I’m refurbishing my Smiths PA speedometer and have found one of the screws holding the face to the mechanism has sheared. Does anyone know the size of these screws (apart from the fact they are minute!). They are so small that I haven’t got a thread gauge that will go anywhere near!
Howard, I am not familiar with the model nomenclature, but likely they are BA. The screws holding the instrument to the dash on my 1930 Ulster are 6BA, Raised Countersunk. I will check my speedo later today to check.
These screws hold the instrument face (the one with numbers painted on it) to the brass odometer mechanism. The screws are very small, perhaps less than a mm diameter. If they are BA then the number would be something like 20BA.
Howard, here is a chart that gives diameters of most of the available BA sizes. I suppose it is possible that Smiths may have cut their own threads, but I doubt they would have done that. Sizes of BA go up(down) to the smallest at 25BA, which I'm not sure my old eyes could even see.
(02-09-2020, 08:48 PM)Parazine Wrote: Likely these are watchmakers sizes, which seem to be a closely guarded industrial secret. I suspect they're a metric based system of some sort.....
Certainly the the later magnetic Speedos etc. used an odd metric fine thread. Someone who had worked at Smiths told me that this came about as a result of Smiths taking over Jaeger instruments and using their designs. There's a huge amount of general thread information on this website: https://www.gewinde-normen.de/en/index.html e.g. Bodmer thread or Whitworth instrument thread etc.
Thanks for all the useful suggestions, especially Zeto for the kind offer. The screw in question is shown below and has a bevel head diameter of 2.3mm and thread diameter of 1.05mm (although I had difficulty holding the thing steady and using the micrometer).
Having thought about it I realised that even if I had a screw I don’t have the correct tap. In addition the speedo has probably been without one screw for some time ....so...
I’ve turned a dummy screw (no thread or slot) and glued this into a hole drilled in the support. It’s a first for me having to use a magnifying glass on the lathe! The instrument face has slots rather than holes for these screws so at least it is held in position at 3 locations and fastened down in two!
