Differential Ratio - Standard 3.9 For Early Mk1 ?

[CNO510B]

Hi all,

From all the information I can find it seems that the "Standard" ratio for an early/A frame Mk1 should be 3.9:1 can anyone confirm this 100%, i.e. extract from a workshop manual for instance. It is stated in my owners handbook, which is original to the car.

The other thing I would like confirmation of is the crown wheel to pinion count of this 3.9:1 ratio. I believe it was 39/10 can anyone confirm this.

The reason I need to know is I need a new/replacement for my car and cannot find anyone who does a 39:10 pair. I can find lots of available combinations which give 3.889/3.875/ etc etc but not the 39/10. Any suggestions for a supplier would be very much appreciated.

I have found a UK manufacturer who will make better than OEM quality (better materials/heat treatment/finishing/shot peening to todays standards and spec's) pairs of 39/10's but unless I can order at least 10 pairs the price is rather out of the park. The price each pair for an order of 10 or more would be approx. £360 inc VAT. I can provide the details if anyone is interested.

Many thanks in anticipation of any replies.

Brian.

 

[Keith Halstead]

Axle extract from Lotus Workshop Manual

 

[CNO510B]

Hi Keith,

Many thanks for the reply and for adding the pic/removal instructions from a WSM. If you have the manual, in the spec's section does it quote the various ratios and the corresponding tooth counts. Here's hoping.

 

[Keith Halstead]

If this is what you need :-

No. of teeth on crownwheel = 39
No of teeth on pinion = 10

 

[MLI]

I believe both the 10/39 (3.9:1) and the 9/35 (3.889:1) were used on the mk2 Cortina?

Yes please, I'm interested in the details about the crown wheel and pinion manufacturer you mention.

I've bought a new repro 3.54:1 crown wheel and pinion set from Burton for my 66 mk1 Lotus.
But after hearing about quality/hardening issues with them, I'll probably source one from another supplier (apparently, Burton source theirs from Turkey...).

So far, I've contacted Quaife re their Tran-X crown wheel and pinion sets.
Quaife's reply:
"We are aware that some of the Burton CWP kits have had issues.
We sometimes supply Burton with CWP kits when they are out of their own stock, and vice versa.
The gear cutting data that Burton use is different to ours, however, hypoid bevel gear cutting is not a process that we can undertake in-house, so we do sub-contract this operation to a gear cutting company that has this capability."

You may have seen this already, but anyways:
For Quaife's Tran-X 3.889:1 CWP, see
https://shop.quaife.co.uk/english-crown-wheel-pinion-kit-3-889
(apparently, this can be "REM superfinished" as well, "giving a much smoother surface finish to the gears").

 

[ray8829]

I have both a 39/10 and 35/9 used diff assemblies if you cannot get fixed up with new.

 

[Bertil]

The 3,9:1 ratio is not a gear that will wear down in a short time of use like for example the 4,44:1 does so finding a good second hand one is easy, that is most likely the reason why it not is reproduced.
The 3,9 was fitted to several cars and is easy to find. I should say it was the most common ratio used.
Escort 1100 and 1300 early cars, later has 3,889 :1
Cortina mk1 with 1500 cc engine both deluxe and GT (not estate)
Cortina mk2 with 1500 and 1600 cc engine both GT, super and deluxe (not estate)
From my records it looks like the 9/35 3,889:1 was introduced in 1969, that is probably why the 10/39 not is listed for Capri. I am not sure regarding late mk2 Cortina having 9/35, but having owned and parted out several Cortinas I have never seen a 9/35 in a Cortina.

Regarding the new ones with hardeneing issues; I have never heard of anyone having problems in a roadgoing car, only pure racers has as far as I have heard had problems, and that goes for the lower ratios, 4,44, 4,7 and 4,9. However the problems should be gone with some extra surface hardeneing and that can easily be done before fitting.


Brian, if you dont find any in uk I have a couple of sets.

 

[CNO510B]

Hi all,

Hi to all who have posted replies to my post, the info., comments and offers of good used parts are very much appreciated. Apologies to all for the delay in reply.

I am now happy that the 10/39 - 3.9:1 is the correct part for my car and although there are several offers of good condition used items I am still going to try and drum up enough interest to have some manufactured from the supplier I have found.

