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MG Midget and Sprite Technical - Speedo - Rebuild or not

I have just fitted the 3.9 diff into the Sprite and what a difference it has made!

Next problem is the speedo. I think I could buy a suitable pinion but they are very expensive. For not much more I could get Richfields to rebuild my speedo and recalibrate it. The cost is £75.

I think I will go for the latter. Anyone got views on the issue?


Jan T
J Targosz

I had mine rebuilt by Richfield when I installed a 3.7 diff in the Frog. Took about a month to turn it round but I was very pleased with the result.
Bob Beaumont

I can't comment on Richfield, but I know Speedycables were quoting 8 weeks and taking even longer... Be prepared to wait!

Malc.
Malcolm

You could try Richard Woolley, who advertises in the classifieds. I bought a speedo calibrated for a 3.9 from him.
Bill Bretherton

For what it's worth, a few years ago I used Richfields for the speedo on my Elan. It took them 3 gos to get it right.

Not sure that I would use them again.
Then again. others seem to have had no problems.
Just my tuppence worth.

Simon
Simon Wood

I know several different diff ratios are available for 1960s BMC cars but don't know if the pinion gear in the gearbox was different for each ratio or if the Speedo heads differed. If different gears are available I wonder if the pinion can simply be changed or if the collar gear in the gearbox must also be replaced. In any event the cost of a pinion gear is almost the same as a reconditioned speedo head, which would be totally accurate, so it is a no brainer. The only thing going for a pinion change is if someone wanted to refit the 4.2 diff they could also use the original pinion. After seeing the way the 3.9 performs I don't think anyone would revert to the 4.2

Jan T


J Targosz

The pinions in the BMC boxes are all the same. The speedo heads were varied to suit the diff ratio. The frogeye for example had part number BHA 4120 for the 3.7 diff, BHA 4122 for the 3.9 diff and BHA 4118 for the 4.55 diff. Personally I think its better to get the head re-calibrated for accuracy. Eg. Tyre diameter varies between say cross ply 520x13 and radial 145x13
Bob Beaumont

The pinion is the same the speedo's are different.

The number on the speedo gives the number of revs I think per mile.

An A series with a 3.9 UK spec is SN6142/06 with a number of 1376.


Its worth picking up a Moss catalogue which is free as it has the original BMC/Leyland part numbers etc and is a very useful reference.

R.

Cross posted with Bob.
richard b

This thread was discussed on 22/06/2021

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