<VV> Some more about Speedo calibration

N2VZD at aol.com N2VZD at aol.com
Sat Oct 5 21:28:17 EDT 2013

Early speedos had 1000revs per mile   lates had 850? (maybe 860 ,  I cant 
remember without looking for my OLD books), this results in a different  
calibration , and odometer drive gear. Spyder and Corsa had a brass drive gear  
in place of plastic to drive the added load of a trip odometer. Some other   
later 60's GM cars had spindle drive speedos also
Calibration requires a machine to drive the speedo at known speeds. you  
need to run it at 3 speeds on a chart , typically 15  45  and 90 mph  while 
adjusting the hairspring and magnetism of the spinning magnet. I had a  
special VERY strong electromagnet that had armatures to fit close to the moving  
magnet. I fed a very high current capacitor bank discharge ,thru a starter  
solenoid , of @12volts to magnetize , and a variac to feed controlled AC  
voltage to it for demagnetizing until speeds were exact at all 3 charted  
calibration points. It is a balance between hairspring tension and  magnetism to 
calibrate.This is a short description of why I suggest a shop set  up to do 
these correctly. I still own the setup , but gave custody to someone  doing 
a lot of antique car speedos as a service to his Chrysler car club guys. I  
am not ready to take it back yet , and have way too much going on here  
anyway's.  I hope this helps you avoid sadness.
 The idea of switching face and case to a late corsa frame is ok , but  you 
still do not know how close you are , unless maybe a check with a GPS. Also 
 , it is VERY easy to break the shaft that the needle is on , so you must  
carefully hold the speed cup while twisting the needle , not just pulling on 
it!  no fingerprints on the face , they will not wipe off.     Somewhere I 
have pictures of my equipment at my old shop.
Regards, Tim Colson

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