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Thread: M-73 Compass

  1. #41
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    Default Re: M-73 Compass

    I just collected my Mk III donor compass from PostNet. So perhaps I'll get the restoration of the Mk 3A finished over the weekend.

    As I said in an earlier post - it's in rather shoddy shape. But that is fine - I am only after the compass card and grub screw - and they both look good. Those are the only parts I need from this one.

    I will however, keep the leftovers in case I run into another Mk III again. I'd actually rather like to have a Mk III in my collection.

    Click image for larger version. 

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    Peter - as you can see, the cover for the prism assembly is there as well as the screws. I'll see whether it fits on the M-73 when I strip it down this evening. I've got no idea how interchangeable parts are. Although they look very similar in terms of style and size etc, they are completely different models with about 40 years in between their "development". I say "development" because although the M-73 was introduced in 1973, they still manufacture them today. The Mk III was introduced in 1932 and production stopped sometime during, or just after the war years.

    Strangely enough though, and this is what makes these compasses so confusing, is that the Mk 1 compass was introduced in 1966. This was a Mk III compass but converted to Mils. The differences in the Mk numbers and dates depend on whether they're roman or normal numbers. The Mk 3A is a bit of outlier in this regard - originally designated the Mk IIIAUST, it was shortened to Mk 3A.

    Anyways, all these compasses were then superseded the M-71, M-72, M-73 and later the M-88, the last two being the only models still in production.

    If the prism cover / screws do not fit, I believe I will be able to find spares for you. Spares are quite readily available for them.
    Ian de Villiers

    Patrol 4.5 GRX
    Jurgens XT65 2x0 with Super Select Zero
    ORRA: H80

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  3. #42
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    Default Re: M-73 Compass

    Thanks Ian
    Beat-up rat rod of a '96 Nissan Patrol that bears the evidence of many wonderful adventures (and a few stupid indiscretions).

  4. #43
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    Default Re: M-73 Compass

    Right, so more worky worky this weekend, but I am almost done now. After disassembling the donor compass, I decided to bastardise my restoration and use the Mk III compass card holder / oil well in the Mk 3A. Yes - this means it is no longer a Mk3A, but I was just impressed with the quality and condition of the oil well internals as opposed to the Mk 3A one. So I cleaned it up, did my doctoring with glow in the dark stuff and sealant and started filling it.

    There is still a bubble in the oil. I'll remove it when I have time. I did not want to fill the oil well too much as I was not sure in what condition the gaskets were in. So I first wanted to check whether they gaskets help up.

    Two things left to do.

    1: I've got to tap a thread where I had to drill out one of the prism cover screws when I disassembled the thing a few weeks ago.
    2: There is a design difference between the Mk III and Mk 3A in terms of the bezel grub screw. On the Mk3 A, the screw is internal to the compass and sticks out the side. The grub screw screws onto this. On the Mk III, the grub screw is machined with a thread which screws into the compass. In short, the grub screw will not work as it. I have ground away the grub screw thread, and will drill a suitable hole for it when I can. No idea where I am going to find a suitable tap for that, but I'll worry about that when I get there.

    Some photos. Note that I had not refitted the prism cover or the crystal guard in the photo where it is open.

    Click image for larger version. 

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    Edit: Oh - I forgot. There is a third thing I need to do. Francis Barker compasses have a rubber ring on the base. The restored one is missing this. I will need to source some suitable rubber, cut this to size and stick it on.
    Last edited by iandvl; 2024/05/20 at 09:42 AM.
    Ian de Villiers

    Patrol 4.5 GRX
    Jurgens XT65 2x0 with Super Select Zero
    ORRA: H80

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