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MG Midget and Sprite Technical - I need gen. mazda RX7 rearend info for my midget.

Im getting my parts togather for my big engine build, coming up this winter....so I am thinking ahead beyound the engine, So the next modifation after the engine is the rearend...im trying to decide if I want to stay with the midget and beef it up with peter may axles, and a phamton LSD,

or go the Mazda RX7 with disk brakes, and fatory installed LSD and bullet proof no worries rearend.

So Im leaning towards the Mazda RX7 rearend

THE MEAT OF MY QUESTION.....
anybody have a clue which year and model I need to do this....(NOT wanting the later model independent axle with IRS) wanting the older solid straight axle assembly


I wont be doing this mod till spring at the earliest...so no hurry on getting one, just getting the info I need so I can be on the look out for a good used assembly and various modification parts

thanks for any info.... websites for this modification would be nice also.

prop
Prop

Prop,

I got some info about his around 3 years ago from a guy in the states. Its for a frogeye but a later car should be the same, maybe easier due to the smaller spring mounts. Email me at djb< at >metal-arts< dot >co< dot >uk and I can send you his contact address, you can contact him for further details if still valid.

Text as follows.

<<<<<<<<<<<<<<
The RX7 diff. has to be shortened, brackets arranged, drive line modified, etc. The good news is it bolts right in, and does not modify the car or the orig. diff. It is cheeper than any parts to do any one upgrade to a stock diff. (I know, cause I went that way and still had troubles... fortunatly I was able to sell all the stuff.) Yes, the disk brakes are not a real necessity, but a nice plus. The unit when complete does not look different nor weigh much more... remember it is unsprung weight anyway

The one possible killer is getting the axels shortened. We've done it with resplinning, and with welding... Both are mechanically sound, but depending on local shops, either may not be possible or expensive. I get the axels shortened and welded for $60... wouldn't even fire up my TIG for that $$$.

attached is a pic of an 84 RX7 GSL (in process) which is the solid diff. with disk rotors that are not the finned ones (which can be used but is heavier and overkill on strength).

If you think you have local services for axel and drive line mods, then we have plans for doing it and instructions to build a jig do get all the measurments and angles right, etc. It comes with or without the new brackets shown.
>>>>>>>>>>>>>




David Billington

Prop,

checkout this site. Very good.
http://the-mite.com/index.htm
P R Clark

I'd be interested in this too for my K-series conversion...let me know what you find out prop!
Rob McGeown ( LMG 5K - '72 RWA)

It looks lighter than my narrowed TR8 rear axle.
Tarquin

My cousin has an RX7 axle in his '58 Frogeye.

He did the work while he was living Stateside - I'll ask him for details.
Deborah Evans

Prop, make sure that you get an axle which has a compatable ratio for your car. A lot of newer cars have high ratio rear axles for milage considerations and those might be a little tall for even a well built 1275. I'd say stick with something around the 3.7 to 3.9 range for best results, or use an axle that has factory ratio options in that range such as the Chevy 10 bolt. If you don't want to get into the hassle of trying to swap out the master cylinder then pay close attention to the bore size of the master on the donor car the axle comes from. If the rear calipers require too much fluid volume then you'll get into pedal travel problems much like I have with my Monza rear. I can compensate a bit by careful adjustment of the drum brakes but with discs there's no adjustment. If you need a machine shop for the axle work and housing narrowing I can recommend one in KC which did mine at a very reasonable price.
Bill Young

I think most rx-7's were geared pretty low since the rotary engines rquired higher revs to generate the power needed.
Trevor-Jessie

guys I really appericate the response and the links...thank you!!!

Hey BILL,

I will be okay on the brake master cly. issue, because I have the tilton master brake clyinders so I can get any size bore from 1/2 inch to just under 2 inch bore size NEW for under $60-$75...from what little I have read, the popular size C&P was 4.22...but I think they did come in 3.7 as well


I hadnt thought of a chevy 10 bolt, but briefly was thinking a ford dana 9 inch rear end...but Im thinking that is just way over kill...not to mention the wieght

So does anyone know what year and model I need to be looking for...I want the straight solid axle, with disk brakes and LSD, .....Not the Independent IRS axle that came out on later years...

agian thanks everyone
prop
Prop

BTW I talked to my machine shop guys...they recommened sending to "mosers" for the machine work....they guesstimated with machine work and parts to rebuild...around $350 - $400,

I found one on ebay I think might work, for $350....but There is a RX7 race shop 4 towns over from me, so i figured Id give them a hollor as well....

but agian Im not in any hurry, still just trying to figure out how the meat goes on the bones for a basic plan at this point.

after all I still got to build the engine 1st this winter

prop
Prop

I like the Chevy just because of the number of ratios available for it, inexpensive limited slip differentials, and the Monza/Vega axle has the same bolt pattern as the Spridget so no extra machine work there to adapt the hubs and discs or drums to the Spridget pattern. As a plus the Monza axles have enough size to not require welding to be resplined when shortened to the Spridget width. The housing center is fairly heavy, but could be trimmed if that is an issue. Rear brakes only do about %15 of the braking anyway, so discs aren't usually needed unless you're racing and getting a lot of heating. Add the complexity of a parking brake hookup and sticking with drums for a street car makes sense to me. The RX7 rear is a good choice though. You'll want the gen 2 axle for the disc brakes, the gen 1 cars had drums and I think the gen 3 cars went independent.
Bill Young

And the Monza axle was stout enough that I don't believe that they altered it for the factory V8. Or did they? My brother had one from new with the 262.5 CID small-block Chevy and a 4-speed. I still haven't forgiven him for selling it on without telling me. I might never have discovered Spridgets, however, so I guess it is ok. If it is a 4-on-4 bolt circle, it would make a very good choice. I don't really need disks back there (more wow factor though) and fewer dirty looks from RX7 owners.
David "weighing the options" Lieb
David Lieb

David, you wouldn't have liked the Monza much. I had one (75 V8 4 speed) and it was a real PIG as far as handling went. Would absolutely ruin the front radial tires because of heavy understeer. I went back to bias plys at the time to get more than 10K from a set of tires. In the wet with radials or bias plys the rear was so touchy if you applied just a touch too much throttle in a curve (note curve, not necessarly corner) it would break loose and come around. It scared the s*** out of me more than once on I70 in St. Louis in the rain.
One other thing to consider when choosing a rear axle is the size of the brakes. Remember most of us run 13" wheels and many rear brake drums or discs are too large to fit inside the wheel. The early RX7s were 13s as I remember but the later ones moved up to either 14 or 15" wheels so might be a problem. The Monza was designed for 13" wheels and fits fine there.
Bill Young

Prop,

You should be able to get to a pick n pull and grab one for about $100 or so. The ratios are huge in variety from 3.6 to 4.778 (search the miata.net forum garage section, as the 94+ miata used the same diff). I believe the 1st gen RX-7's all used 3.90's. They also have cheap LSD's from the second gen rx7- clutch type or get a miata torsen. A pretty sweet, reasonably light and really strong axle setup. I'm trying to remember, but what you want is the non-vented disc brake rear end (the axle differential ends look like q-tips in some and are cut into the axle shaft in others- you'll want the non-q tip ends if you can understand where I'm coming from).

Look up John Carey on the interweb and his site might show you what you need. I narrowed the miata independent, but would go with the rx-7 next time. Too many adjustments that I'm not smart enough to make right now!

Brust

sorry...I needed to get info off my thread ...lacking time

prop
Prop

I have a Midget that uses a RX-7 solid rear axle.
http://mysite.verizon.net/vze7vhc0/

http://www.mgcars.org.uk/v8_conversions/midget/images/rearsuspension/rear-3.jpg

I have also built another axle for a fellow to use with leaf springs that uses a panhard rod, and anti-tramp bars. (Pics are at home on other PC)

The '84-85 RX-7 GSLSE model came with vented disks, LSD, 4.10 ratio, and large axles (can be re-splined rather than welded). I had Moser Engineering http://www.moserengineering.com/
cut and re-spline the axles for about $300 8 or so years ago. I run the stock Mazda rear calipers and they work fine. I do not have the e-brake connected, but it would not be hard to do since it is built into the Mazda caliper. Disks and axle re-drilled for 4 x 4" wheel bolt pattern.

I run about 210-220hp to this rear with full slicks on 15x8 wheels and it holds up just fine to standing starts. It has been running for about 7 years on this axle with only a couple axle seals needing replaced (use the rubber coated units from C/R over the metal shell units). I run Red-Line 75W90NS in the rear.

http://video.google.com/videoplay?docid=1730751518945515654


You can e-mail me at K2Motorsports at MSN dot com.
Brian Kraus

This thread was discussed between 01/09/2008 and 23/09/2008

MG Midget and Sprite Technical index

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