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MG Midget and Sprite Technical - Diff ratios. 3.889 vs 3,9

Some of you who have had Fords in the past, might be able to answer this.

It seems that some Ford diffs are repackaged Moggie thou diffs.

Also, would the difference between a 3.889:1 and a 3.9:1 diff be noticeable at all?

The 3.9 diffs have 10/39 teeth.

The 3.889 diffs, have 9/35 teeth.

Could you even tell which one was in the axle with the difference of ratio being so small?

Lawrence Slater

From memory my 1500 had 11/41 stamped on the diff casing i.e. 3.72/1
but I had to bead blast it to find it. Less than 1% difference between 3.889 and 3.9, bugger all really.
Cheers
Rod
R W Bowers

Lawrence,

If my calculations are correct, at the same revs you would be doing 100 mph with a 3.9 diff and 99.7 mph with a 3.889 ....... I doubt that you would notice the difference.

I have worked on over 40 Spridget and Morris diffs as I used to convert them to fit MG TCs and TC race cars (3.7, 3.9, 4.2, 4.5, 4.8, 5.1, 5.3). I have never seen a 9/35 version. What model Ford was it? What are the differences? Are they interchangeable? I am interested in any details and differences.

Bob Schapel (Australia)
R L Schapel

"It seems that some Ford diffs are repackaged Moggie thou diffs"



Elaborate Lawrence?




I doubt that FMC would have been doing that!
SR Smith 1

Thanks Rod.

Bob. Ford English diff. There appear to be 3.889 and 3.9:1.

The markings on the crown wheel are 9-35 or 10-39.

If you google on the subject of 'Ford English diff', there are many results. I was trying to find out the reason for such a small difference, because I want to fit a 3.9(ish) to my Capri axle and wondered if there was a reason to be specific about the ratio. Why on earth did Ford make a 3.9, and a 3.889?
Lawrence Slater

Lawrecnce,
is there a date difference between the two ford ratio diffs? Perhaps the change wasn't to do with needing a different ratio, but needing a smaller overall diameter diff casing. Or maybe the reduced number of crownwheel teeth meant that they could be slightly thicker and stronger and less likely to strip? Possible that the difference is because it was the least possible change in the ratio, whilst addressing a different problem.

Just speculation, I know nothing.
Guy W

I think the term Ford English is probably used to differentiate from Ford Cologne axles [known as Atlas]. Atlas axles are physically bigger and can handle more torque and were used for high power competition cars.
Doug Plumb

'Why on earth did Ford make a 3.9, and a 3.889?'


Hang on - this rings a bell.

I have a very dim and distant memory of a friend of my Dad saying something about the numbers of teeth on the crown wheel and pinion being changed from one to the other because it got rid of a harmonic vibration that made the earlier (don't know which one) noisy. While maintaining the final drive ratio almost the exact same. Something along those lines anyway. Although I'm not sure it was this diff he was on about; but I do remember him being a Ford nut.

He also said that's more or less the same reason for the cam bearings in the Pinto engine being different sizes. I recall him saying he wished they had made the rear bearing smaller than the front, instead of the other way round as he rallied a Pinto engined Escort (IIRC) and had to cut a hole in the bulkhead and dash to change cams without taking the head off as the camshaft had to come out backwards (into the car).

I remember thinking at the time it sounded plausible, but don't quote me on it; we were in a pub at the time!

Has anyone else heard of this? Does it sound reasonable to you?
Greybeard

SR. I was reading (forgotten where) that the diffs in some old small fords of the same era, used a repackaged moggie diff.

Guy.
I know as little as you. Most of the last 40 years have been more to do with bmc than ford. But I have read something about light and heavy duty diffs, of the same ratio. And even read that there is a 3.91:1 ratio, but that the 3.889, 3.9, and 3.91, all come under the 3.9 class of ratio.

Doug. I wondered what an Atlas diff was.

Maybe greybeard.

What I'd like to find is, a history of Ford diffs and their development. Much as you can find one for gearboxes, when you do any searching for T9 boxes.
Lawrence Slater

This thread was discussed between 07/08/2015 and 08/08/2015

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