I updated my tacho by replacing the 1960s electronics with a new circuit board made by a company called Spiyda trading on Ebay amongst other outlets. Cost of board about £45. Time to fit about an hour once the unit was out of the car added to about another hour running in a new wire from the coil negative to the tacho.
Instructions are pretty good with helpful videos on YouTube. A major advantage of fitting the new board is that the unit can fairly readily be calibrated during fitment. This is fairly straightforward but requires a mobile phone or laptop with a high level audio output. I struggled a bit as my laptop wouldn't produce enough level.
Nice to have accurate measurement and smoother needle movement.
I have a Kienzle clock on my 410 which is very good. Working second hand units can be found on Ebay for less than repair cost of a Smiths.
I had clocks on my 401 and 403 repaired by Speedograph Richfield with electronic impulse units but they didn't last very long.
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