Success! Just road tested and adjusted (re-calibrated) the speedo on my 73 750 commando. (I assume the same process would work for re-calibrating the tachometer, substituting a known accurate electronic tach for the GPS speedometer phone app, and doing the adjustment on the center stand not on the road.)
NOTE: I highly recommend the book "Magnetic Speedometer Repair" by Graham Blighe
http://www.improvingclassicmotorcycles.com/reviews.htm I ordered my copy through EBay.
To summarize all in one post, with great help from Kommando (thank you again!!), here's how I re-calibrated my Smiths ssm 3001/09 magnetic speedometer
without disassembling it:
1) remove the speedometer from the bike
2) looking through the bulb hole, locate the brass calibration adjustment screw. It should be in a straight line with the center of the speedo where the cable attaches. You probably will only see the edge of the brass screw, which on mine was notched like an adjusting star on my old Ford truck's drum brakes.
3) CAREFULLY drill a hole in the outer case over that screw. I was not careful enough and you can see in photos that I dinged up the slot in the top of the brass screw, but it was still intact enough to work. (After posting this, a friend suggested using a dremel tool to make the hole, rather than a drill. If the hole you make is not perfectly centered over the screw, a dremel would be a good way to shape the hole to get better access to the brass screw.)
Whether you drill or Dremel, BE CAREFUL!! The top of the screw is not far below the case.
a) I put a magnet tool into the bulb hole to catch drill filings. It caught a lot of them. I also held the speedo upside down and drilled up so gravity would help filings fall away from the innards. After I was done drilling, I also held the speedo upside down and blew compressed air through the drilled hole which cleaned out a few more filings through the bulb hole. You don't want metal filings on the internal magnet.
b) start with a very small bit and drill a pilot hole to locate as accurately as possible over the center of the screw (I used a 3/32 bit for my pilot hole, but you could go smaller)
DRILL VERY SLOWLY AND PATIENTLY, WITH MINIMAL PRESSURE. Use a sharp bit and let it take it's time cutting through the steel of the outer case. If you drill too fast or with too much pressure, when the bit breaks through the steel case, it will almost instantly drop onto the top of the soft brass screw and damage the slot in it. Wrapping tape around the drill bit to create a depth stop is highly advised.
c) drill out your pilot hole to the size of the head of the brass screw. (I used a 3/16 bit for my final hole, but it was tight and I had to grind off my small screwdriver so it would not bind on the hole. I'd use a 1/4 bit if I were doing it again.)
AGAIN - DRILL VERY SLOWLY AND PATIENTLY, WITH MINIMAL PRESSURE. Use a sharp bit and let it take it's time cutting through the steel of the outer case. If you drill too fast or with too much pressure, when the bit breaks through the steel case, it will almost instantly drop onto the top of the soft brass screw and damage the slot in it.
Graham Blithe's book suggests obtaining a grommet in advance that you will use to plug the new access hole to prevent dirt getting into the internals through the access hole, then drilling the hole to the size of the grommet you have.
d) Once you have drilled the hole over the brass screw, blow out any remaining filings with compressed air, holding the speedo upside down so filings can fall out of the bulb and new access holes (see "a" above).
4) remount the speedo on the bike
5) install a GPS speedometer app on your smartphone. I have an android and used "GPS Speedometer" by cool niks. The free version worked great.
6) Try the speedometer app to see where and how the speed readouts are displayed, then secure your phone on the handlebars as close as possible to the bike's speedometer, making sure that you can access controls for the phone and app and see the app's readout. I used rubber bands, covered by painter's tape (so it would come off the phone easily) and then duct tape for strength.
7) Make sure the screwdriver you have will fit into the hole you drilled and easily turn the brass screw. Mine hit on the headlight mounting bracket and was a bit too fat at the blade for the small 3/16 hole I'd drilled, so I had to cut, grind and bend to make a custom screwdriver that would easily turn the brass screw. (I'd make a larger hole if I were doing it again.)
8 - road test and adjust.
A straight higher speed road with a wide shoulder so you can stop well away from traffic when you adjust the brass screw is highly recommended. Do this at a time when there is as little traffic as possible. It is dangerous to be looking down at two speed readings while tracking traffic and other hazards! I brought a small flashlight and my old guy reading glasses so I could see the brass screw slot better.
9) If your speedo reads too fast, turn the brass screw counter-clockwise (as you look up from underneath the speedo). If your speedo reads too slow, turn the brass screw clockwise.
10) adjust, test, re-adjust until it reads the same as the GPS.
11) After adjusting it to match the GPS, I removed it and put a dab of black silicone gasket maker on the side of the brass screw as insurance against it shifting over time and vibrations.
I also scratched a line in the case marking the direction the slot of the brass screw is pointing with it (now) accurately calibrated as a reference for the future.
And I put some duct over the new hole to keep dirt out. I might replace that with a rubber plug, if I can find one to fit well. Or maybe some silicone gasket maker would plug the hole and be easily removed for later access.
12) reinstall. Happy and safe motoring!
