Identifying gearbox and diff

[michael4]

Our 1000 is a very late car and previous owners have done all sorts of long forgotten things...but at least it runs well.

The engine is 1098 with what turns out to be a 1275 head

The speedo is wildly out reading over 55 mph at 40 mph. II do wonder what sort of gearbox and diff I might have, might have something to do with it.

Without pulling things apart are there any identifying marks or features on box or diff that would help to give me a clue?

[Peted7202]

To get the diff ratio lift the rear wheels off the floor, ensure it's out of gear. Mark a fixed wheel and a fixed propshaft position. Rotate the wheel one turn at a time and note the propshaft revolutions.

Leave the rear wheels off the floor. Remove the spark plugs. Mark the crank pulley fixed position and the propshaft fixed pulley position. Put into each gear in turn and rotate the engine with the starter handle. Note the crank to prop revolutions.

[les]

The diff usually has the ratio marked on the casing.

[svenedin]

There are many reasons why the speedo may not read correctly e.g:

1) Wrong wheels and tyres (giving a different total circumference to standard). Usually this would be fairly small error ratio.
2) Wrong speedo. These are marked with a number which is the tpm (turns per mile). A standard late Minor tpm is 1408
3) Faulty speedo
4) Different gearbox and/or differential

If you can calculate the final drive ratio you should be able to find a suitable speedo that will have the right tpm and hence read correctly.

Stephen

[svenedin]

The diff usually has the ratio marked on the casing.

Which may take some finding in amongst years of old oil!

[geoberni]

The diff usually has the ratio marked on the casing.

Which may take some finding in amongst years of old oil!

And that it's hand stamped on the top... needs to be cleaned and then get your phone in there for a photo...

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Lifted this image from online to show where it is:

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[michael4]

Many thanks for the marking location, I will face up to going hunting for it.

The speedo is marked thus:

s/n 4419 / 04 1376

This is listed on the Potteries website as being for a 4.22 diff from chassis number 1203563 (ours is 1287280, 2 door)

Is this correct?

[svenedin]

So the speedo is 1376 tpm whereas the standard speedo for a late Minor is 1408 tpm.

If the diff is standard then the reason for the over-reading speedo is likely that it is the wrong turns per mile although I am not sure whether it can account for such a significant over-read.

Stephen

[michael4]

I'll have a go at identifying the diff by marking wheel and prop shaft etc but what I do not know is the what number of turns I should be looking for. I know there is an older post about this but just don't seem to be able to find it.

[geoberni]

Well you've certainly got the wrong Speedometer; incidentally the Potteries website is wrong, 1203563 was the last chassis for that part number, not the first.
4419/04 was:

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Your car should be a 4423/06, unless as detailed in the Parts Book, it was factory for with Radials, then it had 4419/12.
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[geoberni]

I'll have a go at identifying the diff by marking wheel and prop shaft etc but what I do not know is the what number of turns I should be looking for. I know there is an older post about this but just don't seem to be able to find it.

Quite honestly, I found it took me about 15 minutes to check my Diff marking.
A lot easier to get it up on a couple of stands and then get under it with a mirror or your phone camera and take a few snaps, than to mess around counting turns.

[svenedin]

What's important is the tpm number rather than the exact serial number of the speedo. I don't think the number after the forward slash is very relevant. My 1969 tourer has 4423/04 with the expected tpm of 1408

Berni quotes 4423/06 for a car of mine's age but in fact I have 4423/04. Regardless, it reads correctly.

Stephen

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[michael4]

I was wondering about the importance of the first number.

The lettering is placed differently on mine.

Regarding the diff ration, I'll dig out my overalls...

[geoberni]

What's important is the tpm number rather than the exact serial number of the speedo. I don't think the number after the forward slash is very relevant. My 1969 tourer has 4423/04 with the expected tpm of 1408

I'm not aware of the exact make up of the part numbers, other than it related to the vehicle and the technical spec., but as you say, the TPM is the critical part.
Just referring back to the parts list I posted earlier, the SN 4406/8 became unavailable, so the 4477/10 was the direct replacement ...

[svenedin]

I don’t really understand Smiths part numbers either but for comparison with, for example, Smiths heater motors, the first number before the forward slash is the model number and the number after the forward slash is possibly a revision number but that’s a guess.

Stephen

[Peted7202]

Michael, get the diff to prop and the gearbox to engine in each gear revolutions and identifying will be easy.

For the 948 gearbox 1st gear the ratio is 3.628. For 2nd 2.374. For 3rd 1.412. Top is 1

For the 1098 gearbox the only difference is second gear 2.172.

For the diff it should be 4.55 or 4.22.

I'll have a go at identifying the diff by marking wheel and prop shaft etc but what I do not know is the what number of turns I should be looking for. I know there is an older post about this but just don't seem to be able to find it.