Checking out a Mk IX sextant


Many sextants, most of mine, have usually featured on eBay. Often they are reputed to be ex Dambuster raid instruments or they are extremely rare Lancaster or Wellington accoutrements.

There were thousands of these superb instruments made, they were used in all the aircraft that carried a navigator. The fact is that while many did in fact take part in the war, most were used for training and other humdrum duties. It appears they were not very popular with wartime bomber aircrew and typical fixes were reconed to be about 15 miles. Coastal command however, were very pleased on the whole with theirs and often reported positions well within 5 miles.

So, whether yours is a fabulously rare one or just a government surplus item that never left stores, you will want to find out whether it is a worker or not. Be you a buyer or a seller, the state of the instrument should be of prime importance. Either way your monies are at stake. Why buy a dodgy instrument or sell one that is not in good condition. Here are some points to guide you in making a judgement of the instruments condition. You will notice many sellers claim they know nothing about the instrument so will not comment. Ask the seller a question and point him here. If he is genuine you should get some sort of answer.

It is a Bubble Sextant. Is there a bubble? Can this bubble be made, and more importantly can it be removed? This is a common though by no means a fatal problem.

Two Mirrors are used by the instrument. Are they in good condition? The adjuster screws often fret through the protective back cover and allow corrosion to take place in the reflective material.

Although for ground use the averager clockwork is not needed, does it run? For the right duration? A common problem with the two speed attachments is that people will move the gear lever when the clockwork is running. This usually breaks an internal shaft and often a clue is that the pointer is not horizontal, pointing to neither I or II, but rather floppy.

There is a small gearbox full of finely adjusted gears, these are mostly made of aluminium. Are they OK? Does the index knob move from stop to stop freely?

Does the 5 increase button move freely with a sharp click? do the shutters over the numbers move to the correct positions? Often people have removed screws of slightly different length and re fitted them in the wrong holes, causing fowling of the shutter.

The lamps used are, as far as I can tell, no longer available anywhere. Do they work?

Finally. The biggest killer of these old sextants is, would you believe, the lamp batteries. I have recovered several completely corroded specimins but have then been stumped with leaking battery damage. I strongly advise checking that point if you are a buyer and would advertise that fact if a seller. That is about the only point that is not obvious with these instruments.


Bubble manipulation


A brief checklist:

Battery box, is it Corrosion free?

Will it make and collapse a bubble OK?

Are both Mirrors OK?

Is there free movement of the index knob?

Is there free movement of the 5 and 10 knobs?

Are both the lamps OK?


I have written a booklet on repairing and overhauling the Mk IX and can confidently say that anyone with, like me, only average manual skills and simple tools can repair one of these beauties, and, because of their very clever design, provided you don't have anything broken you can restore them to perfectly good functional ground use. The clockwork mechanism is another matter. Here some skill with fine mechanisms is required.




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