I mentioned this before when I was taking apart my vette rear suspension. the differential support bolt was on an angle and the hole was bent/elongated to one side. I presumed the car took a hit in the passenger side suspension because that side had a replacement GM trailing arm (not original) and a replacement strut rod. I am the 4th owner & have no clue as to it's past, I ran the Numbers and it has no claimed insurance fixes back to 1995.
anyway I heated up the bent/elongated hole on the support flat again and decided I'd deal with it later. I reinstalled the rear tonight & of course the support bolt does not line up with the hole (for got about it)? It's almost a full bolt width off? there is no obvious evidence of a bent frame as everything under the car looks square & symmetrical. Its like the rear is off towards the drivers side or the diff support is bent to one side or the support that bolts on the differential nose is bent but nothing looks odd or out of place.
I am thinking worst case , bent rear frame but I would think I'd see something with the rear that far off, a repair,frame wrinkles, something?

Paul

 

How are the other bushings....? Not sure what year you have but isn't there other bushings on the left and right....? Maybe one of them is wore out or missing with the bolt thru it tightened down all the way....?

Brian

 

take 2 measurements.

1. from the driver side diff crossmember bolt to the diff carrier hole
2. from the passenger side diff crossmember bolt to the diff carrier hole

these should be equal measurements to the center of the diff carrier hole.

Does the diff look centered in the crossmember ?

Any chance the diff carrier braket is the bent item ?

 

Loosen the four 5/8" bolts that fasten the iron cover to the leaf spring and twist the rear end until the nose lines up with the frame hole. Then after bolting the nose to the frame hole re-tighten the iron cover/leaf spring bolts.

 

cross member bushings are good, even replaced my cross member with another (nice) used one because I thought maybe that was off or bent?
although the center frame support was elongated (the hole) to the driver side, that frame mount itself looks square & centered with good factory welds. the rear bolts on & seems to sit square against the cross member. the cross member go right onto the alignment pucks on the upper frame and when installed the cross member is parallel/squared to the cross frame behind it (not cocked or angled).
I will try your measurements today, not sure if you want the frame points or the same points on the rear diff assembly. I will try both & post my findings later?
Not sure if the drawing I found is accurate but even in that drawing the CL of the car is off center from the differential support about as much as mine looks? maybe my differential support bracket is bent?
almost seems as though I have to open the cross member holes .500" to one side & slide the differential over then re torque?
Paul

 

not sure what this would do as the leafs are hanging in the air & not connected to anything right now. my only fame connection is with the 2 bolts going through the cross member up into the upper frame.

 

Okay, then loosen the four 3/8" NC bolts that bolt the crossmember to the top of the rear end then swing the nose around. The bolts are hard to get to but you CAN get to them with some effort with a 3/8" breaker bar and short 9/16" socket. Just be sure to re-tighten them.

 

I thought about leaving them loose (not torqued) then putting the rear in place & moving it like you said. then I'd make a "hash" mark or two where the cross member & rear end up, tighten it, drop the rear & torque it if the hash marks haven't moved.
I just wasn't sure if there was enough play in the cross member holes to allow it? I will end up trying it, but I did find a member close by that had a differential support for $30 so I bought it off him. looking at mine it may have a slight twist in the front pad? it's worth it to me to rule out that piece for $30 .
thanks.
Paul

 

Ok, dropped the rear/cross member again and loosened the 4 bolts but left them pretty snug. re-installed the rear and knocked the rear over as far as it would go and I can get it with in a 1/4" of the front supports lining up?
after re-trying this over & over (about 7 times) I called it quits for the night. today after 10 hours at work i had somewhat fresh eyes and noticed 2 things. the frame seems square, as far as using a tape measure (by yourself) and measuring what you can. It seems parallel side to side as well? I decided to use a Verneer to check the distance from the center of the bolt on the cross member mount to the center of a factory punched hole in the main frame cross member that is behind the differential cross member. the passenger side is 5/16" off, from the drivers side, so one side it kicked forward (passenger side) or one side is kicked back (drivers side)? which would throw the differential nose towards the drivers side. I also found that when the car took what ever hit it had, it must have got the edge of the cross member as it has evidence of the corner being torn back (on an angle towards the center of the car). And the front sheet metal support that used to be welded to the main side frame from the rear cushion retainer, is gone, the retainer is there but i think it was pushed or bent forward and someone cut or ground off the sheet metal support that used to come from the front to the main frame?
So heres my question,is there a way to move that rear cross member cushion retainer back 5/16", while in the car/body on. or do I leave that alone and open the 4 cross member holes up to enable me to get the last bit I need to center the rear?

