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Metron Speedometer. 2007 - 2011!

(addenda 2016)

When the car arrived from the UK it was fitted with a replica Smiths speedometer which wasn’t reading correctly. Anthony had told me it went silly when he was driving the car to the docks to ship it out.

In the spares was the original Metron unit that wasn’t working. The chassis had broken and been glued in an unsuccessful repair attempt. I managed to locate another Metron in Italy, it was a metric model and needed a full work-over. All the UK cars had a mph unit fitted. I sent the two units off to Howard Instruments in Heidelberg ( with instructions to make one good one out of the two, preferably using the kph dial.

A month or so later back came a very nicely restored metric Metron unit. I had to get a new cable made to match the Metron end, this, of course, took another month. However it still wasn’t reading correctly.

Convinced it was the cable I swapped it for another, still the same problem. Changed the right angle drive, no better. Ordered another cable from Cavalitto, still the same problem. Sent the speedo back to Howard Instruments, on return no better. What was going on?

Eventually I decided it must be inside the gearbox, that was all that was left. Andrew sent me a photo of the gear inside another gearbox. I didn’t realise it was separate from the main insides, so all I have to do is pull the back off and check the gear (38-20921), sounds simple! (See TAV 16a below)

More time on my back under the car, I’ve spent more hours there than driving the thing, seriously! Take off the front flex coupling and then the back of the box. Sure enough the pin holding the speedo drive gear (54186) on the output shaft had broken. With just friction holding it in place clearly explains why it sometimes read correctly but mostly was erratic.

To get the gear out I had to take off the centre coupling to get the required clearance. Ordered some hardened pins, the hole in the gear was 2mm, the hole in the output shaft 3mm. Drilled the gear pin hole to 3mm to match and inserted the new pin. Put the gearbox back together.

At same time I took the opportunity to check the central transmission shaft bearing, that was a bit sick (49003), found a new replacement, it’s a single row cylindrical roller bearing, no. N204. Gave it plenty of grease, the old grease had set hard hence the sick bearing. (See TAV 17a below)

Now it’s all back together the speedo is fine, the drive train a little quieter and even less vibration, almost down to none.

Of course as is usual with this car all is not quite well. At 1001km the odometer got stuck! The speedo reads fine and the trip meter works, so it will have to come out again and be sent off for repair, if that is possible!!

After a third repair the odometer has now reached 2500km and seems to be fine. Plus I swapped the fuel gauge face for one in litres, not original but makes sense in a metric country.

In 2014 the odometer was ticking over to 10,000km and got stuck, again. Once more off to Howard Instruments for a fix!


July 2014

Replica Smiths speedometer as fitted.

Note clock & petrol gauge are in the wrong locations

Broken roll pin in output shaft.

Gear in place with new hardened 3mm pin.

Metric Metron speedometer now fitted.

Clock & petrol gauge now in the correct location

I have since discovered other speedo gears have a keyway machined that slides over the pin in the gearbox output shaft.

Now with metric (litres) fuel gauge.

New transmission shaft central bearing.

Locating hole in output shaft for speedo gear.

Broken chassis from original Metron unit.

(not my repair!)

Addenda 2016.

While going through our collection of gearbox spare parts, I was investigating the mainshaft splines for another problem, I found there are two different versions of the shaft, well three actually. In the photo below the length of the splines on A & B are identical, but in C they have been cut longer. Why is this so? In all the parts books the part number is the same, 38-20001. My guess is the longer splined version is earlier as it has a basic problem, the bearing and speedo gear sit over the cut spline and have the potential to wear. On the shorter spline version the bearing and speedo gear fit over the machined shaft which would give a better fit and not wear.

There is also another difference. On the later version A, the speedo drive gear, 38-20921, has a pin to fit through the machine slot in the gear. In the earlier version C, the speedo gear has a small internal pin that locates correctly it on the longer spline.

But just to throw a fly in the ointment, shaft B has shorter splines but no provision for a locating pin, so how is the speedo gear secured to the shaft?


November 2016