MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Hiohg Ratio Diff

Has anyone had first hand personal experience of fitting a high ratio diff to a TD rear axle.

Unlike MMM axles where diffs can be changed in a trice, the axle on the TD/TF is, I believe more complicated.

Our TF has a high ratio rear axle which, combined with the right camshaft and a properly tuned engine enable us to cruise easily at 65/70 mph.

Bob Marshall
R J Marshall

Yes, first to a 4.875 which I had in for about 20 years then to a 4.55 fitted a couple of years ago. Not what I would call an easy job, you need lots of patience, but its not complicated. There is a good booklet produce by one of the American clubs.
B W Wood

Bob
Not 100% sure what you mean by "high ratio". Assume that is 5.125 vs a lower 4.3:1 that is a common replacement for the TD for highway speeds.
Plenty in the archives look for "Clark Conversion", Rear End Conversion" etc.

Classic tradeoff with diff rear end ratios.
Speed vs torque.

If you have a properly, well tuned 1500 like in the TF with the "higher" (>4.5:1) rear end, cruising at current highway speeds (>60 mph) isn't much of a problem. BUT with the TD 1250 and NOT well tuned (or even decently tuned) it's a struggle to make 60+ with the original 5.125:1 on any substantial uphill grade. On the flats you'll be at 5500+ rpm min for highways speeds.
I did the Clark conversion in my TD to the MGA 4.3:1 and it's GREAT for cruising at 60+, even with a less than ideal 1250. Can run at 4500+ rpm without much strain on the engine only ON THE FLATS.
BUT, BUT, BUT it is terrible for any substantial hills, like alone the mountains of Colorado maintaining highways speeds. Long slow grade hills you'll be able to somewhat maintain speed, usually having to go down into 3rd and 45mph tops.
Anything more hilly and you'll be down into 2nd going 20 mph.
Maybe with a 1250 TD with a supercharger you'll be able tackle big hills, mountains in a 4.3:1.
Just my two cents (or shillings)
Randy
R Biallas

I fitted a 4.55:1 diff to my TF, replacing the original 5.125:1 set-up. I also have a Ford Type 9 box and an uprated engine. Its a very good combination, especially for where I live (Cambridgeshire).
Not an easy job to do, but not impossible either if you read all the background that you can and take your time. I had to make a couple of special tools to pull in the new bearings, etc. I used the Moss kit and replaced a couple of the bearings with better ones. The crown wheel and pinion in that kit (made in India) has been much criticised, and mine is a little noisy, but improving. Better to use MGA parts if you can, but difficult to find and usually costly.
Dave H
Dave Hill

Thank you for your comments.

By high ratio I mean a diff, which reduces the engine speed related to road speed. EG One of our cars does 35 mph for everu 1000 rpm. We won't get this result from the MGA did and it is doubtful if the engine (unless changed for a small block Ford V8) would pull it.

What I really want to know are the practical details of changing the diff.

Bob Marshall

I
R J Marshall

Have you read the guide written by Carl Cederstrand? Not the easiest to follow, but absolutely essential reading. Some aspects of the installation have been better covered by others, and will get to you on those unless someone else does in the meantime. If you are local I can lemd you some kit.
Dave H
Dave Hill

Bob,

This is the document which I used to change from 5.125 to 4.3. It is complete and easy to follow.

http://www.ttalk.info/RearEndConversion3.pdf


John
J Scragg

That's the one that I was thinking of, but couldn't find quickly,
Dave H
Dave Hill

This thread was discussed between 31/10/2021 and 01/11/2021

MG TD TF 1500 index

This thread is from the archives. Join the live MG TD TF 1500 BBS now