2009 - December - Speedo Restoration

During the Chev 4 Tour in October, my speedo broke 3 days into the tour. When I finally got around to repairing it, Xmas was on the door step and it seemed like it would be a simple job to change the inner speedo cable and then back in business.

Wrong. I replaced the inner speedo cable, went for a test drive, no speedo, nothing. The inner cable broke immediately. Ouch, $45 per insert.

I stripped out dash panel again, pulled out speedo which was locked up solid.The worm drive was bent and jammed. The speedo gears are made from "pot metal" which seems like a very soft die cast, and is prone to collapse with age.

Decided to tackle the job in a big way and stripped 4 speedos I had in my spares cupboard, and was able to make 3 good ones, using the following process:

Stripped all speedo wheels back to bare metal.  Applied a coat of acrylic primer and a top coat of acrylic full gloss white. Then used reface kits from Gary Wallace and the Filling station. Worked well and looked great. It's just a matter of getting your eye and hand coordination right. If anyone had second thoughts about trying, I would say give it a go as the end result is great. The trickiest part was refitting the fine spring, which was still attached to the drum, back into the top brass fitting with the wedge. I did this under a magnifying lamp. To recalibrate the completed the speedos I used a laser tacho that I purchased for $18.95 on EBay and my bench drill press.

The Calibration process was:

The formula I planned to use (for USA speedos) was 1000 turns per mile x 35 MPH = 35,000 divided by 60 min = 583.3 RPM as the calibrating speed to set the speedo at 35MPH.

Howver I was only able to set the speed on the drill press at 650 RPM, plus or minus 1 RPM, and had to fit a bigger drive belt on the drill press in a figure of 8 to reverse the direction.

I revised my calculation to 1000 turns per mile x 39 MPH = 39,000 divided by 60 min = 650 RPM as the calibrating speed to set the speedo at 39 MPH.

After a bit of battle I have managed to calibrate the 3 speedos. Rough adjustment was done by moving
the magnet, and then fine tuning with the spring adjustment. I  bench ran for approx 10 Miles.

However, when I installed the reconditioned unit in the car, it was under registering speed by about 15mph. Turned out that the speedo casing affects the magnetic field of the speedo, so out of the dash came the speedo again, and I had to calibrate out of the casing to a higher speed to allow for the magnetic change when reinstalled back into the casing. Took another day and finished with  3 fully reconditioned speedos, with new numbers.

A word of caution, do not misplace the T bolts that hold the instrument panel into the dash, they are  a special thread and are not available.

I finished the job by reconditioning the light switch, oil and amp gauges, polishing the gauge rims back to brass, ad fitting new glass and gaskets.

New speedo has done 1500 miles with no dramas.

Very happy with the result, and have 2 spare reconditioned speedos for future use.

Below are a few photos of the process: