As part of my 73 project, I installed a new poly front differential pinion or snubber bushing.
The bushing cracked immediately after being torqued to the specified 70 ft lbs torque.
I received the replacement poly bushing today and installed it.
While I was torquing the bolt from 40 to 55 ft lbs I could feel the threads on the bolt strip.
I loosened and removed the nut and the entire bushing assembly.
As you can see in the pictures of the bushing assembly, the bolt stripped, the lock washer became disfigured and the thick flat washers bent.

I thought my torque wrench was off because it never clicked so I replaced it with my second torque wrench with the same results.

This is the second bushing I’ve had problems installing and need some help from the pro’s as to try and find the issue.
I just received a new rubber bushing to install instead of using poly.
The new rubber bushing cushion is thicker than the poly replacement and almost twice the thickness as the first Delrin bushing.

Could something be binding up the assembly?
The rear differential is bolted and torqued to spec to the differential crossmember.
The differential crossmember is bolted and torqued to the frame.
The front differential bracket (snubber bracket) is bolted and torqued to the differential.
Trailing arms and shims are installed, the bolts are not torqued.
Half shafts are installed and flange bolts are finger tight.
Strut rods are in and nothing is torqued.

I can move the front of the snubber bracket up about 1/2” easily by hand.

Does anything look off in the pictures or could the parts that I have torqued be causing a bind?

If all looks good, I’ll try installing the new rubber bushing and see if it torques up to spec without bending anything, breaking anything, or stripping the bolt threads.

Thanks for the help!

 

Rubber is better in that area. here's the way the parts are assembled:

 

Here's a better copy:

 

Back in the mid 90's. With my car completely assembled in every way. I replaced my snubber bushing with a poly replacement. I am certain I didn't use a torque wrench. I just tightened it tight. I have inspected it several times recently as I have been fighting a vibration/shake issue. No issues with my poly snubber bushing.
I guess I didn't think there was a critical torque for this mount.
Still don't.

 

I had the problem when installing on the now-gone 69 project, with rubber bushing. The lock washer provided peeled open like yours. I think it is junk metal. So, I found and used an original lock washer that was removed from the car, and it worked perfectly. It appears finding good metal is now a problem. I know 70 lbs is a fair amount of torque, but a Grade 8 bolt should easily handle it.

 

That's a pretty robust bolt to strip out at 55 ft lbs, very weird.

 

Get a serrated flange nut......problem solved.

Jebby

 

Thanks everyone,
Is there any problem that there is as much space as there is between the snubber bracket and the frame or is it normal to have some space?
I’ve watched several videos and read threads where there is discussion about having to pry to two pieces apart just to install the cushion.

Could my problem be that the Derlin bushing is half the thickness of the other poly cushion and the rubber cushion?

I’m just trying to figure this out before I ruin another bushing assembly.

 

Yes.....you have stick a bar in there and open it a bit....slide the new one in and install the bolt.......with poly you want no movement at all.....any movement will shred that poly mount to bits......

Jebby

 

Thanks Jebby.

I went out to the garage and tried to see if the new poly bushing would slide between the bracket and frame.
It clearly will not and I will need to pry the two pieces apart.

The new poly bushing assembly came with two large thick steel flat washers under the cushion.
Does one of the large washers go between the cushion and snubber bracket?
The first thin cushion only came with one.
I noticed the new rubber cushion has a large flat washer imbedded into the bottom of the cushion.

I have the Van Steel Delrin crossmember bushings installed in the differential crossmember with the aluminum pucks.
Would you suggest using the poly or rubber bushing in the snubber?
Van Steel suggests the poly.

Thanks!

 

Poly....you don't want any potential flex on the crossmember bushings....

Jebby

 

Just an observation - if you do a wheelie, this is what lifts the front wheels off the ground. Kind of amazing!

 

Thanks Jebby,

I spent some time on the phone and texting Eric at Van Steel about this bushing.
Eric said the same thing Jebby did and that with the Derlrin crossmember bushings, I need the poly bushing in the snubber.
He told me the short cushion is paired with the crossmember bushing because the crossmember bushings pull the differential up into the frame and the short cushion is to keep the driveline angles in tolerance.

He was at a loss for the reason the bolt failed.
He said it could have been a batch of poor quality grade 8 bolts, nuts or lock washers.
Van Steel is sending me a complete set of new large flat washers, the bolt, nut and lock washer.

 

I will leave the wheelies up to the racers.

 

It ain't just wheelies. Merely an observation as to how much instantaneous force (i.e. 'jerk') is applied at this point at pretty much every hard shift. Myself I went with rubber to minimize the shock. Anyway if your poly bushing disintegrates you will soon know it. As to the bolt failing, well wouldn't it go into tension at every shift? Nothing you can do about that. But again, the rubber that was in compression would now expand at first, softening things in that direction too.

That crossmember is pretty solidly welded to the frame. The crossmember that the diff is bolted to is part of the dynamics here and I believe I have rubber there too. I have seen videos where that member wants to move around a whole lot if not properly attached. All in all the diff is sort of floating on those three points.

I'm a couple of glasses into wine at this point in the evening and mechanical stuff is not my forte so I suppose I could be misinterpreting things but that's how it looks to me.




Oh yeah, those are classic tube SS steel lines that you see. I had to do a little tweaking but it wasn't bad.

 

Ignatz,
I’m a builder type and not a designer.
So if the designer or engineer tells me this is what we engineered and what works, that’s what I build and install.
If Van Steel told me I could put rubber bushings in I would, but they strongly recommend against it.
I’ll go with their recommendation and when it fails whether in two or twenty years, I’ll replace it.

I’ll be heading out your way to drop some parts off to Chris sometime in the next couple of months.
We should get together and meet like we talked about.
You can have a glass of wine and I’ll have a cup of coffee.

 

Would love to get together Greg, I've only met a couple of forum members in real life. Meanwhile having fun following your adventures with your car.

 

My experience with the snubbers is that on leaf spring cars there is significant snubber movement, and if it changes the driveshaft u-joint angle enough, the angle change becomes a problem. U-joints are most efficient at 90 degrees, and are weaker at other angles, leading to easier breakage. (aka Dodge pinion snubbers) But this has almost no impact on a C3.

But the other thing that rubber bushings do is they let the diff rotate, and rebound, with power or traction pulsing on/off. With any inconsistency in tire traction, as in slippage, gripping, hooking-up, pothole, etc. etc. you can get into wheel hop. C3s can get into serious wheel hop with rubber bushings, and there is no shock absorber in this direction that dampens this oscillation (as Mustang quad-shocks do), so once it begins, it can get quite severe. And this hammering will break already marginal IRS parts. Spinning the tires is not the problem, it is when they are beginning to hook up.

The 70s Baldwin Motion C3 solution was a diff mounted C3 traction bar.

I'm going delrin diff mounts for this one reason, to prevent wheel hop, and protect my IRS parts.

 

As to the differential mounting, if you want to go completely hard core, here's what's in the Chevy Power guide. A bit too harsh for me. As to why, I suppose this eliminates any compliance change with the chassis that affects wheel alignment. That's about all I can come up with.

 

This is the same thing Van Steel and others have told me.