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MG TD TF 1500 - MG TD MK2 Speedometer?

I recently had a look at a couple of TD speedometers and found one with 1600 turns per mile printed on the face and the other with 1525. The ratio of these two numbers is very nearly the same as the ratio of the rear end gearing of the standard (5.125) and the higher MK2 one of 4.875. This leads me to believe that the 1525 speedo is from a MK2 or possibly from a TD sold with the optional higher gear ratio. Both these speedos are the later magnetic type with the main beam indicator at the bottom of the face. The 1600 speedo also has the numbers s.s.598 and x65022/34 printed on its face whereas the 1525 speedo has the numbers s.s.606 and x 65022/36.
Has anyone seen this on a MK2 (Tom??).
Hugh Pite
H.D. Pite

Hugh - what a puzzler! My initial reaction is that the tach comes from another car, since both the standard TD and the Mark II use the same tach drive reducer on the generator, and since Section R show different pinion and speedo gears for the transmission of Mark II cars. These gear parts match the part numbers for alternative 9/43 (4.875) ratios that could be ordered for the standard TD. So the change seems to be in the speedo gears in the transmission, not the speedo itself.

But that being said, Section R does list a unique Mark II speedometer - different from the stock unit - under p/n 500829, introduced at car number 18989. I don't know what this change is all about.

And finally, just to confuse matters, there is another small point. We all know that there was a change at chassis 10779, where flat-faced instruments were replaced by dished ones and the 20/20 ammeter was replaced by the 30/30. But then there is yet another speedo change at chassis 17548, where part number 500616 replaces 500527. Anyone know whjat this difference is?

Tom Lange
MGT Repair
t lange

Tom - According to the information production change section in the T Series Handbook, published by the New England MG 'T' Register, the change in the speedo at car #17548 was the addition of the high beam warning light. Cheers - Dave
D W DuBois

Tom: My TD, number TD8986 which I have owned since 1977 has a dished speedo and tach and a 20-20 ampmeter. I believe all these are original. Where does the flat to dished instrument change info come from?
John Quilter (TD8986)

Tom - An additional note regarding the change over to the dish faced tach and speedo. Again, according to the T Series Handbook from the New England MG 'T' Register, the change from flat faced instruments to dish faced occurred at chassis #10751 (Oct 5 1951) and the 20 amp ammeter changed to a 30 amp ammeter on LEFT HAND DRIVE CARS. At the same time, the on/off panel light switch was changed to a rheostat. These same changes were introduced on RIGHT HAND DRIVE CARS commencing with TD 10779 (Oct 8 1951).

John - I suspect that the speedo and tach on your car were changed by some previous owner, probably to get away from the chronometric instruments. Cheers - Dave
D W DuBois

John - The factory "Illustrated List of Service Parts" is the source of change serial numbers, for better or worse. I have always found it surprising and unlikely that changes took place at a fixed serial number, since there must have been remaining stocks of the old part to be used up. I confess when I wrote "We all know..." I was being tongue-in-cheek...

Chris Couper, on the other hand, references 10750 as the car where the switch-over happened, not 10779 as I note above - I wonder which number is correct?

I would not be at all surprised, John, if your instruments had been changed. I would not worry (unless your car is a show car) abouot the change since the dished instruments are better and more accurate.

Dave - thanks for that clarification - I am still puzzled. Section Q (LHD export) shows the speedo change-overs at 10751 and 17548 only for kilometer speedos and does not list mile speedos at all. I wonder if your source is right, based on the parts book? Am I missing something (I frequently am...)?

Tom Lange


t lange

I just had a look at a TF and a YA speedometer. They look identical except for the mounting tabs. The TF speedo has 1525 turns per mile on its face corresponding to the 4.875 diff ratio whereas the YA has 1600 turns per mile corresponding to its 5.125 diff ratio.
It would seem to me to be much easier to change the speedometer calibration and face than to change the speedometer drive gear in the transmission. Has anyone actually seen the speedometer drive gears from a MK2?
Tom, I think you are incorrect about the magnetic speedometer being more accurate than the chronometric one. The magnetic ones are much less expensive to produce but rely on the delicate balance between the force caused by a magnetic eddy field induced in an aluminum cup and a hair spring. The chronometric ones depends on a clock type excapement for its timing which is very accurate and the rest of the mechanism is a set of meshed gears.
Hugh Pite
H.D. Pite

Hugh - Yes, I have had a Mark II trans apart fairly recently, and the speedo drive and pinion are indeed different from the stock parts.

My chronometric instruments are probably old and need rebuilding; they jump around much more than I would have thought.

TRom
t lange

Where can I find a pinion and gear for my now 4.55:1 rear end? I have the early gauges...

Bill Chasser Jr
TD4834
W. A. Chasser Jr

Mark II had a 4.875 rear end, so that's not a reliable source. I suspect you will look long and hard before you find a 4.55 speedo gear and pinion set, since they were only available as an option, and never standard on a TD.

You are better off getting a gear adapter box for the trans; look in the archives. I know Speedograph Richfield in the UK offers them, and I believe there are others.

Tom Lange
MGBT Repair
t lange

Thanks Tom , for the information. I have bookmarked it for reference. Contact me with the MK II pics when your able...

Regards

Bill Chasser Jr
TD4834
W. A. Chasser Jr

Bill - You might contact Butch Taras at mgyowner@yahoo.com about building a ratio box for your speedometer. Cheers - Dave
D W DuBois

Thank you Dave I will try him first
W. A. Chasser Jr

This thread was discussed between 12/11/2014 and 15/11/2014

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