brush]

If you have had a damaged screw in there, what about the state of the threads in the cell?

 

[BrendanC]

Good point, but it really was the head of the screw that was the issue. It had been dodgy for a while 

 

[Mandy D]

5 minutes ago, BrendanC said:

Good point, but it really was the head of the screw that was the issue. It had been dodgy for a while 

 

Dome head socket ("Allen") or Torx screws would be my preference for replacements, but not in stainless steel as it reacts with aluminium.

[BrendanC]

Argh, I did not know this. I'll try and cancel the order! What material would be best? 

And in fact, seeing as I'm clueless with this sort of thing, if you could provide a link to what I need, that would be amazing.

Edited August 16, 2023 by BrendanC

[BrendanC]

How about these? 

https://www.gwr-fasteners.co.uk/m4-x-6mm-socket-cap-head-screws-din-912---aluminium-7075-91248-p.asp

I'm finding it quite a challenge to find anything that isn't stainless steel. How about placing washers or grease between the screws and the scope?

Edited August 16, 2023 by BrendanC

[Clarkey]

To be honest, because you have steel in the tube and an alloy of some sort for the mirror cell, trying to prevent galvanic corrosion completely is tricky. One or other will be affected. Chrome plated mild steel is probably as good as anything - which I believe is what SW normally use. SS is probably the worse choice.

Not sure how strong they are, but you could try nylon.

Edited August 16, 2023 by Clarkey

[Oldfort]

5 hours ago, BrendanC said:

How about these? 

https://www.gwr-fasteners.co.uk/m4-x-6mm-socket-cap-head-screws-din-912---aluminium-7075-91248-p.asp

I've used GWR before without problem.  Accu.co.uk also do M4 alluminium bolts, but the shortest they do is 10mm.

I'm not sure that mixing aluminium and stainless steel would be a big problem in your case, as there isn't going to be much force applied, and anyway you can get anti-galling compound (often called anti-seize compound).