Morris Minor Owners Club

I think you are missing the point. The 140ft-lb torque is a guideline when using a collapsible crush spacer. Every crush spacer is different. The required pre-load on the pinion bearings is set by measurement and measurement only. Anything else is a guessing game. It is very easy and expensive to over torque a crush spacer if you are not familiar with differential set-up. It is explained in the video linked above.
You torque the nut to approach the pre-load and stop once within spec. -not the other way around.
Regards
Declan

I have been offered a diff supposedly from a midget which has 9/33 on the crown
I havent seen you guys talk about this ratio
Does that sound right ?

That seems to be the 3.7 diff-and is quite difficult to source. There is not a huge difference between the 3.9 and the 3.7.
I find the 3.9 makes my speedo spot on.
Regards
Declan

I know it works out at 3.7:1 but the actual gears being 9/33...
I cant find any mention of that . 41/11 and 10/37 I have seen on other threads but not 9/33
Does anyone have any info on this ?

As far as I know the Austin Healey 100-4 BN1 used the 9/33 diff. I doubt if that is a Midget diff that you have been offered.
Regards
Declan

The correct ratios are:-

3.727...………………….11 pinion teeth/41 crown wheel teeth. Commonly referred to as 3.7.

3.9...……………………..10 pinion teeth/39 pinion teeth. Commonly known as 3.9.

There are some BMC Special Tuning 'A' Series Midget differential assemblies around and you may have been offered one of those.

A picture of the carrier would help. The carrier shown does not on the face of it appear to be of the 'A' Series Midget.

There are different marques of MG Midget not just the 'A' Series Midget so possibly a misunderstanding of the item being offered.

So the 9/33 isnt suitable ...bu@#$er

From the various Special Tuning booklets that I have, and also looking at the Marina as well, none of them use a 9/33 (3.66:1) cwp ratio, so it does look that it comes from something other than an A-Series engined car?

Richard
There are some BMC/BL Special Tuning Differential assys that were not listed in the BMC/BL Special Tuning booklets or were not made available ex factory. They were one offs produced especially for the BMC/BL works teams. At the demise of BMC/BL Special Tuning I suspect that they went into general circulation.

I went to near Guildford last year to collect a NOS Revolution 7J wheel and the seller (who was in the motor trade and had raced BMC cars) had a BMC/BL Special Tuning diff assy that I had never come across before. If I remember correctly it was a 3.4. I did give it a through examination to confirm that it was suitable for a 'A' Series axle and it was.

It had already been sold and was awaiting collection otherwise I might have purchased it.

Phil

Wheel size, tyre aspect ratio and gearbox ratios will all make a difference. Marinas were on 13” rims?

I ran a 3.78 diff in my Escort with relatively low aspect tyres (of the day) back in the 1970s. It was a good match with the 2000E gearbox and the engine I was running at the time (no, not a 1100cc!).

alawrence10360 wrote: ↑Tue Mar 05, 2019 7:15 am I know it works out at 3.7:1 but the actual gears being 9/33...
I cant find any mention of that . 41/11 and 10/37 I have seen on other threads but not 9/33
Does anyone have any info on this ?

It is being sold with half shafts rear springs and a RWA midget wing all from the same vehicle apparently

Are you sure that it isn't simply badly stamped - and actually it is 9/38?? Perhaps more likely?

The diff ratio is also stamped on the diff carrier alloy housing and can be found on the top offside of the housing as shown in your picture above.

If it does turn out to be 9.33 which is 3.66 - I would suggest that the ratio is too high and your engine will struggle to pull the car along in top when faced with the slightest grade..

If you already have a 3.9:1 diff, why don't you have a go at rebuilding it yourself with new seals, bearings and crush tube? If it was mine, I would be itching to have a go.

The 3.9:1 I have has a damaged CW&P
Im not sure I fancy rebuilding it to that level but it worth thinking about if i can get the bits

Here's where I bought my 3.9 CWP. (246.46€).
The tolerances were not the best (Made in Turkey) and this company is distributing them all over.
https://shop.bastuck2.de/index.php?&cat ... =1&lang=en
Regards
Declan

https://www.ebay.co.uk/itm/MG-Midget-3- ... Sw7MNcWeqb

You must use a BTA carrier and not a Minor ATA carrier.

Declan_Burns wrote: ↑Wed Mar 06, 2019 2:39 pm Here's where I bought my 3.9 CWP. (246.46€).
The tolerances were not the best (Made in Turkey) and this company is distributing them all over.
https://shop.bastuck2.de/index.php?&cat ... =1&lang=en
Regards
Declan

thanks Declan

philthehill wrote: ↑Wed Mar 06, 2019 4:33 pm https://www.ebay.co.uk/itm/MG-Midget-3- ... Sw7MNcWeqb

You must use a BTA carrier and not a Minor ATA carrier.

Thanks
What does BTA mean ?

BTA part numbers , such as BTA 806 for later halfshafts on Midgets, denotes a sturdier part. In the case of Diff it means a thicker web round the crownwheel carrier. Basically it means an ability to accept more Oooomph.