Smiths Speedo Rebuild

By Judy Goodrum

 speedo1The irregular movements of “Jane’s Speedo had given us concern that we had blamed the drive cable for, but with two new cables fitted the problem persisted so it just had to be investigated further and the speedo head was removed. With the mechanism revealed it was discovered that the cast frame housing was fractured allowing the shaft containing the centrifuge weighs to move uncontrollably. This section of the casting had been repaired previously. With the speedo already dismantled, the reason for the malfunction was clearly identified. Hence it was a straight forward matter to clean the old adhesive off and prepare it for repair. As it would be difficult to hold the speedo mechanism in a suitable position a crude stand was made to hold it in the inverted position. The first stage was to join the two halves of the alloy bridge section, this was done and left to set for six hours. At this point a number of extra balls were found in the interior of the speedo but at that point there was no apparent location where they had come from. After inserting the race into the casing and seated on the tapered shaft the assembly was glued back in place and weighted to settle the adhesive. The adhesive was then left to set over night.

With the aluminium frame once again repaired, and rigid, quite a lot of side play was found in the upper bearing of the centre shaft, so the extra ball bearings found there home. However it was found impossible to re-insert these balls with the central shaft in place, and no confidence was gained that we actually had the correct number present hence a complete strip of the speedo mechanism was required. A tool was made to unscrew the upper bearing race, however it was still impossible to release the shaft with the operating arm of the speed indicator hand in place so the hand, and dial, had to be removed before the link arm could be removed. With the arm removed the upper bearing could be completely unscrewed and the centrifuge shaft assembly withdrawn. With the upper bearing extracted the suspected shortfall of ball bearings could be verified, there were actually five missing. They were not to be found anywhere within the casing or trapped on the mechanism, so we can only assume that they were missing when assembled and may have been a factor in the early failure of this unit. New 1/16″ Dia balls were ordered from ‘Simply Bearings’ Halton House, Greenfold Way, Leigh Business Park, Leigh, Lancashire, WN7 3XJ. With these fitted everything once again to functioned correctly.

It would appear that the area of the lower casting is a problem area, as this had already been broken twice on this unit, and comments from other members indicate that the ball bearings falling free is a common occurrence. As this can only be caused by excess clearance in the adjustable bearings the frame fracture may be a design weakness in these Smiths speedometers hence my experience may prove useful to others.


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