View Full Version : MGB rear axle ratio change.

31-03-05, 06:16 PM
I am in the process of changing my MGB axle ratio for the V8 one and have all the bits sitting in the workshop.
My request is for any information about the process to change.
I have no problems in setting up crownwheel and pinion and backlash but I have heard that this change will require some machining to the diff carrier. Hence the reason for wanting to clarify this point and maybe find someone with some direct experience.
The main MG suppliers are of liitle help on this and the local transmission engineers have no recent experience of this. I would like to have the information to hand if I approach a local workshop for a setup rather than pay for them to find out.
My early investigations would suggest that the carrier needs machining so that the crownwheel can sit a bit further back.


David Large
31-03-05, 06:50 PM
I had mine changed a few years ago and indeed the carrier does have to be machined. I had mine all done and rebuilt for me so I cannot help practically
Where in the country are you?
You will probably get more info off the MG website than here.

31-03-05, 06:55 PM
I have been searching the web and established the carrier does need machining as you say.
I would like to know how much! then I may have a crack at it.
Where did you get yours done?
I'm in South Norfolk by the way


David Large
31-03-05, 07:50 PM
I got mine done in Newport, Shropshire. Can't remember the guy's name. I have a card somewhere but it was about 10 years ago and he was talking about retirement. If i can find it I wil try the number
Gerry Hawkridge told me that he uses Hardy Engineering in Leatherhead to produce their axles; they may help you.

31-03-05, 10:57 PM

David is correct, check with the guys at Hardy, I have phoned them a few times and they helped each time (Hardy on 01372 378927). You may find that the case will need to be increased in size.


David Large
01-04-05, 05:38 AM
There were no alterations needed to the case on mine as I recall.
When I changed mine it was because I had to but actually I preferred the lower diff I had previously. Unless you have a lot of torque the 3.07 diff is a bit high.

01-04-05, 06:23 PM
I found the standard ratio too low for cruising.
I am running a Rover 4.0litre with hi torque cam. I plan to set up on a rolling road eventually so i will be able to tell you the figures then but I think it is putting out toward 240hp.
So far i have found up a spare diff carrier and have machined 2mm from the crownwheel mounting face. Not had a chance to try it yet but I am confident this is the right way to go.
I will follow up on the contacts give by you, many thanks.

Andy S

David Large
01-04-05, 10:05 PM
Just replied to your pm before checking the thread so sorry for asking questions in the pm you have answered here. Must learn to do things in the right order.
I too run a 4.0 litre Rover 240bhp/255ftlbs and that is good with 3.07 but my last 3.5 engine was well down on torque and the 3.07 was too tall for it.
Nice to hear from the 289 minority.

02-04-05, 07:10 PM
Things are looking promising.

I managed to drop the assembly back into the housing today (less the 2mm of machining).
Things are now meshing correctly and in fact the backlash happens to be about right!
However this is with no shimming on one side and the old bearings so I will have to machine a bit more off.
The crownwheel now has excessive runout as I suspected so I will have to get my original carrier machined out of house when I have established the amount to remove.
Have not yet had a chance to chase up any of the contacts but will.



David Large
03-04-05, 08:28 AM
Sounds like good stuff. Important to get it spot on or after few miles your diff will be able impersonate a police siren. That is what happened to me. The original diff I had (MGC) ran silently but used to clunk a bit when I engaged reverse. In a moment of "improvement blindness" I took the axle out and sent it to be reset. When I put it back in the car it whined like my mother in law!
Axle out again and new diff! Different ratio which didn't suit the car so well.

Rule 1 applies; If it ain't broke don't even think about fixing it

20-04-05, 07:37 PM
Right, It's all back together.
Ratio is spot on. Now pull 30mph per 100 revs. It's like gaining a whole new gear over the standard ratio.
However I would probably not go down the MG doner route again. One simple reason, Cost.
It has cost the best part of 500 to do this job. It may well have been easier to get an SD1 axle narrowed for much less. The only reason I kept the MG axle was because I had designed and fitted an A frame four link system on that axle and didn't wish to have to set up another one to fit.
Anyway, car runs well. Slight whine but I will go to a thicker EP oil shortly. It will be good to have a test run to Stoneleigh.
Many thanks for the heplful info.


20-04-05, 07:39 PM
Not a Hawk
Or a BRA
It's an AS

David Large
20-04-05, 08:04 PM
Well done Andy. Hoping to be at Stoneleigh on the sunday. Have you seen the 289 weekend advertised on the 289 register site?

20-04-05, 08:07 PM
Not a 289 register member (yet).
What details?

David Large
20-04-05, 08:12 PM
30th July, probably around Malvern area. Being organised by the sales manager for Cobra beer!
You can go on the website anyway I think......look under general forum.


Tony 289
23-04-10, 10:27 AM
Guys, I converted my MGB axle to a 3.33 which is the ratio from the sherpa van back axle as the 3.07 was for the MGB V8 running 14" wheels and too tall. I calculated that 3.3 - 3.5 to be the best ratios. My 3.33 gives 2500 revs at 70mph and very quick acceleration see me burn off a skyline at YouTube - 289 Cobra vs Skyline Drag at North Weald You will see he catches me towards the end as the engine starts to run out of grunt, my 4.6 build will stop that. With my 4.6 I will still use the 3.33 ratio even if I swop to Jag rear end. Hope this helps

29-04-10, 11:50 AM
Here too?

YouTube - AC Cobra Drag Race 289 verse 427