Now knowing the parts were original to many Mk1 and Mk2 Cortina’s etc in addition to the LC, I am going to contact all the other clubs to try and drum up interest.

I have attached the details of the possible supplier.

I firmly believe they would be able to supply a UK manufactured parts which would be better than OEM quality pairs of 39/10's (better materials/heat treatment/finishing/shot peening to today’s standards and spec's) but unless I can order at least 10 pairs the price is rather out of the park. The price each pair for an order of 10 or more would be approx. £360 inc VAT.

Being an ex. Rolls-Royce Aerospace troubleshooting engineer I know quite a lot about what makes a good gear and what does not, having seen lots of failures and know how we fixed them. Super finishing is all well and good but it will never do anything to make a bad gear (produced from poor material/incorrectly heat treated/poorly finished) any good. If gear is made from the correct mat’l, is correctly heat treated and finished the best thing you can do to it is shot peen it. This shot peening process improves the surface finish and most importantly further strengthens and increases the wear resistance of the teeth. NT&G used the same provider for their shot peening as RR, I had a chat with NT&G’s top man.
Cost Comparisons
Quaife 9/35 – 3.889:1, £280 not REM superfinishing, £380 inc REM super finishing

Burton £278 REM not offered

NT&G £296.20 ex. vat £355.44 inc vat – UK made, latest tech. materials/heat treatment/finishing/shot peening.

So, if anyone is interested in new, top quality 10/39 English CW&P’s please let me know as soon as possible.

 

[Pete G]

Interesting stuff!
I was looking at the options last year after not being able to find/fit a quiet diff. Bear in mind that any used one is going to be approx. 50 years old with an unknown history!
When I priced up Burtons it came to £349 including seals, washers etc.
I have one more diff assembled by a reputed engineer which hopefully will be fine, but if not I will be after a new one as well.

What exactly is 'REM superfinishing'?

There was more than one mention in the write-ups of the development of the LC of the importance of 'running in' the diffs otherwise they become noisy very quickly,- although no mention of how that is done. Maybe just keep the speed down for 500-1000 miles maybe?

 

[CNO510B]

For REM super finishing see - https://www.remchem.com/services/isf-process/

For shot peening see - http://www.gearsolutions.com/article/detail/6730/an-introduction-to-shot-peening-for-increasing-gear-fatigue-life

REM really does only improve the surface it does nothing to the material structure.
Shot peening not only improves the surface finish but it also improves the material structure increasing wear resistance and fatigue life.

Running in of gears used to be necessary as the surface finish of a component years ago was not as good as it is today. The rubbing of two parts against each other smoothed them both, rub two rough stones together and they are hard to slide the two touching surfaces become smooth as the high spots/debris is removed then they slide nicely over each other, this was how it used to be. Today you have two smooth stones they slide easily no debris released.

Running in is done with light pressure to start with then slowly increasing the pressure, for a car its :- for the first 500 miles light throttle max 3k rpm, over the next 500 miles slowly increase the throttle opening and rpm a bit every 100 miles. Do the process too quickly, full throttle from day one, and lumps get ripped of each surface doing damage which can never be undone.

My view is that running in, when the gears are either REM or SP, is not necessary, as long as the meshing of the gears is set up correctly during the build.

 

[Pete G]

Running in is done with light pressure to start with then slowly increasing the pressure, for a car its :- for the first 500 miles light throttle max 3k rpm, over the next 500 miles slowly increase the throttle opening and rpm a bit every 100 miles. Do the process too quickly, full throttle from day one, and lumps get ripped of each surface doing damage which can never be undone.

My view is that running in, when the gears are either REM or SP, is not necessary, as long as the meshing of the gears is set up correctly during the build.

As it is the diff it is road speed that needs to be controlled, not engine revs!
Surely the crown wheel and pinion would be case hardened?
Set up and meshing is certainly critical!

 

[CNO510B]

Speed of rev's whichever you like its all about load on the components being run in, low load to start then progressively increase the load.

They are heat treated which comes in many forms, nitriding, tuftriding, case hardening and many other forms of heat treatment either one their own or in combination with each other. Shot peening is an additional and beneficial process after heat treatment.

Agreed.