 

I stole this picture from another post just to point out what part of my passenger side retainer bracket has been cut off /removed from my frame by a previous owner (circled in red). this is from a 60's vette frame so I think my support is not a wide as this but it is missing when compaired tot he drivers side that has one.

I assume I can buy an new sombrero(retainer) and frame mount to weld in, I hope!
Paul

 

You can check that crossmember to see if it is square by measuring,then the cushion bolts if it won't go. if that thing is out of whack, then loosening the entire crossmember at the cushion bolts would get that amount of distance, but then I would wonder what else is bent?and get a tapered tool and pull it into alignment!

 

long story short I bought the car ('74) as a project & knew it had rear suspension problems, just not this much? the passenger side had a GM replacement Trailing arm assembly (not he original like on the drivers side), I also found a replacement lower strut rod on the passenger side & heavily damaged u-joints on the half shaft on that side, + the missing frame support off the sombrero/retainer & evidence of the old weld wear it was ground off. all of that is in conjunction with the diff not lining up with the center mount (the original center mount bolt was on an angle & elongated/torn the frame mount hole to towards the drivers side (see pics above). I don't see evidence of body repair under the car or stress cracks or main frame damage or repair, so i think it took a direct hit to the passenger side rear wheel and it moved the mount etc.

other then that the car runs and has all the correct pieces and is worth saving but I have no intention of taking the body off. I will be replacing the diff center mount with new poly pieces as well as new shocks,rebuilt trailing arms,new adjustable lower struts,new brakes/calipers & lines,all new u-joints etc. so the whole back end is new so this problem is my hurdle from putting the car back down on the ground.
thanks for any and all input!
Paul

 

I definitely won't get discouraged!

Unless you can pinpoint whether something is bent or not square. You do want the nose to driveshaft as inline as possible. I doubt that the nose mount on the frame would have been bent, but there is always the possibility that that entire crossmember where that mount welds too is tweaked if you aren't seeing it at the rear diff crossmember. Do the brake lines look Ok? I now insist that you not even loosen the cushion bolts like I suggested a minute ago and start really checking to see if that is square. I would prefer to not have the rear diff off at an angle, even if it essentially runs through halfshafts and a driveshaft and there is some flexibility of the rear spring mounts.

Do what you can to identify if you are square, but honestly also-- you don't look too far off to me, so I feel that distance with a little wiggle of the rear end crossmember on one side smacking it with a mallet should get you that distance, but then I don't want you crossthreading the cushion bolts either, so leave those well engaged if it comes to that!

 

will do, I am going to remove the rear/cross member one more time to go up and inspect the mounting plate that the cross member retainers are spot welded to (the sombrero's). I found other posts on this site with pics of how those flat plates that are stitch welded to the main frame have been known to tear off or crack with age & hard use?
I need to find a member that has a good frame to measure some center lines off there frames so i can compare it to mine using factory frame spots (see pic).
all of my brake lines & fuel lines were original & still factory "straight" alone the frame. as stated above I even got another cross member thinking mine was bent and still had the same outcome so I think my problem is in the 2 retaining mounts since they position the diff. also tried another differential front mount in case mine was tweaked, same problem.
I will post my finding later tonight.
Paul
measurements I need:

 

just up dating this post with the results of my measurement request post, for those that were following along.

here is the other post from this weekend:

https://www.corvetteforum.com/forums...surements.html


here are some frame measurements I did with a tape measure laying under the car (so it's not pin point laser geometry LOL )

 

With all those diagonals off on the passenger side seems to me it got smacked right where that crossmember is the front diff bushing goes in. I am not sure if you have the ability, but may need to get it to a frame shop.

 

I would say you are square at least. i bet you feel better now on that part! But are these built with an offset for the drivetrain? i think so, but i can't get to mine to measure, so i hope you get some help, because this whole thing is shifted (but squarely) to the passengers side. Also this car could have been jigged up forever like this as evidenced by the hole having had someone hog it out. Interesting! I am going back up to study the stock frame dimensions and measurements! The bracket is one inch toward the passenger side, but you have a two inch shift on the crossmember. Plus i would measure the donuts distance from the very rear crossmember each side also to see if we can see if it was hit or how hit, if it was hit!

Does someone have the hole OD for him also??

 

take measurements of the crossmember ends to the mounting holes for the diff to see if there is an offset there!!

 

is this the measurement you want me to take?



i also presumed that the drive train was offset to the passenger side on c3's and would account for the different angled measurements?


what puzzles me is if the frame is bent, wouldn't the body mounts (all or some) be off just like the differential mount was? all the (visible) body mounts are straight , not jogged or at any angles ,and centered on there mounts? also no obvious signs of body cracking or distortion. all the original brake & fuel lines were in there factory installed positions with no signs of replacement or re-bending from a hit etc

 

or this measurement?