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MG MGB GT V8 Factory Originals Technical - NarrowingaMiatarear end.

I originally posted this on the MGB board. Thought some one on this board may have some insight: I want to slide a Miata rear suspension under a 61 sprite. The unit is too wide as it sits. The housing and A-arms pose no real problem other than some geometry. The axles look to be another problem. I have had many solid rear axles shortened, but I am not even sure if independent rear axles can be safely shortened. I fear that custom made axles would put the project way out of budget. Thoughts?


Pete, if my memory serves correctly the rear half shafts of a Miata have a u-joint on one end and a constant velocity joint at the other. The CV joint should be attached to the shaft by splines and either a bolt in the end or a retaining ring. The ideal way to shorten this type of axle shaft is to have new splines and retaining threads or grooves machined on the shaft after it is cut to length. The shaft could be cut in the center and then welded, but you would certainly loose a lot of strength. You might check some Mazda FWD cars, perhaps you would find a shaft which would fit and be the correct length.
Bill Young

Pete, the axles on the Miata are just like a drveshaft therefore they can be shorten like a driveshaft.

Youe real problem is going to be the goemetry set up for it to work correctly.
Bill Guzman


Why do you see the geometry as a problem? By making custom A-Arms and moving the mounting points, I should be able to find a reasonable camber curve. The rear of the car is being "clipped" with a tube frame, so existing metal should not be an obstacle. I have never done this with an independent rear, so there may be some issues I am not aware of. I do know that if done wrong, the handling will be scary. Any and all advice welcome.


I think the Honda Vtec powered Minis use a cut and welded driveshaft to combine the one piece of the honda with the stub of the Mini setup in order to produce a workable unit.. You are just shortening, so it should not be as much of a problem. Not my favourite thing. :-)

BMC Brian McCullough

Interesting that this subject should come up, I was researching the possibility of sliding a MX5/Miata rear suspension final drive under my BV8 conversion.
The main problem is the gear ratio is far to low although it may still be possible to get a dif from a Mazda B2000 deisel van which is 3.308-1 which would be ok and is a straight swop.
Pete can you tell me what is the width of the miata unit wheel mounting to wheel mounting faces,
K.R Jackson


Sorry, but I do not have the rear in my posesion. I did own a Miata up till last year, and I can tell you that it was too wide to go under a B without some shortening. I believe the rear was 3.9 ratio, so should be ok with a T5 and .63 overdrive. I will check on Miata board to see about ratios ( although buying a new set of gears will put this project into the big $$$ department)


After a bit of reserch I have concluded that this one is beyond me. For whatever gain I may find, the geometry and cost would spiral out of hand. Not that the Miata rear (same pumkin as rx7) would not be neat! By the way, there is a 3.6 gear set for the diff that may make it attractive to a B.


I've been thinking about this one as well as I've got both a 72bgt presently under the knife and a 90 miata in the middle of a DIY Turbo buildup.

Taken from

Miata Wheelbase 89.2 (2265)
Miata Track: front/rear 55.5/56.2 (1409/1427)

The track of 56.2 is to...? the outside edge of the tires or flange to flange?

IIRC the MGB rear is about 49 or 50 inches wide, so maybe this is a real good fit if you are going with a sebring Kit?? If you can wait til the weekend, I'll measure it for you disc hat to disc hat. I don't know what the stock offset is for the miata wheels though if you want to maintain the aixs of inclination.

From 1990 to 95? the miata was offered with an optional Viscous Limited Slip Differential (VLSD)- a clutch pack diff. I have one on my 90 and it works great. After 96 or so, the miatas come with a Torsen LSD as and option and then standard equipment after 99?? Point is they are out there and easy to get. These things will hold up against 225 to 250 RWHP without much trouble. FOr all-out efforts, you could swap in a late 80s mazda rx7 turbo II diff which should handle upawards of 300 withgout blowing up.

The stock units are used frequently behind Flyin Miata turbo kits which put 225 to the rear wheels with no trouble at all.

While I love my 72 BGT, the Miata is a sweet handling car. I don't really care if a well-sorted leaf spring suspension is pretty good, I just want to build something a little different and this would be a fun challenge if the dimensions are close enough.

Anyway if someone has the dimensions from drum face to drum face for the MGB axle ( mine is in a barn up by the brother-in-laws house) I'll get the same number from disc hat to disc hat.


Brian C.
Brian Corrigan


Many thanks for your response, a dimension between wheel mounting faces would be most useful.
From information found it would appear that the post 94 rear end is stronger 7" against 6" for pre 94's
The idea is convert my GT to Sebring body work and use my existing 6" Minilite wheels with 195/65 x 15" tyres
I do have a Rover SDI axle which could be narrowed to the right dimensions to fit the extended wheel arches
but still leaves me with a cart sprung rear end and I like yourself would prefer something a little more modern if it can be achieved at a reasonable cost, there are IRS kits available in the UK but cost in excess of 3000 inc. recon diff and disc brakes!!!
As i mentioned before obtaining a CW&P for the Mazda diff may not be that easy as the only one was a 3.308-1 for the B2000mn Diesel Van and I think that it was only an option.
There is some very useful information on
Look forward to your further response with interest


Kevin Jackson
K.R Jackson


I doubt a SD1 conversion will be cheap these days.
An alternative
Tim Fenna at Frontline has been considering a rear suspension redesign.



There is a 3.6 rear ratio. This should work ok with an overdrive transmisio. Maybe not the ideat cruiser, but good for acceleration.


The TR7/8 track is approx 5inches wider than an MGB [its also approx 5inches narrower than a SD1 Rover] came with 3 ratios 3.9 for TR7 3.45 late USA injected cars, and 3.08 in automatics -and all TR8s[rover SD1 CW/pinion swaps staight in], If shortening the tubes, think about using Rover axles-stronger,heavier in the resplining area, same part no bearings, Rover drums [wider shoe,]bolt straight on [use rover backing plate, its slightly different] and Will fit under Sebring guards."cart"spring it ,or play with trailing arms.

This thread was discussed between 20/02/2004 and 29/02/2004

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