all of the sudden this morning, the front end of my diff started banging up against the bottom of the car every time I accelerate. its a loud clunk, and a buddy of mine said he saw the front end of the diff (where the drive shaft bolts on) banging up every time it clunked.

what happened? is there a mount that broke? is the diff shot?

 

change your diff bushings

 

The front diff/subframe mount broke where the bolts are... Common problem... Either replace entire subframe or weld it solid...

 

so both of those bolts in the red circle? the heads just snapped off?

 

No... The piece of metal the bolts go through cracked along an imaginary line through the middle of each bolt (if that makes any sense)...

 

Replace the entire subframe? No.

From the archives: https://www.rx7club.com/2nd-gen-archive-72/easy-way-change-diff-mount-39026/

 

Replace the entire subframe yea, because it's usually the bracket on the subframe itself that ends up broken... Of course whn mine broke, I just welded an L-shaped bracket of angle iron to support it... Worked great!

 

All the ones I have seen the rubber part was the failing point.

 

Jack up the car and look. It seems more likely to me that since it all of the sudden happened, then its what 13b4me said, but you'll never know till you look.

 

Depends on what causes it... Wheel hop during launches will completely snap the subframe bracket right off from around the bolts, leaving the actual mount intact... And usually failure of the rubber mount won't cause the diff to bang up against the floor of the car... That's a telltale sign of a busted subframe... If my camera was working I'd go outside and take a picture of mine to show you what I'm talking about...

 

ah. well thats convenient! can I weld it with it still in the car?

 

it looks like its the rubber part thats broken.... but when I wiggle the diff up and down the bracket seems to be moving around the heads of the bolts so im not quite sure

in the mean time I just stuffed a few rags between the diff and body so it doesnt knock so hard!

 

Oh yes it does, I can assure you of that. I've only just replaced my broken front mount. The diff would hit the body when applying power and hit the subframe when removing it.

And there are far more threads on the forums about broken mounts than there are broken subframe brackets, so I would consider that the most likely cause. It's easily checked anyway by using a big bar to lever the mount upwards and see what moves.

 

I guess I've just gotten lucky with the mounts then... My friend Jason broke his subframe mount on the Dragon, and then a couple weeks later I broke mine in the same exact spot on the bracket... Guess it made for an easy fix though...

 

well the rags failed miserably. big surprise..

where exactly is the diff hittin the body? I cant tell

i guess its necessary to drop the subframe a few inches then right? and I can just weld it solid?

 

well when my RUBBER mount let loose it made a nice dent in the tub of the car. if your mount has been hitting the bottom of the car pretty hard there should be a dent from it..

 

I broke three of those things on the RXX-7 after putting in the 20B. After putting a torque brace on the motor, the problem hasn't re-occurred. If it does, I'm going to check with Mazdatrix. I believe they offer a competition-grade mount.

 

Dittos. I've replaced three of those mounts that banged on the bottom. Busted rubber. Three seperate cars. Old car. Fifteen year plus OLD cars. Stuff breaks after fifteen years.

 

Just got done replacing...

Its that part circled (wish I saw that pic before I started)

All the bolts that attatch to that crossmember need to be undone..
Some say "lower" the subframe, I ended up taking it completely out!

Be prepared for an alignment after your done!

 

Yes, mazdatrix does sell a stiffer competition grade mount. I've actually got all of the competition diff mounts sitting in a box in my garage. I've never seen a stock mount to compare to, but the "rubber" on the competition mount is MUCH stiffer. I can barely make a dent in it with my fingernail. I've heard it advised not to upgrade just the engine or transmission mounts to competition units because of the extra stresses, but I guess that shouldn't apply to the diff mount since the driveshaft can move in and out of the tranny some.

 

mk. so I strapped down the front of the tranny with 4 rubber bungees , yes it is still doing the same thing. how strong is the friggin mount to be able to hold all that down!

theres nothing else it could be right?

 

Nope.

Of couse we could go through a bunch of hypothetical situations etc... but no. The mount is broke.

Anyway, while we're on the subject. Does anyone know where I can get another solid steel diff mount like the ones Scalliwag used to sell?

Edit: Nevermind, found his homepage.

 

Very strong. A quick calc will show the peak shear force applied to the mount is over a ton, so your bungees never stood a chance.

 

I tied them all from the most forward part and its a lot better. still clunks on decel, but accels smoothly usually. no surprise that this mount broke!

so I only need to undo the two forward bolts that come off the diff to lower it enough to get at the bolts?

 

i shoved a 2x2 in there its worked for now until i fix it.