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MG MGB GT V8 Factory Originals Technical - Jaguar rear end

I am thinking of using a Jag rear end in my MGA V8 project. I know it is heavier, but seems to have some advantages, i.e. IRS, simple installation, easy to narrow, Posi and decent gear ratios available. (Actually I have my eye on an E-type rear end which is only 1-1/2" wider that the stock MG rear end, 3.54:1 gears and Posi.) Does anyone have experience, positive or negative using the Jag unit under an MGB?
Kurt Schley

Kurt, most of the experience using the Jag IRS in other cars comes from the Street Rodders and Cobra Kit car folks. The narrowest ones came from the E types but the rear hubs were for wires, so adaptors for bolt on wheels had to be welded on. Not difficult and the rings were available from some of the specality speed shops. If you are going to have to narrow the unit and want bolt on wheels I'd go for the XJ version which was a little wider but came with 5 lug hubs which I believe were Chevy pattern. Combine those with MGB front spindles and some of Bill G's 5 lug hubs and you'd be in business. From looking at my MGA frame I don't see any problem areas to start with as far as any clearance issues except possibly the battery holders.
Bill Young


The jag IRS conversion is probably easier to do in an MGA rather than a MGB because you have a chassis.

There are virtually kits available from the hot rod suppliers to do this.

Basically you discard the original cage and use a weld in top mount crossmember for the diff which are available cut to size either straight or with a kick up in the centre dependent on your ride height requirement.

You will need a pair of anti climb bars which locate on the diff bottom plate and then go up to the chassis tp stabilise the diff and a new pair of radius arm which ideally should be triangulated with the lower wishbones on the same inner pivot axis.

The later XJ6/12 suspensions are a lot better than the earlier E type units and can usually be obtained with a variety of ratio's and powerlock diff.

I posted quite a lot of information on the British V8 forum for Jim Blackwoods BADASS project as he is using a Jag rear end.

Heres a pic of a similar installation.

Kevin Jackson

Thanks for the info guys! I have decided to use the Jag unit. From all the reading I have done over the last week, it seems to be a perfect rear end and easily modified and installed. Makes one wonder why they are not used more often? Kevin, the imaged rear end you sent is really a clean set-up! The MGA install should be easy.
Thanks again.
Kurt Schley


Glad that was of some assistance, if you need anything else don't hesitate to ask.

It is important to triangulate the lower control arms don't be tempted to run them straight forward to the front leaf spring mount, as this will cause unwanted bump steer, it was always a problem in the original Jag set up and that was why they had such a large doughnut bush to allow compliance and avoid binding as the suspension moved through it's arc of travel.

The kit you will require is the new crossmember, straight or with kick up, with or without isolator bushes, a top and bottom plate for the diff,a pair of radius arms and a pair of anti climb bars which are angled up to a pair of brackets on the chassis to stabilise the diff housing.

All these parts are available from quite a few of the hot rod specialists and are not particularly expensive, all you need to do is calculate the required ride height to set the height of the diff in the chassis, then measure the required length of the radius arms and anticlimb bars.
I would suggest for the radius arms using rubber or poly bushes at the chassis end and rod end at the diff end to allow you to set the final length and avoid any binding in the suspension movement.

The inner pick up point for the radius arms can be established once the diff is in the chassis by running a suitable rod through the inner wishbone pivots to a position forward of the diff to a convenient place on the chassis, this in itself is not critical.

The additional weight of the system is not a problem because the unsprung weight is significantly less and the additional weight of the diff itself is right over the rear wheels which is no bad thing for traction with the additional power of a V8.

Hope that a help.

Kevin Jackson

This isn't done more often because it is a fair bit of work for dubious gain. Weight and complexity go up, brake cooling is inferior to stock, and the IRS doesn't get you much on the street over a properly set up live axle with an LSD.
Bill Spohn

Really agree with Bill ---not to mention the expense of getting and rebuilding the Jag rear end.
Gil Price

But the "neat factor" is really up there!
rick ingram

yeah Rick maybe --but Ted's rear end ( pardon the expression)
would be my choice if it came to that !!!
Gil Price

To all the guys that do not believe in IRS. How come that lately almost all the new car manufacturers are changing to IRS ? If you drive a car with IRS over bad twisty roads you know and feel the difference. I am planning on converting to IRS on my 77 "RB". The John Hoyle IRS is a magnificent piece of engineering. Just too bad it's lots of $$$. I do have his front coil over modification. What a difference compared to the old lever arm shocks.
What about the IRS sety up from a Miata ? anyone tried this before?
Werner Van Clapdurp

Werner, I agree the ride and handling with an IRS are usually better, but is there enough improvement to warrant the expense of installing one in a B? As for the new cars going to IRS, a lot of that is because of the increased interior room available as less room is required in the floor pan for an IRS rather than a live axle. I heard that Ford saved over 3" in height and leg room in the rear of the Explorer when they went to the IRS. With the external dimensions shriking because of drag, any improvement in interior space is worth the engineering. I'm all for Kurt installing the Jag if he wants it, but it's something that really requires a lot of thought to determine if the hassle and expense is worth it.
Bill Young

I'm with Bill. I could build another MGB V8 for the price of a Jag IRS & Ted's front end (I dearly luv it, though.) I'll switch when someone proves they are actually faster.
Carl Floyd

rick ingram

Hey Carl ---its the driver !!!!!!
Gil Price

The Driver!, that could be the solution to my problem. Carl seems to do much better in my car, than myself. Gil, I was thinking all along the car may be the culprit.
A IRS would be off little help in my situation.
BTY, how is the weather, NY now is COLD & Snowy. how is the sand?
kelly stevenson

Bill is right about why manufacturers go to IRS - otherwise they wouldn't as it is more expensive.

Some of those units are real weight hogs too - my frind just put a Lexus diff in a TR-6 (don't ask me why he felt the need to change - to get a limites slip perhaps) and it weighs a bloody ton.

I spent a lot of years racing two cars, one with a live axle and one with beautiful double A arm IRS, using the same drivetrain, and I can tell you that while the IRS certainly gives a nicer ride on rough surfaces, it isn't any faster on the track, and for the ones that stick the brakes on the diff unit like the Jag, you create some serious brake problems as well.

A nice idea, I suppose, although I doubt many people will fawn all over the car at a car show because you have something that no one can see (hey - there's an idea - just TELL them you have IRS), and I'm sure all that money could be put to much better use in other areas.
Bill Spohn

Hi Guys,
The arguements against the Jag rear end are certainly valid, but I just like the idea. After all, if being rational entered into all decisions, no man would be married!
Kurt Schley

Well.. you do get tired of hunting all the time... But then again...

Oh, yes, IRS. That's right. Look guys, none of us is getting any younger, even Carl. So it's not all about lap times. Eventually the ride is just as important, maybe even (blasphemy!) more so. And if you can get the same lap time with a better ride that's a real big plus. The IRS is expensive, no getting around it, even if you do all the work yourself. But is it really that much more than the live axle swap? I guess it depends on how you go about it.

I think it's the next big thing for our cars. 5 years ago the attitude towards the IFS was the same but now Ted has sold about 50 units and we all want one. When you consider that we had about 65 cars at our last annual meet that's really saying something.

So I'm with you Kurt. I'd be happy to pass along anything helpful that I pick up on the way.

Jim Blackwood

This thread was discussed between 12/02/2008 and 26/02/2008

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