dave.vitesse Posted April 11, 2019 Report Share Posted April 11, 2019 From experience a worn bearing can cause over-reading as it allows the magnet to become closer to the disc. Dave Link to comment Share on other sites More sharing options...
NonMember Posted April 11, 2019 Report Share Posted April 11, 2019 Also from experience: residue from ill-advised attempts to clean / lubricate with oils or WD40 can also cause over-reading, often by a huge factor. 120mph in 2nd anyone? Link to comment Share on other sites More sharing options...
martin62 Posted April 11, 2019 Author Report Share Posted April 11, 2019 Seems like a prudent next step is to try another tacho. There is one local to me on eBay at the moment so that maybe worth a go. Link to comment Share on other sites More sharing options...
johny Posted April 11, 2019 Report Share Posted April 11, 2019 yes this article is quite good although its for a speedo: http://obswww.unige.ch/~wildif/cars/docs/Smith-jaeger_speedo_repair.pdf Looks like the coil spring can become weaker so allowing the instrument to read proportionally higher across the complete range or possibly a lack of lubrication in the bearing between the magnet and cup might have the same effect.... 1 Link to comment Share on other sites More sharing options...
martin62 Posted April 12, 2019 Author Report Share Posted April 12, 2019 I definitely believe the spring is the issue. After re-assembling and a little manual testing, I found the needle returns to the bump stop at a different rate if the gauge is upside down. Seems, at least in part the needle is currently returning under gravity not because of the spring. Link to comment Share on other sites More sharing options...
johny Posted April 12, 2019 Report Share Posted April 12, 2019 hmmm not easy to adjust the spring. I take it youve not found any dirt? How about getting a drop of thin oil in the bearing between cup and magnet? At least you can test it easily unlike a speedo..... Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 12, 2019 Report Share Posted April 12, 2019 the basic drive in the tacho is identical to the speedo if the needle has been banging the end stop its likely its moved , so its worth a try at hold the disc and move the needle back to retension the hair spring, apart from its pure size i wish i kept our smiths head drive tested , you could remag and re calibrate anything, but it was in a box about 2.5 ft square like all good things it went in the skip there becomes a problem with hair springs and needles you can get the reading correct at on speed but the hysterisis of the spring bu**ers up all other readings ....can be lots of fun getting it right for the whole range keep at it Pete Link to comment Share on other sites More sharing options...
Mad4classics Posted April 12, 2019 Report Share Posted April 12, 2019 It's not impossible to re-calibrate NEMAG gauges yourself; encouraged after reading Anthony Rhodes document posted above and based on the pointers given in the the very old US War Department document TM9-1829A, I decided to re-calibrate both my speed and tacho; it's a fiddly task, ideal as a winter project but in the end you can get them spot on and have some confidence in what they're saying. Took me several magnetizing demagnetizing cycles to get it right the first time. You do need a few pieces of kit to do it; most notably a large electro magnet which I had to make based on the pictures in TM9-1829A, a small geared electric motor and an optical tacho or some means of measuring the cable RPM. David TM 9-1829A.pdf Link to comment Share on other sites More sharing options...
martin62 Posted April 18, 2019 Author Report Share Posted April 18, 2019 Well the old rev counter was beyond what I was capable of repairing, but a replacement unit (from a Spitfire of the same period), is working just fine. Link to comment Share on other sites More sharing options...
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