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MG MGB GT V8 Factory Originals Technical - Rear axle ratios

I have read that the rear axle ratios are not High enough for a v8 conversion. Why is this? would the car not be faster up to the top speed of a normal MGB or can the V* not rev as high as a standard MGB engine. If this is not the case what are the alternatives the only 2 I have heard about is a MGC back axle which seems to be hard to come by and a cut down SD1 axle which seems a lot of expence ( I also hear 80% have axle whine). Is there a easier way? (cheaper)
Mick Black

I have exactly the same problem at the moment. The crux of it is that the standard MGB ratio of 3.9:1 is way too low, making first gear virtually unuseable unless you want to climb walls (or drive around at 6000 RPM all the time).
The MGC came out with both 3.3:1 and 3.07:1 (standard V8 ratio) and as you say they are as rare as chickens fangs.
The SDI option (3.08:1) is a goer but there was only one company I know of in the UK doing conversions (A1 Fabrications), however, I have been unable to get a working phone number for them. These are rare now anyway as the extension housing has to be replaced with the one from the TR8 which is hard to find.
The other option is a 3.07:1 crown wheel and pinion that is available new from numerous sources. This does lead to another problem though, the carrier face that the crown wheel bolts onto has to be machined 220 thou to fit into the casing. A number of people can do this for you, Clive Wheatley is one.
I have looked at shortening a Volvo axle (3.3:1) here in the UK but cannot find anyone who will do the work.
The cost of any of the above options is going to be a good 500 pounds if it is done properly.
Me?.....I am waiting for one of the above to fall out of the sky into my lap or watch this BBS until a pearl of wisdom helps me. Sorry about the sarcasm but I have gone round in circles so many times on this one that I'll take advice from anyone.


I've been using the standard 3.9 diff. for the last 9 years in my GT. A lot depends on the gear ratios. I'm using a Toyota 5 speed box with a 5th gear of .82.I find 1st ,2nd, & 3dr very usable. If anything, 4th is the only superfluous gear, but I still tend to use it.My other car has a 3.08 Ford diff. & is quite sluggish in comparison until it gets into top gear.Why not use what you've got until you find something different ? HTHs Barrie E


Here in the U.S. we have a borg warner T-5 that was used in a camaro 305 c.i. V-8 that has a .63 O.D. & with the B 3.9 rear you get 4 gears for drag race & 5th. for cruse
Glenn Towery

Thanks guys, much do you charge for the T5 as whole kit plus shipping to the UK ?

Stainzy mentions 'rare as chicken fangs' and 'Clive Wheatley' in the same post, but Clive supplies bucket loads of them - 01902 330489.

Paul Hunt

T5 gearboxes are I believe available in US and european forms, and the two are not the same. US T5 and T56 are available from Real Steel in Uxbridge who will happily tell you what ratios are in the boxes. European T5 is used in fords as heavy duty replacement for MT75 ( Sierra ?, prehaps cosworth ). Warning - many secondhand T5 from Sierra have broken or damaged 3rd gears and don't use the bearings with "plastic" cages.

If you have an ordinary B axle case then accept that you will have to pay to do it properly and buy crown wheel/pinion and cage unit (Quaiffe ?). Talk to Clive Wheatley, Malcolm Beer, Doug Smith et al or your local mg specialist - its surprising what they can come up with sometimes.

My current choice ( if I started again ) would be a ZF or Gertag 5 speed box ( BMW 5 series ? ) and 3.3 or 3.07 diff for the road and a modified Rover SD1 3.45 for competition - regs permitting of course.


Dave Brooke

Mick, BIG SUBJECT! 1. What type of rear axle to use? At present I am using one from a 85 Camero with rear disc brakes, and posi traction, cost $150. cut to fit MG $85, Cut, and weld spring brackets $40 Axles $85 Calipers kit $45 Turn rotors $30 Axle ratio 3:23 with .79 gear ratio 5 gear. Camshaft output 2500 rpm
Other axles can be use; Volvo, Ford, Jeep 3:73
Avg cost is about $500 depending on gear ratios, posi, and type of brakes.

Formula to figure out what ratio is needed.
mph X 5 gear ratio X Final Ratio X 336
Tire Dia.

Example 70 X .68 X 3.90 x 336 Divided by 23" = 2700 rpm Replace the 5 gear ratio and plug each gear ratio to give you the rpm in each gear.

T5 transmissions come with .68/.79/.83 plus other optional ratios the one from a V8 would have higher ratios than a V6
.79 is common on V6 and .68 on a V8
Bill Guzman

Thanks for all your replys it has been very informative I will have to admit Bill answer was a bit complex for my feeble brain I think I will try the standard axle and try from there.
Mick Black
Mick Black


For my conversion, i used a compagny called Heathrow MG who is near London. They switched the gear in the axle to MGC specs. I planned a holiday in the UK with the MG and brought the car in the morning and picked it up in the evening. It was not cheap at GBP 600,- but haven't had any problems in the past 3 years and no wining.
Peter van de Velde

You posted some prices to for getting an axle resized to fit the B. If I'm reading it correctly the axle housing was cut and resized for $125 ($85 to cut and $40 for brackets) and the axles were $85 (I assume per pair for resplining). This seems real reasonable. Was the work done locally in CA?

I had planned to have a local shop resize the housing (my case a Ford 8 inch) and send the axle halfs to Curry in the midwest. I thought they charged ~$85 per side for $170 for the pair.
Chris Moog

This thread was discussed between 28/10/2000 and 02/11/2000

MG MGB GT V8 Factory Originals Technical index

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