Repairing Jaeger and Smiths Speedometers

An Article by Anthony Rhodes
Copyright 2000

Page 3


Repairs of the speedometer and odometer sub-sections can be made by exchange with intact/functional parts from other Smiths or Jaeger speedometers. Many parts will be common across a broad range of models and years. There are four main variations (that I know of) that will influence the possibility of exchange. Within a specific type, parts seem to be completely interchangeable. "Old" models have all metal construction except for the worm gear and also have separately driven main and trip odometers. "Intermediate" have plastic odometer wheels, and the trip odometer wheels are more widely spaced. The "new" models have mostly plastic construction and the trip odometer is driven by a gear from the main odometer, so there is only one worm and pawl. The spindle bearing in the magnet wheel is more shallowly set in the "new" type of speedometers.

The Triumph cars seem to have had a slightly different variant speedometer than the MG's. The primary difference is that the spindle to which the pointer is attached is longer (.180" vs. .150") and has a somewhat narrower taper (.032 to .035 vs .030 to .035). This makes it less than optimal to move the works from an MG to a Triumph because the Triumph pointer fits slightly loosely. The move of a Triumph works to an MG is even less possible due to the MG pointer being too tight to fit on the TR works. The taper of the long and short spindles is approximately the same, so the longer one reaches a more narrrow tip. It is possible to shorten the MG spindle to the Triumph length and thereby have the diameter correct for the Triumph pointer. Use a file or rotary stone on a dremel tool to shorten the spindle by a 30 thousandths or so, and try refitting the pointer. If it will not slide on, there may be a burr on the tip, so use a fine file to chamfer the edge.

Depending on the calibration required, the worm on the input shaft may have 20, 25 or 32 teeth (there could be others but I hav not seen them). It appears that 32 teeth were very commonly used on the "old" and "intermediate" versions, with 20 and 25 also seen. 20 and 32 teeth were used on the "new" styles. There was a wide variety of gears used on the odometer wheels to provide the final calibration. The calibration of the odometer is the number of teeth on the worm gear multiplied by the number of teeth on the odometer wheel shaft. This gives the number of input shaft turns for each odometer shaft turn.

Of course parts are completely interchangeable between identical units, but many parts are carried across a broad range of speedometers, and will be completely interchangeable. For instance there are only two types of magnet wheels that I have identified. One type has a shallowly set spindle bearing, and the other is more deeply set, so the magnet wheel can be interchanged quite freely. The main speedometer frame is identical across all models as far as I can tell, and are completely interchangeable. In the "old" and "intermediate" units, the spindle/main odometer frames are interchangeable as long as the pointer fits properly. In the end, it is usually possible to obtain sufficient parts to repair your speedometer without great difficulty.

KPH and MPH speedometers are essentially the same and parts exchange guidelines apply here as well. As far as I can tell, the actual speedometer function is exactly the same. Only the printing on the dial face is different. The odometers are also essentially the same. The KPH units have 62% fewer teeth on the gear mounted on with the odometer wheels so there will be more turns of the odometer for the same number of miles. It is easily possible to convert a KPH speedometer to an MPH unit. All you need to do is exchange the dial face and install the proper odometer gear to set the desired calibration. If you have a KPH speedometer and want to convert to MPH, you can calculate the desired calibration by multiplying the calibration printed on the dial of the KPH speedometer by 1.609. The reverse calculation may be made by dividing by the same number. This number will usually not correspond to an actual calibration. You need to round to the nearest 20,25,or 32 (depending on the number of teeth on your worm gear). For example, a common TR6 KPH speedometer has a calibration of 740. This corresponds to an MPH calibration of 1190.6. This is just about centered between the two possible calibrations of 1180 and 1200. To settle the issue of the what calibration you REALLY should have, you ought to calculate your ideal calibration as described later in this manual, then translate that a MPH/KPH calibration and then look for the best possible calibration available.

List of Smiths/Jaeger types:
Old Style:
Separate main & trip odometer frames and drives (all 120 mph?) TR2 thru 3A
Type 1: 25 tooth worm gear
Type 2: 20 tooth worm gear
Intermediate Style:
Separate main & trip odometer frames and drives (all 120 mph?) All 32 tooth worm gear (?) TR4 and TR4A (probably TR3B)
Type 1: Narrow trip odometer wheels (early)
Type 2: Wide trip odometer wheels (late)
New Style: Single main & trip odometer frame and drive (TR5/250? and TR6)
Type 1: 32 tooth worm gear (all 100, 120 mph) (MG only?)
Type 2 20 tooth worm gear (all 140 mph?) (TR only?)

Note: This summarizes the extent of my experience with TR and MG speedometers Any additional information will be greatly appreciated and incorporated into the next edition of this manual.

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