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MG MGB GT V8 Factory Originals Technical - LT 77 identification

I got a SD1 LT 77 gearbox along when I bought my Rover engine. It does not have any production number stamped to it. I have seen other LT 77,s which have the number stampes on the lower righthand side of the casing. Is there any alternative place? I need the information for speedo calibration purposes.

Erik
Erik

Erik,

All the LT77 boxes that I have seen have had the serial number in the same place but even if you have the number it won’t help with speedo calibration. The same box was fitted to many different cars with different speedo drive gears (including electronic). The colour of the gear indicates how many teeth (you could also count them of course).
Orange gear = 20 teeth
Green 21
Red 22
Black 23
Blue 24
White 25

From 1976 to 1982 the 3.5 SD1 used a 20-tooth gear (orange) and 3.08:1 axle ratio.
The 2.6/2.3 a 23 tooth gear (black) with 3.45:1 axle ratio.

1982 onwards the SD1 had an electronic speedo.
3.5 - 20 teeth (orange) 3.08:1 axle ratio.
2.6/2.3 - 23 teeth (black) 3.45:1 axle ratio.
2.0/2.4TD - 22 teeth (red) 3.09:1 axle ratio.

Although this may be interesting it doesn’t help you calibrate your speedo, you need to know the axle ratio and tyre size that you are using and the turns per mile/kilometre that your speedo needs.

Geoff
Geoff King

Geoff,

Thank you,
I have black gears on the detachable part where you connect the speedowire. Guess I have to start counting output shaft revolutions in relation to speedodrive revolutions.
And also, -Thanks for letting us have Henrik Larson back and in saying so, I hope you are not a Rangers fan.

Erik
Erik

Erik,

Sorry, I’m not a Rangers (or any other team) fan and I have no idea who Henrik Larson is or what he does.

Geoff
Geoff King

Geoff,
Came to think of the obvious question a little too late.
How many Turns Per Mile is the mechanical SD1 speedo set up for? My "American" ´79 B has a 1000 tpm speedo.

Cheers
Erik
Erik

Erik,

I don’t know, it is probably 1000 tpm but it doesn’t matter, what you are interested in is the tpm of the speedo you are fitting – in your case also 1000 tpm, then you need to consider tyre size and the axle ratio.

Calculate the turns per mile of the tyres you are using, multiply that by whatever axle ratio you have, to give the speedo drive gear tpm, then select the speedo driven gear to give 1000 tpm. Unfortunately, although I know the number of teeth on the various driven gears I don’t remember how many there are on the drive gear so you will have to count them to calculate the ratio.

Incidentally the driven gears are available from Rimmer Bros. http://www.rimmerbros.co.uk/

Geoff
Geoff King

Geoff,
Thank´s

Erik
Erik

This thread was discussed between 14/06/2004 and 18/06/2004

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