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MG Midget and Sprite Technical - 4.2 Diff

It turns out you cannot get new crown and pinion wheels for a 4.2 diff. All the recon diffs of this type use good condition crown and pinions as they become available.

The standard replacement is apparently now 3.9. This does have a new crown and pinion wheel.

I understand that I would have to change the speedo over to a 3.9 unit to make it read correctly. Is there anything else that would need to change?

What sort of effect (apart from a drop in revs) would the 3.9 have on performance?

R Williams

It will give slower acceleration and a higher cruising speed - when it gets there.
Dave O'Neill2

How much of a difference to acceleration would it make?

My 4.2 is rather worn now :(

For what it's worth I have a 1275 engine and gearbox.
R Williams

Most 1275s had 3.9 diffs anyway, it was only the early ones that had 4.2s
Dave O'Neill2

That's true,

What would be the percentage error on the speedo? The difference between the 2 ratio's is rather small?

R Williams

hi all,
is it very noticable or worth it, thought about changing my 65 diff to a 3.9 but just put a 3.9 in my 78 midget which was a 3.7 and to be quite honest i cannot tell the difference.
regards bob.
bob taylor

The lower ratio gives you 15.4 mph/1000 rpm and the higher 16.5 I believe.
That would be a measure of the speedo error, on top of the "normal" errors. It doesn't seem much but believe me it is noticeable.
I changed from 4.2 to 3.9 and the car is much quieter at 60mph with the higher ratio. Around the lanes I find I need to drop a gear more often as bottom end tracktability is reduced a bit.
Having originally had the 4.2 axle with a 3.9 speedo, and then fitted the correct speedo and subsequently changed to the 3.9 dif (and correct speedo) the improvements were worthwhile all the way through.
Noisy gearbox seemed much quieter.
GraemeW (Kent!)

new 4.2 CWP? often on ebay

David Smith

I had a '68 1275 in the seventies as my daily car and changed from a 4.22 to 3.9 - great improvement - really didn't miss the acceleration and 2nd and 2 - 3rd becomes much better ratios to use the power of the 1275.
As Guy notes quieter cruising in top also.

IMO the 1275 should have had the 3.9 from the outset.

richard boobier

Car was origibally 1098
Alan Anstead

I am more inclined to go to the 3.9.

However being young :) Would I miss the acceleration from the 4.2?

Also, is it absolutely necessary to change the speedo to a 3.9? Is the read error that noticeable? I don't fancy rummaging around the back of my dash to change it!
R Williams

IMHO you need to change the speedometer. Buy on Ebay, 20 min change over. As well as the speed reading, the mileometer innacuracy gets really annoying too.
GraemeW (Kent!)

Check which speedo you have first.

It's not uncommon for them to have been changed for the wrong one.
Dave O'Neill2

I changed my frogeye diff from a 4.2 to a 3.9 and the speedo is now more accurate at low speeds (according to my sat nav) than it was before!! Indicated 30mph was an actual of 27mph, now an actual of 31mph. Suggest that you check before changing your speedo if it bothers you.

SA Wood

yep my frog was the same when I changed the diff to a 3.9 But I found when cruising over 60 it was less accurate. The original cross ply tyres would have been bigger in diameter than modern 80 ratio radials and the effect of changing the diff could be less pronounced??
Bob Beaumont

My speedo is now within 1 or 2 mph across the range. Before with the wrong speed for the dif it was significantly out. Rylan knows the correct speedo references, so it's easy to check.
GraemeW (Kent!)

What would a serial number of SN 6142/09 relate to?

I cannot get to the document on the members area of the main site that will tell me this sort of information.

R Williams

6142/09 is not listed in Horler; other 6142/nn could be either for 4.2 or 3.9.
What is the 4-digit number for TPI, that should enable us to identify it.
David Smith

1000, that won't be correct, I shall keep searching for a 3.9 speedo
R Williams

According to Moss, 6142/09S is early 1500 Midget.

Is your speedo 100 or 120MPH version?
Dave O'Neill2

yes I think that is an early 1500 speedo so for a 3.9 diff.
David Smith

It may be for 3.9 diff, but 1500 speedos were 120MPH IIRC.

