Differential gear teeth

[akuchanny]

Hi I wanted to know if the wheel gears inside the diff casing have any effect on the gear ratio, I always thought it was the large gear and the small gear at the base of the diff that contexts to the prop shaft that controlled gearing.

I have one diff that has 15 teeth on the gears that the half shafts go into that will fit a minor from a 4.5 diff, and a 16 teethed gear from a 3.9 I have tried to fit, only to find the half shaft splines are not the same. Have pulled the diff cages out, and removed the bearings and large gears on both, so I could swap the differential cages over and make the 3.9 have the right half shaft splins. I just want to know if this will effect to gearing at all.

Thanks in advance

Andy.

[bmcecosse]

It won't affect the gearing - but obviously one of them is NOT from a Minor........ ALL Minor(and A35/A40/Spridget/Riley 1.5/Wolseley 1500) final drives take the SAME half shafts.... so have the same splined outputs......

[akuchanny]

Thanks for the info, I was under the same understanding. This diff has the same size case and mounting points as well as gear sizes the only difference is the halfshaft splines.

So once I get two stubborn bearings off I will swap the cages over, and drop the gearing on the 1275 from 4.5 to 3.9. Which will make life much better!

[philthehill]

If you swop over the differential cages you may have to reset the interface between the crown wheel and the pinion otherwise it may whine or have excessive play. Just because you can swop the differential cages it over it does not mean that you can do it without resetting the clearances and backlash. See Wksp Manual Section HH7 for details.
If you have facilities to press out the pinion pin it may be simpler to change over the sun and planet differential gears as it is the differential gears into which the half shafts fit and if you refit the original differential cage with the same shims in the same places you should have no requirement to reset the crown wheel.

[bmcecosse]

Perhaps the 3.9 is ex Marina?

[akuchanny]

I guess that's the most likely candidate. As everything else is the same. Was hoping for an easy swap but its been a pig! Am going to get a garage to get the bearings off as they won't move after lots of penetrating fluid and persuasion. The little buggers!

[philthehill]

I would still go with changing the sun and planet gears (differential pinions) because if you have got to get a garage to pull the bearings you might as well get them to remove the pinion pin instead. To me it would make life a lot easier in the long run and get the job done much quicker.
If you read Wksp manual sections HH4 'Removing The Differential Pinions' and HH5 'Replacing The Differential Pinions' you may decide that you are able to do the job yourself as it is not too difficult and does not require any special tools and you would be saving money by not employing the garage to pull the bearings. The bearings are pressed onto the carrier and cannot be removed without using a hydraulic press/puller.
There are several posts relating to BMC/BL/MG 3.9 rear axles on the web that suggest that changing the sun and planet gears is the easiest way when faced with the same problem with axle shaft splines that you are encountering. Later MG midgets had fine spline half-shafts where fitted into the differential pinions.
See links for details and photo relating to change of splines and sun and planet gears:-

http://www.mgparts.co.nz/advice/technic ... ear-axles/

and

http://www.mgparts.co.nz/advice/technic ... -strength/

Lots of good information on the above web site!

[bmcecosse]

Oh! Didn't know about 'fine spline' shafts -so not all 3.9 final drives can be used on a Minor?? Maybe this is one of them? First time I've heard of the problem. Frankly - for the change from 4.22 to 3.9 ratio -I'm not at all sure it's worth the effort.......

[IslipMinor]

Andy,

I don't think there were any changes in the halfshaft spline in the Minor/A30/35/40/Sprite/Midget at all? Looking at the Sprite/Midget, it uses the same halfshaft part number all the way through from Frogeye to last 1500 - BTA806 steel wheel and BTA807 wire wheel.

Frontline do offer 'fine spline' halfshafts for Spridgets, but these have about twice as many spline teeth as standard.

Is the 'new' diff definitely a 3.9? The Spridget 3.9 has 39 teeth on the crownwheel and 10 on the pinion. What does yours have?

[philthehill]

Richard
You may be right but when I trawled the web last night for information there were quite a few postings regarding fine splined half-shafts fitted to later MG midgets and none mentioned that they had Frontline half-shafts and differential gears fitted as a modification. There was quite a discussion regarding the fitting of Frontline half-shafts to Midget axles and separately discussion regarding the fitting of the 'A' series 3.9 diff to Standard Triumph axles which was quite informative.
As you said in your post more information is required and photos would not go amiss.
But I still stand by my assertion that if the diff is a 'A' series BMC/BL/MG 3.9 the easiest way to over come the half-shaft problem is to change the sun and planet gears

[bmcecosse]

I agree with the idea to simply change the sun and planet gears.

[akuchanny]

I did switch the gear cage over, but it was so noisy and ate its shims so ended up getting another 3.9 diff to go in. I had a 948 diff so it was a 4.5 originally as shot off like a scolded cat and topped at out 60 mph. so things have been a bit better since the swap. Only problem is now I don't have a filler hole, so either have to swap read axel cases for a midget one I have or tap a hole.

[bmcecosse]

Well done - or just put a measured amount of fresh oil in through the breather hole.

[katy]

How many splines on a "regular" moggy half-shaft?
I have a 3.9 unit that I want to put in our '61 2 door w/1275 engine and want to make sure things will fit before I pull the existing unit out. Easier to ask a question than pull a half shaft out to count splines.
TIA

[mike.perry]

You will need to change the speedo, I would suggest that a 1408 would be suitable. If you want the original colour then a speedo from the early 1098 car SN4421/12 1408 would fit

[philthehill]

A regular half-shaft fitted to a Minor has qty 10 splines.
The sun gears fitted to the MG Midget 1.2 & 3 and the sun gears fitted to the Minor all have the same part number: ATA7037.
Both the Minor & Midget half-shafts fit into the sun gears on their inner ends.
So there should be no difficulty in fitting a MG Midget 3.9-1 diff to a Minor.

[IslipMinor]

The splines are definitely all the same for Minor, Spridget etc., but be careful as the halfshaft length is shorter on the Spridget, A30, A35, A40 and will not fit a Minor axle. Wolseley 1500 and Riley 1.5 are the same as the Minor.

We fitted a 3.9 diff into the original Minor axle casing and have drilled the rear of the casing to take a welded-on tapped boss so that we have a 'standard' oil filler.

[katy]

Thanks for the replies. As I said, I have a 3.9 unit, but, I don't know what it's out of. It has no filler plug hole so it will get drilled and tapped before installation.
I also knew that I would have to change the speedo, I just have to source one.

[bmcecosse]

Maybe not! What tpm is your speedo ?

[katy]

What tpm is your speedo ?

1525 Car has a 4.5 differential in it right now.

Checked the 3.9 unit and it has the 10 splines axle gears.