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    Re: A-12 Bubble Sextant (Octant)
    From: Douglas Denny
    Date: 2010 Sep 23, 11:43 -0700

    I have had to refurbish my Link A12 too.

    The bubble chamber unit is a push fit into machined holes in the body of the instrument, and has a bowed spring clip, presumably to hold it in position.

    NOTE: Removal cannot be done without first removing the lamp assembly which fouls the bubble chamber being removed.

    Removal _should_ be easy as it just pushes out, but of course with these old instruments the aluminium alloy body can corrode, and even if only slight can act like glue. The only way is to be increasingly fierce with a brass drift of the correct diameter and a leather (hide) mallet from the opposite side, after using an engineering 'easing' fluid (such as "Plus Gas" here in the UK) around the edges of the holes. Introduce the easing fluid a number of times over a couple of days before, first, for it to creep in by capillary action.

    Actually a mixture of diesel, light lubricating oil and kerosene 33/33/33 makes a good easing fluid.
    Heating gently the aluminium body can help expand the aluminium - but care is needed not to heat damage anything.
    -----

    Removing the grub screw is not easy too for similar reasons. Having failed to remove it with suitable tools on mine, I just forced a screwdriver-like tool into the hole to jam it then turned that to release it and luckily it released. I then made up on the lathe a new screw in brass and this time used a PTFE washer I made with flat-headed screw pressing down onto this. The sealing is supposed to be by a needle into tapered hole but is not foolproof.

    If releasing is not possible by any means, then the next thing to do is drill out and re-thread with a slightly larger thread. Use sharp drills only. (learn how to use a grinder and sharpen drills - it is worth doing as there is nothing so frustrating as a blunt drill). Use BA taps for instrument work. If you are very careful and use good centring in drilling-out, and using the root diameter of the threads for the drill you can drill out the screw leaving the original threads intact, which can then have remaining screw material removed and the threads cleaned with a tap. This is not easy but can be done. Using a small nylon or PTFE washer and standard flat headed screw works well for sealing.

    Having a small engineering centre-lathe is a huge advantage for any kind of work like this.

    Douglas Denny.
    Chichester. England.
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