Henry Hughes and Son Bubble Sextants Mk IX (all models)

Freeing a stiff Index Mirror Bearing

 

Many of the Mk IX sextants, along with many others of this vintage, have got sticky or seized index mirror bearings. You may be able to feel this as a stiff wheel. A sure fire method of checking the index bearing itself is to select a bearing of about 25°  and then carefully, with one finger only, press down on the ten degree selector plate just on the corner above the letter “D”.

The sector plate should move down with a click, and more importantly back up under the influence of a spring with another click. The distance is only about half a mm but if all is in order with the index mirror bearing the clicks being present means any stiffness must be elsewhere. This is good news because stripping the sextant is normally required to clean and lubricate the bearing. A stiff wheel can be caused by several other faults but most can be rectified by careful examination cleaning and lubrication.

With many sextants to free off, I studied this problem and came up with a solution that has worked with more than twenty of my instruments so far. You might like to try it. And I put these photos forward as a guide.

First open the sextant. You might like to dust off any dirt at this stage, there is usually some lurking in the recesses, I find a lot of straw particles presumably part of the original packing.

Next, remove the 4BA alloy split nut on the index bearing spindle. I use a small screwdriver in the slot. Take care this is soft alloy!

 

With the split nut (1) and square centred alloy washer (2) removed, undo the nut (3) and remove it and the spring (4) revealed below it.

You can now see an alloy spindle with a square end. This part is stationary and the threaded circular boss around it is the index bearing housing. This boss has a thread on it (the nut 3 came off it). I have made a simple extractor tool which screws on to the boss and when firmly seated a screw is done up against the alloy spindle carefully (alloy is very soft!).

Look carefully at the extractor in this picture. It fits inside the mirror mount onto the index mirror bearing. The mirror mount can move up and down a small amount (say 1mm) with the extractor fitted.

In cases of really stiff bearings I flood the extractor with oil, apply LIGHT pressure with the screw and then blow hot air over the whole assembly. Again take great care and do not use too much force or heat. Eventually the bearing should break its seal. You can now flush through with a light oil and work the bearing back and fore many times to polish up both surfaces.

This is obviously not as good a method as a complete strip, but will do no harm other than get oil everywhere to trap dust, but you may decide not to worry about that and so this may well be all that you need to get your Sextant working again.

In the second photo I show the gearbox and the four screws holding on the cover I have marked with red spots. There are a lot of other parts that will have to be disconnected before the cover can be fully released. However, with the four screws removed and the screw in the centre of the shades knob turned 180 degrees anti clock, you can prise up and move the cover a couple of mms to the rear, then squirt some WD40 or other thin oil in. A really cheap and dirty method I must agree, but again, it will save you a lot of time and will do no damage apart from, again, getting oil everywhere!

Here is rough sketch of the extractor tool I made and so far have used with complete success.

 

EXTRACTOR

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