Also, is the gearbox output on a 1500 the same as a 1275?
Dave O'Neill2

(AFAIK from previous research)
6142/09S - the S is for Sweden, I've no idea what the differences would be, perhaps Europe had 5% error allowance for new vehicles as opposed to our 10% was it, or perhaps one for Sweden need different mandatory markings?

I know what I'd advise Rylan to do but as I'm not a member of the Kent MG or MGCC Mafias I'd be wasting my typing finger
Nigel Atkins


All advice is welcome no matter from what angle.
R Williams

I'd go with what Simon's put - stick with your existing speedo (it doesn't sound like it was original to the 1098 anyway)

when you fit a 3.9 diff then you'll see how inaccurate (or not) it makes your existing speedo

you can always pick your speed from the rev counter anyway

buying a s/h speedo could make things more inaccurate unless that speedo has recently been reconditioned by one of the recognised chaps - I don't know where the big suppliers get theirs reconned

if you want to have your existing speedo reconned and recalibrated later then you can

I don't think you'd notice much acceleration difference in swapping to a 3.9 but it would be a bit quieter because of the drop in revs and possibly an increase of mpg over long journeys

IIRC your engine picked up a lot of untapped power when you had that rolling road session which suggests to me you didn't realise how much it was running under potential so with regular use(?500 miles in 2 months?) and regular servicing and maintenance you should pick up more power if only because you learn how well the car should go)
Nigel Atkins

I am going to stick with my existing speedo.

I shall probably stay with a 4.2 diff as the increase in cost for a 3.9 is rather large. The 4.2 does have a 6 month warranty so if it is duff then I will go for a 3.9.

I did gain a extra 26bhp when the car was rolling roaded. I have managed about 1500 miles since february this year, but I plan to at least double that next year.

With a new gearbox, diff and tyres I should be sorted for a little while :)
R Williams

the S is not for Sweden, it refers to UK market early 1500s, it IS in Horler, p 124. (I missed it before as I don't often venture into the 1500 section)
David Smith

once again my memory has proven to be faulty and I doubt it'll be the last

I was thinking the same as Dave that it'd be a 120mph speedo for a 1500 - so in theory the speedo is wrong for the present set up but would be OK for the change to 3.9

bit of an ego gauge from the PO given a 26hp increase on a 1275 engine that the manufacturer claimed was only 64hp new (even allowing for different methods of measurement)

new tyres is good but the g/box and diff might have soldiered on for more years given your low annual mileage, they're not important anyway compared with brakes, steering and suspension especially now the 26bhp has been recovered - then there's all the electrics, I'm betting you've got a way to go yet ;)

3,000 miles per year isn't too bad provided it's spread fairly evenly over most if not all of the 12 months, doing one trip of 1,500-2,000 miles then the other 1,000-1,500 over the rest of the year wouldn't necessarily speed up the ironing out the wrinkles from the car's (re)build and/or lack of use
Nigel Atkins

The key to whether the speedo is correct is given by the digits above and to the right of the Smiths logo. It indicates turns per mile. THe 3.9 is marked I believe, as 1376 and the 4.2 as 1472
GraemeW (Kent!)

as Rylan has put its marked 1000 it suggests his present speedo might need calibrating and as he's going for a 4.2 diff this will also add to the mix

but at say in top gear 15.5mph/1,000 rpm (to round things up) then at 2k revs you're doing about 31mph (discounting rev gauge errors that are normally small)

so 2k revs about 30

2.6k revs about 40

3.2k revs about 50

3.9k revs about 60

4.5k revs about 70

or just think 15mph/1,000 revs

you soon get to know the speeds from the rev counter - purely as an assessment of speedo error to be corrected of course, not regular road use

Nigel Atkins

Well my 1275 with a Toyota box wouldn't pull with a 3.7 or a 3.9 its only happy with a 4.2 and the engine loves to rev.
Ken Harris

This thread was discussed between 19/11/2013 and 30/11/2013

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