A question for those who have been there and done that.
I have acquired from a reputable source a 3.9 diff that is said to be perfectly useable with maybe the only issue being with the pinion oil seal.
My plan is to just fit it and see how things go; in the belief that should the seal need replaced then it is easier to do after installation due to being able to use the handbrake to hold the pinion steady whilst undoing then refitting the pinion nut to the correct torque which I glean from previous posts is 132ft/lb.
Would anyone care to talk me out of this plan due to hands-on experience?
Once I have read the sage advice on this thread I may ask the same question on Facebook mainly for amusement.
Fitting a 3.9 diff to a 1969 2-door Minor
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unclealec
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Fitting a 3.9 diff to a 1969 2-door Minor
Alec Gatherer
Manchester-by-the-Ditch.
Manchester-by-the-Ditch.
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philthehill
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Re: Fitting a 3.9 diff to a 1969 2-door Minor
Your plan is sound. 
The pinion nut is 140 ft/lb.
Start at 132 ft/lb and work up to 140ft/lb.
Do not exceed 140 ft/lb because you could have a compression spacer between the pinion bearings and if tightened in excess of 140 ft/lb you could be increasing the pre-load on the bearings without realising it. Check the rotation of the pinion occasionally as you build up the torque.
You should have some resistance to the turning of the pinion but not too much.
Good luck
Phil
The pinion nut is 140 ft/lb.
Start at 132 ft/lb and work up to 140ft/lb.
Do not exceed 140 ft/lb because you could have a compression spacer between the pinion bearings and if tightened in excess of 140 ft/lb you could be increasing the pre-load on the bearings without realising it. Check the rotation of the pinion occasionally as you build up the torque.
You should have some resistance to the turning of the pinion but not too much.
Good luck
Phil
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unclealec
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Re: Fitting a 3.9 diff to a 1969 2-door Minor
Many thanks Phil.
Mechanical prowess is one thing, but having done it before always trumps theory in my book.
Mechanical prowess is one thing, but having done it before always trumps theory in my book.
Alec Gatherer
Manchester-by-the-Ditch.
Manchester-by-the-Ditch.
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philthehill
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Re: Fitting a 3.9 diff to a 1969 2-door Minor
Happy to be of help.
As regards upping the diff from 4,55 or 4.2 to 3-9. The 3.9 diff gives the car longer legs without sacrificing overall performance. IMHO the 3.7 diff is too high for the 948cc or 1098cc 'A' Series engine. The 3.7 is more suited to the 1275cc 'A' Series engine.
For comparison I had a Marina rear axle (with a 4.11 diff) fitted in my Minor Ser 2 way back as part of a CS Auto-Classics upgrade.
The front suspension was also Marina based including the disc brakes.
The engine was a standard 1098cc engine. With the 13" dia wheels fitted all round and with 155 x 13 tyres the car went very well.
It went even better with the 3/4 race 998cc engine latterly fitted and would cruise at 70mph plus all day without fuss.
I was doing around a 1000 miles a week in the car mostly up and down the M6 from South Hampshire to West Cumbria.
The car was a good drive and easy to maintain and more reliable than my then new Citroen BX 1.9 turbo diesel.
For those wanting to fit the Marina axle you will need to weld on the revised axle seats (available from ESM) and change the rear U/J for a Marina item as the bolt PCDs are different. I have several Marina rear prop-shaft drive flanges available if anyone wants one to fit to their Minor prop-shaft.
Fitting the Marina rear axle is an easy job and with the 13" wheels tyre availability is good.
Wider wheels are available sourced from the Marina TC.
Looking back I should have kept the Marina rear axle
As regards upping the diff from 4,55 or 4.2 to 3-9. The 3.9 diff gives the car longer legs without sacrificing overall performance. IMHO the 3.7 diff is too high for the 948cc or 1098cc 'A' Series engine. The 3.7 is more suited to the 1275cc 'A' Series engine.
For comparison I had a Marina rear axle (with a 4.11 diff) fitted in my Minor Ser 2 way back as part of a CS Auto-Classics upgrade.
The front suspension was also Marina based including the disc brakes.
The engine was a standard 1098cc engine. With the 13" dia wheels fitted all round and with 155 x 13 tyres the car went very well.
It went even better with the 3/4 race 998cc engine latterly fitted and would cruise at 70mph plus all day without fuss.
I was doing around a 1000 miles a week in the car mostly up and down the M6 from South Hampshire to West Cumbria.
The car was a good drive and easy to maintain and more reliable than my then new Citroen BX 1.9 turbo diesel.
For those wanting to fit the Marina axle you will need to weld on the revised axle seats (available from ESM) and change the rear U/J for a Marina item as the bolt PCDs are different. I have several Marina rear prop-shaft drive flanges available if anyone wants one to fit to their Minor prop-shaft.
Fitting the Marina rear axle is an easy job and with the 13" wheels tyre availability is good.
Wider wheels are available sourced from the Marina TC.
Looking back I should have kept the Marina rear axle
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moggiegeek
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Re: Fitting a 3.9 diff to a 1969 2-door Minor
Renewing the seal sounds easier with this method of keeping the diff in its carrier on the axle, using the road wheel.
I did the suggested spring balance and string method and after crushing the first spacer by sailing past 140ft/lbs in an instant. My advice would be to buy some spare crushable spacers! It was a fleabay 3.9 diff where they neglected to mention the completely shot nose oil seal. That said it's given no trouble to date.
I did the suggested spring balance and string method and after crushing the first spacer by sailing past 140ft/lbs in an instant. My advice would be to buy some spare crushable spacers! It was a fleabay 3.9 diff where they neglected to mention the completely shot nose oil seal. That said it's given no trouble to date.