GT6 for 302 conversion

[theonlyiceman53]

Hey Bill Y,
Thanks.  I'm taking lots of pictures but not sure I would reccomend until taking it for a spin.  I had a minor set-back on install.  Just before welding in the spring support I took a gander at the location and double checked with tape measure.  Everything checked out as far as centered but then for funs checked the centerline to tire location and they were an inch different.  Needless to say that put me into a tizzy!   I had noticed some parts in the rear end that looked like they had been disturbed from original but figured it was just a U-joint change out with a hammer!   It appears that one of the axles had been changed out and they had managed to put an earlier model 1" shorter axle in place.  This explains to me why the car felt wierd when I took it for a spin after getting the original motor running.   I'll get everything in place and see which axle looks the best for tire fit and then get hold of a matched set.   I'm sure glad I made the decision to change out the diff as that shorter axle would have been real tough to find!   Could have been chasing that for years!  
Have a great Memorial Day!
Bill

[dtindell]

is there any frame that a gt6 body will it, wheele base same?

[Graham B]

Hi Bill,

Just out of interest, which model of BMW did you get the differential from?
I'm currently running a 74 Spit with 3.5lt Rover V8 and I am still using the stock rear end. No problems yet as I don't go drag racing and try not to light up the rear tyres!
But ultimately I would like to beef up the rear end some day.

Cheers
Graham

[Bill Young]

David, you have a personal message regarding the frame. Didn't want to hijack this topic with that information.

[KENATENCIO]

Hey guys is know some of you are using the BMW rear end but i was wondering. is there a way to biuld the stock rear end up to take a aggressive 302 or where do i need to start looking for one that will take the power

[Bill Young]

Kenny, from my experience rear differentials are sized according to torque capacity, for a small V8 such as the 302 with some power and aggressive driving you need around a 7.5 to 8" diameter ring gear to handle the power. The stock GT6/Sptifire ring gear is not quite 7" as I remember so is really far too small to handle so much torque. It's a factor of size, stay small and get the gear ratio you want and the teeth have to be small as well, go larger in diameter and you can have thicker cut teeth with a great deal more power capacity.
You could start in the local salvage yard looking at vehicles with V8s or V6s and IRS. A lot of later SUVs have such suspensions and might be a source of a good differential. The BMW is pretty strong depending on what car it came from, a 5 series or V8 model would be great.

[theonlyiceman53]

Hi Graham,
I used the diff from a "BMW E36 Differential Diff 3.91 325 328" as was stated in the ebay ad.   I used it because of the nifty aluminum rear cover, built in axle stubs, mounting tabs on the back and it was cheap!     I've added some material on the rear frame rails so if it needs a fully independant system in the future there will be some place to mount it to.  

Bill

[theonlyiceman53]

Rear in place.  Just have to remember to put lube in it before running it!   Won't know the spring situation before putting full weight on it but think it will need a change in rate from the look of it!   Really tempted to take a WAG as it's such a pain to pull the body on and off or pull the spring when the body is on.   Anyone know what the body weighs so I can simulate with bags of cement?   I can picture what the neighbors will think when they see a Spitfire chassis being pushed around the yard with a bunch of bags of cement on it!  Actually if it doesn't work I can use it for yard work!  

The driveshaft ended up being about 20" long.  The back half is  from an MGB and the front is from an old Corvette driveshaft I had in stock which happened to be the same size tubing.    Attached is  a picture of the axle adaptor.  The drive shaft adaptor is also pictured.  It just lines up the centerlines of the BMW rear and the MGB driveshaft.  Clamped it in place and just drilled through the MGB pattern.  

A lot of custom parts were needed for this swap and there are some questions left about the spring.  Only way to find out is to drive it. Have to get the computer working for the engine/transmission.  Tranny won't work without the computer so kind of forced to get it to work.  I certainly wouldn't do it voluntarily!   Carbs work just fine for me............


Bill

[jellison]

Christ is that what a GT6 rear is like - I had heard they were an awful design, but holy crap - that looks like death trap with instance massive toe in if you lift a wheel!

[Bill Young]

Jon, what's not shown in the photos is a link from the lower portion of the hub carrier that runs forward to the rear edge of the body. Since it attaches to the sheet metal there's no way to fit it to a bare frame so the lower section looks completely unsupported and without any strength. The down side is the area where that link attaches to on the body is prone to rust so usually has to be rebuilt and reinforced when the body work is done.

[theonlyiceman53]

The current swing axle configuration will be changed to a full IRS setup.  Being a firm believer of the KISS principal the swing axle really appealed to me.  But, it is being abandoned because the BMW stub shaft is held in place by one snap ring (circlip)
A snap ring is not a suitable retainer for an axle on a swing axle.   It takes entirely too little pressure to pull the stub shaft out of the differential.  

Obviously the snap ring is fine for full IRS as proven by countless BMW's.  

Bill

[theonlyiceman53]

Just to clarify a bit:  the snap ring is in the same position as the original spitfire axle but for whatever reason the BMW axles take very little pressure to pull away from the snap ring.
Bill

[theonlyiceman53]

After considering all the possibilities and the amount of time that was invested in this conversion I went with option "W".   There are alternatives to option "W" but the compact design of the rear make them pretty questionable for a $70 differential.  Still want to see how the monoleaf spring works with the swing axle.  

Option "W" makes the disassembly of the stub axles impossible and perhaps there will be some other ramifications from this option.  It's a limited option "W" as the differential action still works, only the axles cannot move.   Even with just the original snap ring the worst that would happen would be some really weird handling issues and perhaps oil leaking from the differential as the axle slipped in and out of the carrier.  Depending on wear of rubber bushings I can't see that the axle would have moved more than an inch out of correct position.  

It's really surprising at how little force it took to "pop" the stub shaft from the carrier.  It's a great design for IRS with the tapered roller bearings but my limited experience with differentials has been with heavy "C" clips or pressed on bearings to act as the retention device for the axles with maybe a snap ring to act as secondary retention.  

Attached are some pictures of the snap ring, stub shaft and rear end back together.

Apologies for resorting to option "W" and perhaps Curtis would want to edit this thread so it minimizes the possibility of others going down the wrong trail.

Bill

[spitfire79]

do you have any more information on this, I'm considering using a Sabaru differential or a bmw.  i have a 1980 spitfire, was a spitfire leaf spring used in your design, and as for the axles, i couldn't tell if the spit axles were used, or welded to the bmw.
thanks
wes

[theonlyiceman53]

Hi, Sorry delayed on this post.  A corvette comosite spring was used.  The axles were spitfire with adaptors between the BMW and the spit flange.  The BMW is a little big to fit in there without a lot of modifications.  It looks like the Subaru would fit a lot better but the price was right for the BMW.  I'm at the point of stuborness on using the swing axle but it very well may end up as a full IRS.  I had to weld the stub axle to the spider gears to insure the stubs didn't pop from their snap rings.    Considering how my Jag full IRS goes hog wild when shifting I may even go to a non independant rear.  It all depends on how it feels once the power hits it.  Work has been in the way a lot lately so not sure when it will get on the road.  
Bill

[rampant racing]

so how is this build going now.waiting to see.

[Mañana]

I know this is an ancient thread but anybody ever hear what happened to this car?

I am considering a BMW Final Drive as well and really like the way Bill got it up in there.

Would love to see how he made out in the end.

Steve