Skywatcher 250px screw for primary mirror clamp

[Swift]

Hi all,

Just in the middle of a mirror clean and ... 🤦🏻‍♂️ Hoping someone might be able to save me a lot of work and point me towards a replacement screw for the broken one - I know it’s a long shot!

Many thanks in advance 🙏🏻

[Swift]

Update: this has gone BRILLIANTLY 😖😖😖

[tooth_dr]

5 minutes ago, Swift said:

Update: this has gone BRILLIANTLY 😖😖😖

Oh dear 🙁 I’ve the same scope too - not an ideal situation at all.  Should be easily removed with vice grips and replacements will be pretty standard M4 or M5 (guessing from pics) bolt and easily obtainable.  Not trying to rub salt here but was there a reason you needed to remove the mirror from the cell? 

Edited October 29, 2020 by tooth_dr

[Pixies]

measure up the unbroken bolts and get new ones here:

https://www.accu.co.uk/en/64-low-head-cap-screws#elasticsearch_id_feature_955463=955463_473&id_elasticsearch_category=64&orderby=ranking&orderway=asc

 

I'm assuming they have been held in with something like Loctite Red. Can you get localised heat to the stuck bolts - with a soldering iron, say? Heat 'em up then try removing them with a big pair of pliers?

[Swift]

Hasn’t been cleaned in a decade and it’s in fairly rough shape. I know I don’t want to be cleaning it by the minutes but it was way overdue - I’m still unsure whether the marks will turn out to be stubborn dust or something worse due to damp conditions. Now, of course, cleaning’s going to have to wait until I can re-house it. I reckon that’s responsible for the degradation of the screws too.
 

This is why I wanted a pro service - I emailed someone reputable local to me but he said he wasn’t taking on any more scopes and urged me to watch some good video tutorials and do it myself ... now I’m sitting here with a trip to a dark sky site booked for next weekend and no scope 😔 Hope I can get these screws worked out and replaced by then! Bit worried about the short one.

[Pixies]

1 minute ago, Swift said:

Hasn’t been cleaned in a decade and it’s in fairly rough shape. I know I don’t want to be cleaning it by the minutes but it was way overdue - I’m still unsure whether the marks will turn out to be stubborn dust or something worse due to damp conditions. Now, of course, cleaning’s going to have to wait until I can re-house it. I reckon that’s responsible for the degradation of the screws too.
 

This is why I wanted a pro service - I emailed someone reputable local to me but he said he wasn’t taking on any more scopes and urged me to watch some good video tutorials and do it myself ... now I’m sitting here with a trip to a dark sky site booked for next weekend and no scope 😔 Hope I can get these screws worked out and replaced by then! Bit worried about the short one.

I'd give him a shout and se what he suggests. He might have come across something like this before.

Could be worth asking him if he knows of any similar scopes that might be loaned out?

[Swift]

Cheers Pixies. I’ll try B&Q in the morning to see if they have the right thing ready to go, and your link if not 👍🏻

[Pixies]

Beware black-painted bolts. The paint can strip off, if you're not careful, and you'll be chasing it all around the cell. Try and get black stainless steel.

If you want to use threadlock when you put them back, use Loctite blue so they can be removed in the future.

[johninderby]

Screwfix does M5 and M6 stainless socket head bolts so might be worth checking their website. 

Edited October 30, 2020 by johninderby

[Davey-T]

1 hour ago, Swift said:

Cheers Pixies. I’ll try B&Q in the morning to see if they have the right thing ready to go, and your link if not 👍🏻

You'll be lucky to get anything similar in B&Q Kays on EBay do quick delivery.

Dave

[johninderby]

Just check stock at your local screwfix online and collect from Screwfix same day. 

[DaveL59]

damp conditions could well have been a cause as the different metals involved will cause things to corrode,  looks that way from your pics and see that on cars a lot too esp with salty roads here. Assuming you don't need to lock the threads with loctite blue as Pixies suggests, then a very very light smear with grease on the lower threads (vasoline will do) then wipe the threads clean before reassembling will help stave off future corrosion and make maintenance easier. Stainless bolts won't prevent corrosion in that you still have 2 different metals involved. Black japanned screws should work but as mentioned, avoid painted ones.

To get the remains out, heat helps, a small butane torch will get things hot quickly and a light spray of WD40 at the threads will then wick and help crack the corrosion. Of course you'll then need to use degreaser to clean up before you refit the mirror. Heat alone may do enough but try keep it focused in the area you're working on so you don't distort things. Might have to retouch the paint after too.

[wulfrun]

Suggestions: first - PATIENCE - if you break the stubs off you'll be in a worse position (obvious, I know). Try and get a small butane blowtorch on the alloy (?) parts and keep the stubs as cool as possible. Use something like mole-grips or, where there's enough thread left, get two nuts and lock them tightly against each other. Technique is everything; if you can get them to move, even a tiny bit - STOP - re-tighten slightly and repeat, rock them back and forth going slowly in favour of "undo". Penetrating oil (like Plus-Gas, not WD40) will help, get something on the stubs now and leave it at least for a day before you even start trying. Diesel works well as a penetrating fluid but take care with that blowtorch then.

A light smear of coppaslip or equivalent (copper grease) on the new screws will reduce the chance of siezure in future, as will wrapping the threads with teflon plumber's tape - if you can get the threads back with it in place. I would replace them with allen-headed blacked screws if you can get them (should be easily available). If you have access to someone with engineering stuff, a "plug" tap run into the threads would be well worth the trouble, to clean the threads out.

Edited October 30, 2020 by wulfrun
added info

[Swift]

Thanks again all. Got the job done, nice clean mirror safely bolted back in and collimated 👍🏻