Fitting 3.9 diff

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mollythemoggie
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Fitting 3.9 diff

Postby mollythemoggie » Mon Dec 05, 2011 11:44 am

Hi I have been given a 3.9 diff out of a midget.. just wondering is there anything I'll need to swap beside the diff when fitting? i.e. axels mounting the tailshaft etc?

Cheers,
Tom

chrisryder
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Re: Fitting 3.9 diff

Postby chrisryder » Mon Dec 05, 2011 12:00 pm

nope. just a straight swap.

be sure to have all the gaskets you'll need to hand. not just the big diff one, but the halfshaft ones too, as these need to come out to get the diff out.

i presume you're aware it'll affect the speedo reading?

Peetee
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Re: Fitting 3.9 diff

Postby Peetee » Mon Dec 05, 2011 12:01 pm

It's a straight swap - although it's sensible to replace the halfshaft/hub gaskets and rubber seals at the same time.
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mike.perry
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Re: Fitting 3.9 diff

Postby mike.perry » Mon Dec 05, 2011 12:47 pm

Check your speedo with a sat nav before you change the diff and again with the new diff, you may well find that it is more accurate with the new diff, mine is.
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mollythemoggie
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Re: Fitting 3.9 diff

Postby mollythemoggie » Mon Dec 05, 2011 12:52 pm

Thank you all for the replies! most appreciated! yes am aware! my problem is i have a 5sp box and nissan motor and the stock diff gives me high revs and mild speeds so just trying to correct that.. the previous owner did so using 195 tyres whereas I've since gone back to 175's

with regards to the seals/gaskets.. could i get a shopping list please! never done anything on the rear of the car before short of replacing shocks!

Peetee
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Re: Fitting 3.9 diff

Postby Peetee » Mon Dec 05, 2011 4:17 pm

Rear hub "o" ring rubber x 2
Rear hub gasket x 2
Gasket differential to axle casing x 1
Older and more confused than I could ever imagine possible.

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Re: Fitting 3.9 diff

Postby linearaudio » Mon Dec 05, 2011 4:58 pm

Also beware that the Midget had the oil filler on the axle housing, whereas (at least my Molly) had the oil filler on the diff housing. So you can end up with no normal way to fill the axle....!
As it was late afternoon and I had already had a row with someone trying to sell me a different electric supplier whilst I was under the car, I did the only practical thing... Fitted one halfshaft, drum, wheel etc, and lowered that side of the car (hence the car was sloping to that side). Then poured the correct amount of gearoil down the remaining open ended axle tube before finishing assembly. If you have time/tools/enthusiasm and a suitable BSP tap then making a new filler in the Midget diff is the sane answer! I was working on the principal that the oil wouldn't leak out to make topping up neccessary, and as for changing the diff oil as part of the routine maintenance...err.. don't think I have ever done that yet on any of my cars :oops:

bmcecosse
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Re: Fitting 3.9 diff

Postby bmcecosse » Mon Dec 05, 2011 5:08 pm

I ended up with TWO filling points.... And yes - do add a bottle or two of EP 90 to that shopping list.
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mike.perry
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Re: Fitting 3.9 diff

Postby mike.perry » Mon Dec 05, 2011 5:42 pm

If you are changing from a 4.22 to a 3.9 diff then you should not have a problem
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mollythemoggie
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Re: Fitting 3.9 diff

Postby mollythemoggie » Mon Dec 05, 2011 7:59 pm

Wow thank you for the heads up re the thread tap very good to know!!

And thank you all for such comprehensive answers MOST appreciated!!

welshrat
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Re: Fitting 3.9 diff

Postby welshrat » Mon Dec 05, 2011 9:47 pm

I recently changed my 4.5 to a 3.7 and ended up with no oil fill option, used an old gear oil bottle (with plastic tube) to add the correct amout via the breather on the top. No way of checking level now so I place a cardboard sheet under the axle when parked up to check for leaks.


mollythemoggie
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Re: Fitting 3.9 diff

Postby mollythemoggie » Mon Dec 05, 2011 10:25 pm

how much oil do they take out of interest?
Cheers,
Tom

mike.perry
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Re: Fitting 3.9 diff

Postby mike.perry » Mon Dec 05, 2011 11:51 pm

1.5 pints Hypoid 90
Engine oil in the gearbox
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mollythemoggie
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Re: Fitting 3.9 diff

Postby mollythemoggie » Tue Dec 06, 2011 12:22 am

Thanks Mike!

Chipper
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Re: Fitting 3.9 diff

Postby Chipper » Sat Dec 10, 2011 12:33 am

And what are the effects of fitting a 3.9 diff over a 4.22? Very noticeable, or barely any difference?
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bmcecosse
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Re: Fitting 3.9 diff

Postby bmcecosse » Sat Dec 10, 2011 10:24 am

Slightly noticeable -every little helps -but 3.7 is perfect.
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mike.perry
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Re: Fitting 3.9 diff

Postby mike.perry » Sat Dec 10, 2011 10:58 am

With a 1275 engine a 3.9 diff is a good balance between acceleration and higher speed cruising, however with a 5 speed box the 5th gear is already an overdrive so the car may well be overgeared in top. Others may disagree.
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dellerie
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Re: Fitting 3.9 diff

Postby dellerie » Sat Dec 10, 2011 1:17 pm

mike.perry wrote:1.5 pints Hypoid 90
Engine oil in the gearbox
my manual states 1 litre.

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Re: Fitting 3.9 diff

Postby dellerie » Sat Dec 10, 2011 1:19 pm

welshrat wrote:I recently changed my 4.5 to a 3.7 and ended up with no oil fill option, used an old gear oil bottle (with plastic tube) to add the correct amout via the breather on the top. No way of checking level now so I place a cardboard sheet under the axle when parked up to check for leaks.
you could re-fill via one of the half shaft holes when removed.

mike.perry
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Re: Fitting 3.9 diff

Postby mike.perry » Sat Dec 10, 2011 5:24 pm

dellerie wrote:my manual states 1 litre.
BMC Morris Minor Workshop Manual AKD 530A & M General Data Page 7 and Australian Morris Minor Workshop Manual Page 7 General Data Rear Axle 1.5 pints (0.85L)
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