Differential ratio

[DEREK ORMEROD]

Good afternoon
My stag has a noisy diff (above 40 mph) I am not TOO bothered about it but have considered getting a new (exchange) one with the higher ratio- I believe the engine is more than capable of pulling higher gearing. However, could anyone tell me how to establish what ratio diff is in my car at the moment. It has had a massive amount of work done over the years so not sure if diff is original (mk2 p reg) If current diff the higher ratio i would probably learn to live with the noise, but if it is the lower ratio I might consider changing on the grounds of improved mpg-at least thats how i would sell the idea to my wife!! Only joking
Many thanks
Derek

[DJT]

There should be a serial number stamped in the flat surface of the underside of the diff. Fairly large numbers and letters, but usually quite faint. If they start with LD then it is probably an original 3.7:1 Stag diff. Sometimes diffs from the big saloons are adapted to fit (as in mine) and the ratio is 3.45:1. The serial number might start with MG.

[jbuckl]

Consider using TD fitchetts for a diff.

[Ian F]

Simple check, what revs are you doing at 70 mph in overdrive top? Assuming it’s a MOD?

I had a TR6 diff installed into my Stag diff casing many years ago, by Hardys of Surrey, so my casing may say it’s a 3.7 but it is a 3.45.

I’m on 15” wheels and at a 70 mph, 65/66 indicated, it’s reading about 2400 or 2600 revs if I remember correctly, but will still pull like a train at 30 mph in overdrive top!

Ian F

[richardthestag]

keep the standard 3.7:1 diff if you have a manual gearbox else you will be moving 10mph at idle speed in 1st gear.

3.45:1 saloon diff is a popular and easy way to lower revs on the automatic transmission stag. be aware that the stag back plate doesn't bolt straight up to the saloon diff. 2 bolt holes are misaligned. so you really need the best saloon diff you can find and then the backplate from a 2500 estate. This will then iirc fit the stag mount plate and ensure the thing is relatively leak free

[DEREK ORMEROD]

Thanks foradvice everybody

[MandM]

Triump Diff Ratios (1).pdf

just for info in this thread, maybe helpful to someone on a search?

[Tdafforn]

I have a 3.54 on my stag. Probably the best thing I have done on the stag. Vv leisurely cruiser. But then I also have 200 bhp and 250 ftlbs from my 3.9 RV8.
Tim

[MandM]

Thought I would post on this thread rather than start a new one, I have been given a Diff to play with before I tackle mine, It was stated as a stag diff but it is a diff from a 2000MK2 which I found out after cleaning the outside etc, number MEC 19831 3.7 ratio.

I pulled the back off and cleaned inside a bit and started have a play, the back lash is between 0.004"-0.085" checked in five places around the crown wheel, the run out is good, the Sun gears and Planet Gears look quite worn and the planet shaft shows heavy wear., I tried checking the pattern but could not get a good idea from using engineers blue. so I ordered some gear marking paste the yellow stuff.

These are the patterns I got from one position and the gears etc, may have to add a couple time as you cant post may photos


P1100530.jpg P1100529.jpg P1100537.jpg P1100544.jpg next

[MandM]

P1100551.jpg P1100552.jpg P1100553.jpg P1100554.jpg P1100555.jpg

will post some more checked in a different position on the crown wheel

[MandM]

the other position on crown wheel

P1100558.jpg P1100559.jpg P1100560.jpg P1100561.jpg

so the questions are what do you read in to these patterns

[MandM]

I should add that this Diff is a shim type with the castellated nut .

[trunt]

I'm about to do this so need some practice..

So took a look at the charts and .. Pinion too shallow??

https://sierragear.com/differential-repair-help-topics/ring-and-pinion-gear-pattern/

[MandM]

Hi Terry, as this diff is quite worn I was thinking if the pinion bearings were changed this would move the pattern more towards the middle heel and toe ,how much who knows, a bit like adding a thicker shim.

I also have a pattern chart that came with the gear marking paste , I will take a photo of it.

I think my Diff has a collapsible collar as it's in a 76 car, not taken it out yet waiting for the screen to be fitted if it comes in!

[MandM]

diff patterns.jpg

This is the Patten sheet that came with the gear marking paste