The MK3 Nemag
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Once you have the speedometer apart and examined it. It is likely that you will need to clean the various bits. Remember that the magnet will attract any small ferrous bits on the
bench so make sure that any cleaning is done in a ferrous free area. Even a ‘clean’ bench will have a residual element of small filings and swarf so I suggest using the kitchen table……..
Initially it is best to go for a very gentle cleaning regime using cotton buds, cocktail sticks and washing up liquid. A combination of these will remove most of the dirt around the speedo mechanism
without doing damage. Starting with a gentle process means you can always get a bit more aggressive as time goes on. Putting the whole thing into a shot blaster is not recommended, neither is an over
enthusiastic use of the air line, take it slowly and steadily and there is less change of damage occurring.
It is up to you to decide what sort of finish you are aiming at with the dial and the bezel, as these along with the glass and pointer are all that will be seen. My personal preference is for an
instrument to look as if it has been on the vehicle since new and so a slightly tired or sleepy finish is what I am after. A dial which has been refurbished to an ‘as new condition’ tends to shout
at you and can spoil the overall finish of the instrument cluster.
If the speedo mechanism has got damp then it is likely that the magnetic part may show signs of some blisters of rust, this should be carefully removed a soft brass wire brush is a good tool for this.
The alloy casting should not require much more than a brush with a soft brass wire brush and the cross holes cleaned out with a cotton bud. If there is serious damage or dirt, then a soft steel wire
brush can be used, take care to avoid the bits from the wire brush remaining. The 9BA screw holes can be carefully cleaned out with a 9BA tap. Similarly the 3BA mounting plate holes can be cleaned
with a tap.
Before cleaning and before removing the needle note the position of the drag cup with the needle at zero, I use a felt tip pen for this. The aluminium’s cup should be cleaned with the cotton buds
and washing up liquid it then should be dried and polished, again using cotton buds. I use solvol autosol again on a cotton bud, but other fairly gentle polishers can be used. Make sure you get
rid of all the abrasive polish before reassembly.
The hair spring should be examined with care, and trying to clean this may well result in a broken spring, but gentle brush with a small artist brush will remove any bits of spiders web or similar,
make sure that any brush hairs that fall from the brush are also removed.
The dial may be cleaned with care, I generally avoid using any chemical and just use a cotton bud to remove any dirt. It is very hard to get at the facts but I would take some care not to get
contaminated with the luminous paint dust as some authorities suggest that this might have very small but never the less present radio-active particles in it. We have to remember that the dial
and the luminous paint might well be pre 1948 when general concerns about the addition of the radio active element to make the paint luminous was not an issue.
If the dial is very badly corroded there are various methods that can be used to repair the situation. The best is to have the dial re painted by a professional clock dial restorer while may be
costly the results are impressive. Other methods involve using a computer scanned image of a dial which is printed on an adhesive backed card or fabric. The new dial is then stuck on to the old
dial plate. This will give a good but perhaps not perfect result.
The pointer or needle can be remade, a time consuming job, but an evening with a small section of brass and a set of needle files will result in a reasonable shape.
Glass can be either cleaned or if very badly scratched can be replaced a horological glass supplier can match the glass with little trouble.
The bezel, some motorcycle factors will have replacement 60mm bezels in stock, I believe these come from China or India. I have not had to use any as yet so I cannot comment on their quality.
The fibre gear driven off the large brass worm will need to be cleaned I use a cocktail stick and clean out the groves between the teeth, a bit like flossing!
Once the whole thing has been cleaned It needs to be reassembled, and as the good book say ‘reassembly is the reverse of disassembly’. If you have got this far then there is not much
point in a whole load of photos reversing the disassembly.
Refitting all the parts back on to the cradle making sure that a small amount of lubrication is used on the pivots of the aluminium cup. I would suggest a very light oil be used. I use a Mobeus D3
which is a synthetic clock oil but it is stupidly expensive. I understand that good results are achieved by using a synthetic bicycle chain oil which is about 10% of the cost of the Mobeus.
Some people might prefer to use a sewing machine oil. I have had very variable experienced from using 3 in 1 oil and so I would avoid using, it but it is a personal choice.
Once all is lubricated and reassembled and you have put the pointer back to the mark that you made when you took the speedo apart all, should be ok.
If you took the speedo apart because it was not working then by now you should have found and corrected the problem. However, experience shows that a high percentage of speedo faults are actually
caused by the speedo drive cable.
The Smiths ‘care of instruments’ book suggests that the speedo cable should be well secured but not pinched with no bends within 2 inches of the connection to the speedo.
The book also suggests that every 10,000 mile the inner cable is with drawn cleaned and degreased, then re greased sparingly with Castrol L.M. or ESSO TSD 119 grease. Do not use oil. The last 8
inches of the cable nearest the speedo head should be withdrawn and wiped clean.