Cracked trans crossmember

[fesser]

I have been prepping and painting parts recently and primed a bunch of parts when it was too cold and damp. Not to bore people with details but I decided to knock most of the questionable primer off some parts. The transmission crossmember from my 68 Coronet R/T was one of those parts. It was only when removing the primer that I noticed three cracks. They aren't that big so I circled them in yellow to help people see them. I recently bought a transmission that came with some odds and ends, including a transmission crossmember. I decided to check it for cracks. I didn't see any cracks but I noticed it has a lot more structure/bracing than my original. Some p.o. chromed it, and the person I bought it from added a tab to the back end of it (not sure why). I have two questions.
1. Are the cracks on my crossmember common?
2. Is the chrome crossmember the one I should use based on the additional structural parts?

My original part...

Both for comparison...

 

[eldubb440]

I'd weld those cracks up and fuggetaboutit.......... you could reinforce it like the other one while you are there

 

[fesser]

I'd weld those cracks up and fuggetaboutit.......... you could reinforce it like the other one while you are there

Is the other one a factory part? I mean with the bracing. Not the chrome and the tab.

 

[eldubb440]

Is the other one a factory part? I mean with the bracing. Not the chrome and the tab.

not sure, I never looked at one that close

 

[not so famous bob]

I'd weld those cracks up and fuggetaboutit.......... you could reinforce it like the other one while you are there

agree^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
those look like they were made when the metal was bent at the factory that made them , weld em up !!

 

[j-c-c-62]

Interesting. The cracks indicate to me the real world "weakest link" in the design. I suspect the mirror pair of cracks are likely caused by the TB's repetitive loading. During fabrication those would be maybe compression bends, not likely to cause cracking? The single crack is puzzling, because I am uncertain of what upward force in use would overload this area to cause a crack to start, and a gravity load in compression to cause a crack would require incredible loading and unloading cycles, and then both sides should be cracking. I don't see transmission torque transmitted thru a single rear mounting point great enough to crack the mount unless car had a failed engine mount for a number of miles, and even that would be a stretch. I do see some very small indication of a crack maybe starting on the other side, not sure. Welding would likely solve the problem in the near future. However, welding will just enhance a stress riser in an already proven overloaded area of the design. It would not take much added metal to reinforce the area to gain high confidence. Unless this is a small batch of poor metal, or the car was a stunt double General Lee ramp jumping car, we should have similar reports of like cracking. Cosmetic chrome plating is frowned upon on highly loaded structural items in most cases. if for no other reason, is it hides from detection the initial cracking slightly.

Good eye by the OP BTW.

 

[fesser]

I apologize for the low quality pictures. I am still curious about the additional metal on the one in the background. If you look closely you can see that it has a flat piece that bridges the center and gets welded to the channel portion a few inches either side of center where the one in the foreground just has the flat piece that is as wide as the pocket that the trans mount sits in. There are also two other "braces" that extend onto the falt center section. I know my description lacks clarity, but I am still wondering about the differences and when Chrysler decided it needed to use the reinforced one.

 

[j-c-c-62]

We can assume I believe if those reinforcements are indeed factory, they discovered the problem after the first rendition was in the field, and decided it only needed a small upgrade to resolve, the when and how of the that is above my pay grade.

 

[Bird 426]

I wouldn't use the chrome one, because of Hydrogen embrittlement. Unless you know that it was heat treated right after chroming to relieve it.

 

[j-c-c-62]

I wouldn't use the chrome one, because of Hydrogen embrittlement. Unless you know that it was heat treated right after chroming to relieve it.

I was under the impression chrome plating traps hydrogen embrittlement and resists the low temp baking corrective process?
High temp discolors the chrome.

 

[Bird 426]

It's not a high temp, 450°? For 45 minutes or so? Some places call it Aircraft chrome. Hydrogen embritlement is real, broke chrome parts on my Harley all the time. A friend had his steering parts on a race go kart all chromed, till a knuckle broke and shattered his ankle. Permanently messed his ankle up.

 

[j-c-c-62]

It's not a high temp, 450°? For 45 minutes or so? Some places call it Aircraft chrome. Hydrogen embritlement is real, broke chrome parts on my Harley all the time. A friend had his steering parts on a race go kart all chromed, till a knuckle broke and shattered his ankle. Permanently messed his ankle up.

I was under the impression at 450F chrome is beginning to yellow, but that was not my primary concern here, it was more that the chrome process effectively seals the hydrogen in, preventing the normal post plating baking in getting Hydrogen out of the metal.
Bottom line, regular chrome does not belong anywhere on a proper race/HP car, unless it's hard chrome on the crank.

 

[Bird 426]

Well , that's not the way that I remember it. I baked the chromed valve covers on my airplane at 450°, no discoloring. Seems like there was a window after the chrome dip for the heat treat. I'm not an authority on this. The person that I asked about way back when, that had a chrome business, is dead on a mountain top in Alaska, never found him and X wife, it was on the national news. Anyway, I probably wouldn't chrome a cross member.
I'm out.

 

[j-c-c-62]

There is a window, and it must be a prompt one, sooner the better. Not sure why a valve cover even needs baking, we are assuming here they were steel. And baking is not any form of heat treatment, in that the metal is not changed in anyway, and why it's a process for steel recommended without any physical downside, assuming there is no quenching by somebody in a hurry.

 

[fesser]

Fwiw, I don’t intend to use the chrome one