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MG MGB Technical - Speedo too slow after swap too OD gearbox

Hello,

Another question; My 64B had 4 sync non-OD box fitted, with side filler hole for oil(No dipstick)

I have now pulled out this gearbox and replaced it with a dipstick/black label OD gearbox, and the speedometer now shows approx 60 when I am only doing 50 mph.

The 90 degree angle speedo drive on the non OD would not and was not necessary on the OD box, and it seems to be 1:1 ratio angle drive.

I guess my problem is the 1000 vs 1280 tpm issue.

There seems to be room for a corrector box betwwen the speedo wire upper end and the speedometer. Is there any readily available source for this, for ex a Speedograph Richfield product or will it has to be custom made?

regards

Jan
Jan Emil Kristoffersen

As far as I'm aware all 4-synch gearboxes were 1280 tpm until rubber bumper when they changed to changed to 1000 tpm. 1020/40/60 were 3-synch.

It could be that you have blue-label gearing inside an OD with a black label.

The right-angle drive is 1:1 as you say, and isn't required if your speedo cable is long enough to make the bend in a gradual manner.
paulh4

Thank you Paul,
here is a photo of the black label.


Jan Emil Kristoffersen

Jan, I encountered this problem when I installed a Black Label 4 synchro/OD transmission into my '67 B. To solve the problem, I used the internals of a speedometer that matched the transmission, inside my stock Smiths gauge. It works perfectly and you can't tell that the speedometer has been modified. RAY
rjm RAY

Jan - I don't doubt it has a black label, I'm just wondering if it has blue-label internals. I don't see what else could account for a 20% increase in speedo reading when changing from a 4-synch without OD to a 4-synch with OD. Speedo suffixes did change then, but the TPMs didn't.

Out of interest what is your speedo reference number and what are the TPMs printed on the dial? A Mk1 would originally have had one of several SN6124, 6125 or 6144 depending or market and MPH or KPH. Mk2 chrome bumper with the 4-synch would have had one of several SN2144, and rubber bumper one of several SN52nn.

The other possibility is the previous 4-synch gearbox internals had been changed to match the original speedo.

"I encountered this problem when I installed a Black Label 4 synchro/OD transmission into my '67 B"

Isn't that a change from 3-synch to 4-synch?
paulh4

I think Jan had a rubber bumper gearbox previously, so a blue label OD box would have been a better bet.
Dave O'Neill 2

If you pull out the speedo drive cog and can see the drive wheel in the od/gearbox, if I remember correctly the rubber bumper ones are red in colour - not sure if the speedo drive is also red.
richard boobier

"I think Jan had a rubber bumper gearbox previously"

In that case if it was accurate before the speedo had probably been changed/recalibrated to suit that.
paulh4

Scrap that!
paulh4

Hello,
Ishavet been away for a few days but went out into the garage now to check. My speedometer is probably the correct for a 1964 B, a Jaeger instrument . It is marked SN 6125/04 and 1060.

I would not want to remove this speedo, and I do not think I am technically capable of rebuilding it with other internals.

The easier fix seems to be a corrector box fitted on the rear side of the speedo. Speedograph Richfield may have an off the shelf solution?

Thank you for your advice,

Regards

Jan

Jan Emil Kristoffersen

Yes, original for a Mk1 other than Germany. So a corrector box seems the next best solution, however both the gear and the pinion for the blue-label OD do seem to be available from the usual sources.
paulh4

But changing the gear and pinion sounds like another lift-the gearbox-and -engine-out-of-the-car session?

If so I would rather go for a corrector box :-)

Regards

Jan
Jan Emil Kristoffersen

You can remove the speedo gears with the engine and gearbox in place, although the rear bearing in the overdrive will need to be replaced as it is destroyed in the process.
Get an SP bearing puller of the right size to remove the bearing and then the worm can be removed.

The speedo ratio could easily have been changed in the OD somewhere in its 40-odd year history, so best to check the actual gears rather than rely on label colour.
Within ~3%:
21 tooth pinion is the same ratio as the 4 sync non-od gearbox.
20 tooth pinion is the same ratio as the 3 sync non-od gearbox.
Paul Walbran

From the info I have gleaned:

Rubber bumper 4-synch non-OD (which is probably what Jan had to start with) has a 9-start gear and a 30-tooth pinion to give 1:3.333 and 1000 tpm.

Black label 4-synch OD (which he has now) has an 8-start gear and a 21 tooth pinion to give 1:2.625 and 1280 tpm.

To get 1000 tpm just by changing the black-label OD pinion you would need a 26.67 tooth pinion. 26 was used on the CB 4-synch non-OD and auto, 22B654, and would give a ratio of 1:3.25 i.e. a 2.4% error, but they are NLA.

paulh4

Thank you everyone for contributing to a very helpful thread ! Seems that a corrector box is the way to go for me.
Jan Emil Kristoffersen

This thread was discussed between 07/11/2016 and 16/11/2016

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