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MG MGB GT V8 Factory Originals Technical - Is a Jag rear end worth the trouble

I am on the fence as to whether to put the time, effort and dough into grafting a jag rear end under my soon to be 302 powered MG. Does anyone have an idea on cost of fabrication and aprroximate time involved? Thanx in advance

isn't it 300 lbs? awfully heavy large rear end, talk with evan a. on here , i think he has this set up in his car, see how he likes it, jim
james madson

Geoff watch the geometry of the jag rear, the only reason it works at all is it's all rubber mounted, the leading end of the trailing arms should be mounted at the same distance from the center line of the car as the lower a arm pivot point.


Installing the Jag IRS is really a pretty simple job, almost a bolt-in. The only difficult and/or expensive item is the narrowing of the unit. The half-shafts and the lower control arms have to be shortened, which is not really a DIY proposition.

The steel cage holding the unit is very easily modified at home, if you have access to a welder.

RM is right about the mounting of the trailing arms - the center line of the pivot points needs to be in line with the center line of the inner pivot point of the lower control arms, but this is easily accomplished. You can get by without doing this if, as RM suggested, you use soft rubber mounting, which is exactly what Jaguar did with the cars the IRS units come from. This gives a somewhat cushy ride, but not the most precise control.

James is pretty close to right on the weight, but keep in mind that most of this weight is sprung weight, which will not have much effect on the handling. The unsprung weight will be a lot less than the live axle that's in there now. The best thing you can do for your handling is to reduce the unsprung weight.

Look for the third edition of Roger Williams' book "How To Give Your MGB V8 Power" which is due out in the next few weeks. He has an excellent write-up on using the Jag IRS in this edition.

I've talked with three people who have installed a Jag IRS, and all three are happy with it. I'm putting one in my own MGBGT V8 conversion.

Pick up a copy of "Street Rodder" magazine and check out some of the IRS vendors in there for more info, and be sure to get a catalog from Concours West Industries, as they are the leader in Jag IRS conversions. They don't advertise in the magazines, but you can find them at:

Put the Jag IRS in back, and a Fast Cars IFS up front, and - WOW! what a ride! Add a Ford 302 and you'll be king of the hill.
Dan Masters

For a cheaper option, has anyone considered the Frontline 5-link kit that they've recently put together? 4 trailing arms, a panhard rod and vertical coil overs plus it reuses the original axle.

So you get a new diff, possibly a LSD, some uprated half-shafts and this kit. I can't find the price at the moment, I saw it in Retro Cars Magazine over here in the UK but it was significantly cheaper than anything else I'd seen and it attached to existing mounts.

Rick Haynes

It was over 1K wasn't it? - Seems a bit steep to me.
Stuart Robson

wasn't justin working on a more modern ford set up? cannot remember the car it came from, jim
james madson

Yes I am. In fact, I checked back in with my welder friend on Wednesday night to see about welding the cage into my car. It's from a Factory Five Racing Mk II Roadster (Cobra replica) and is based on the Ford 8.8" irs out of a Ford Thunderbird Supercoupe. There's plenty of info in the archives. Maybe in a month or two mine will be done?


I'm looking forward to seeing that. I've made no real progress on mine, though I am a little closer to having the mill back together. Been playng with an old caterpillar dozer instead. If anyone wants to go that route though, look for a Lincoln Mark as a donor since they had an aluminum third member.

Jim Blackwood


Looking for that ultimate weight savings for your crawler?
greg fast

OK Greg, you got me. Lightweight dozer indeed!

Jim Blackwood

How 'bout a Corvair transaxle in a B?
Dan Masters

If you could find one you mean? Then there's the sloppy shifter... I had two corvairs, trust me they are sloppy. Better perhaps to ditch the corvair tranny but use the diff and a standard 5 speed. It's been a long time ago, is this a late model axle?

Jim Blackwood


I have no idea, I just stumbled onto this while searching for something else. I'm sure there are better choices for a transaxle than the corvair if one wanted to go that route.

I just tossed this out as food for thought.

I also had a Corvair way back in '63, but I don't really remember much about it now. I do remember the sloppy shifting, but it was not nearly as sloppy as many of the modern front wheel drive cars with cable shifting manual transmissions.

How could you separate the tranny from the transaxle assembly? I have a book on Corvairs in my library, so I'll go look up the details on the transaxle just for grins.
Dan Masters


Hawke engineering/Hoyle have a rear end setup (IRS) that uses the Xr4ti rear suspension. However it does have upper and lower A arms with coil overs within a subframe they manufacture which is pretty slick. The problem you come up against, or at least it is an obstacle to overcome, is where will the exhaust system pass by the new IRS? If you use the jag cage you really have to hog out the openings where the exhaust used to go when it was in the Jag. I think a new sub frame going across the car and attaching the top of the center of the diff to this crossbar using the existing attaching points and some brackets welded to the cross bar ( say a 2 x 3 piece of steel stretching the width between the inner fenderwells and bolt the diff to it) Then weld on some foot long sections to each end of the new cross piece at a 90 degree angle running back to front,that can attach (bolt)to the spots where the lever shocks used to bolt on,drill additional holes a foot or so back from the existing damper holes for the rear attachment points. The existing MGB front spring hanger brackets could be used for additional cross bracing or trailing arms mounting points with a fancy heim joint as you would not be using the leaf springs anyway. The same piece of 2 x 3 could also be used for the coilover upper attachment points. Just weld on some suitable brackets. I have been exploring this option as well, and have been doing some "what ifs" This may be the answer for creating some space for the exhaust(s) to pass by the irs without going too low and sacrificing clearance and knocking the next speed bump with your new flowmasters. FWIW Alyn

JUSTIN - What sort of mods did you have to do to that Factory 5 unit to get it to fit?? THe Cobra seems like it would be a good bit wider than our cars. I know I want to maintain the 52" wheel to wheel width..

I am looking at $400 for get my 8.8 live axle done.. or sell it and do the factory 5 etc.... Are you HAVING to move your fuel tank?

I am not sure the Merk rear would be strong enough, they only run a mild 2.3L turbo ford from the factory... Yes guys are doing 5.0's into them, but it is not a 8.8 that I know of..
Larry Embrey

This thread was discussed between 30/03/2005 and 03/04/2005

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