Not a happy camper right now......really down!

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71coupe

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New Braunfels Tx
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1971 mustang
just got back my 9" rear w 3.70 gears....broke it in...4 spins...with wheels off the ground....12 to 15 minutes in drive.

decided to take the car for a ride around the neighborhood. rebuilder said to take it ez for first 500 miles....so here I am cruising at 50mph....then BANG!!!! all hell breaks lose.

FMX trans cracked!! :mad:

Real down and out right now :( Just when things were going good..this happens. What a major setback.

I've heard both sides about FMX...their good and their bad.

Well...I think its safe (for me at least) to say that FMX are crap!!! just CRAP!!

Now I need to know what a good solid C4 will cost me!!!

trans1.jpg

trans2.jpg

trans3.jpg

 
Wow... Fmxs are not bad just heavy, same ratios as the c6.

For that to have gone like it did I would want to find out why.

Trans mount?

Did the rear end lock up and twist the trans through the driveshaft?

Engine mounts?

Hope things improve from here

 
Wow... Fmxs are not bad just heavy, same ratios as the c6.

For that to have gone like it did I would want to find out why.

Trans mount?

Did the rear end lock up and twist the trans through the driveshaft?

Engine mounts?

Hope things improve from here
Click to expand...
Dan, going on memory here... I think the C4 and C6 have the same gear ratios and the FMX is the odd one out. I believe the FMX and AOD share the same gear ratios - which are not optimal. The C4, C6 and 4R70W have really good ratios. Dan may be correct though so do some research.

BTW, the 4R70W is a great swap if you can afford to go that route. Should be able to find a used one for $200 - $400 range.

 
Don your correct I got a little mixed up.

The c4,c6 are the same and

the fmx and aod are the same

C4/c6: 2.46, 1.46, and 1.00

The FMX Aod :2.40, 1.47, 1.0

Very little difference but they are not the same.

I swapped a c6 in mine from the fmx because I could not get parts for the fmx at the time and the c6 had the most support. The drive shaft had to be swapped as well.

Oh yeah the nuetral safety floor shifter and linkage is different between the 3 transmissions also.

I used a fmx floor shift with my c6 and I had to swap the shift linkage to a c6 but lost the steering lock because of the swap. I used the fmx neutral safety switch that works fine with the c6

 
As said above by 72 H code, nothing really wrong with a FMX, just heavy but a fairly strong box, I know of one guy who has one behind his high 10 sec XY Falcon and has never had a problem with it and quite a few others over here in Australia use them as well (I still prefer a C4 any day) What it looks like you've experienced is a problem I've seen before with a mates Monaro many years ago. He put in a 9 inch in the rear of his car, he was running a pretty stout small block Chev and a turbo 350 trans, got the tailshaft built to suit took it for a drive and bang same problem you seem to have. Ended up being the yoke was in too far because the tailshaft was slightly too long and when he went over a bump, bang goodnight transmission extension housing and damaged the trans as well, all because the yoke could not go any further forward, locked the trans and it looked exactly the same as the problem you have encountered now. So when you put the tailshaft in how far did the yoke go in before bottoming out, because I think that maybe your problem and not the fault of the transmission. Just a thought let us know what you find out.

 
Damn.. Sorry to hear that, if i could afford my TKO conversion, I would just give my c6.

 
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As said above by 72 H code, nothing really wrong with a FMX, just heavy but a fairly strong box, I know of one guy who has one behind his high 10 sec XY Falcon and has never had a problem with it and quite a few others over here in Australia use them as well (I still prefer a C4 any day) What it looks like you've experienced is a problem I've seen before with a mates Monaro many years ago. He put in a 9 inch in the rear of his car, he was running a pretty stout small block Chev and a turbo 350 trans, got the tailshaft built to suit took it for a drive and bang same problem you seem to have. Ended up being the yoke was in too far because the tailshaft was slightly too long and when he went over a bump, bang goodnight transmission extension housing and damaged the trans as well, all because the yoke could not go any further forward, locked the trans and it looked exactly the same as the problem you have encountered now. So when you put the tailshaft in how far did the yoke go in before bottoming out, because I think that maybe your problem and not the fault of the transmission. Just a thought let us know what you find out.
Click to expand...
very good point!! I used the driveshaft from the 9" I got from a '72 mustang, but I think I used the yoke from my original 8". They seemed to be the same length...yokes did(see pics). However, the driveshafts were different lengths. I used the driveshaft from the 9"...I remember...it went into the trans just fine..all the way in..then I had to pull about 1.5 inch back to connect to rear end. I thought that this was ok. I believe..if I remember correctly..I will have to double check this weekend..that the 8" driveshaft was to long.

the rearend didn't lock up....I towed the car home tonight and the rear wheels spin just fine. I txt the rear axle rebuilder and he said it would take "a lot" to destroy the axle. He is pretty confident that the rear axle is solid.

yoke1.jpg

yoke2.jpg

 
Yeah mate all you can do now is go back through everything to see what has caused the failure. Was the trans working fine before you changed the diff, but I seriously don't think that's your problem, also changing the diff from a car of similar type, wouldn't think would've given a pinion angle issue and if you say, you pulled the tailshaft back a good 1.5 inches or so should be plenty enough not to cause a problem. Also your diff guy is right, it takes a bit to kill one of those things and as you said you towed it back and as long there was no clunking noises coming from the rear end and it rolled freely, then all should be fine there. The joys of cars. Keep us informed with what you find and hope it's a relatively easy fix.

 
From the pictures it looks like what broke is the tail shaft housing, which is replaceable. Pull the driveshaft and the tail shaft housing and look for problems with the output shaft, bearings, and bushings. Also, recheck the new driveshaft for proper assembly, dimensions, and balance. I hope it is a cheap fix. Chuck

 
Like chuck said... the extension housing will be an inexpensive fix, The driveshaft was too short if you had 1-1/2" of clearance on the rear end with the slip yoke all the way in. Shoot for 3/4" clearance when you get a new driveshaft built.

 
4R70W man, you get all that fixed and its still spinning hard at 80 mph. I used 3.70 gears x 26" tall tire and 4R70W and 2200 at 75mph. Low gears dig like mad too. Whole swap will run 12-1300. $200-400 used trans, controller wiring mounts 850+-

 
here is the latest update..small video

found a C4 for sale...might go this route (cheaper).

The C4 was just rebuilt 1400 miles ago. It's still in the car, the guy said he is leaving it in the car for anyone interested in driving the car to test trans. He also said that he has a B&M 2500 stall that he can throw in for $50. He used it a few times and just didn't like it so took it out. All for $525.00

my question to you guys....When the tranny blew...I know my motor just quit. I haven't started it. since then. My question is: How do I know I didn't damage any engine components?

My fear is this. I drop the new tranny in and fire it up, then I hear some funky noise coming from motor. I have to believe that it would take a whole heck of lot of crazy power to ruin the motor.

Here at this link you can find the small video of trans.

https://plus.google.com/104596430548161246095/posts/MWbJvg4sMQo

Thanks.

 
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Man I feel your pain been there before, A lot of others stated above that from my experience it can be engine and or trans mounts not aligning up or like showed below from running solid mounts, vibrations and putting stress on the drivetrain. The way it cracked looks like it is from stress from being pulled two different directions in my opinion. Also I don't think that was mentioned is a misaligned unbalanced driveshaft could do that at cruising speed, did you feel any vibration? Also if the driveshaft isn't bottoming out make sure you check the input shaft on the driveshaft goes smoothly in all the way of the tail shaft of the transmission and the groves aren't warped and causing a bind there.

P1011188.JPGP1011202.JPG

 
here is an update....



here is an update....
Click to expand...
finally got the fmx out. Will be picking up my C4 this Thursday or Friday.

Waiting for torque convertor (2500 stall) from monster transmission.

a quick look and it looks like the fly wheel from fmx and the c4 are the same.

will keep you posted.

20150414_181154.jpg

 
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