Im pulling my trans and noticed this little bit is busted. For one, what is it called?
Should i use jb weld, a real weld or a replacement?

 

It's usually called a differential spacer. There's no way JB weld will hold that together. Real welding cast iron can be a pain in the dick and you would have to machine the surface after so that's out IMO. Find a used one either here or on ebay.

 

Differential PPF spacer. Get a replacement.

EDIT: I posted on top of SpartanSV at nearly the same time..

 

Mazda part number is MA02-27-158 which might help a search.

 

Unpopular opinion, just send it as is. That little bolt is just there to hold it in place while you assemble it. The bolts through the ppf, spacer and diff is what holds it all together. It'll be annoying but not structural.

 

I had one break in the same spot. If it isn't structure why do they break there?

 

Rust and the fact the bolts are loose when you try to pry the ppf off of it. Highly doubt it was broke on the car with everything torqued.

 

Look at the cracked area. It's corroded along the entire thing. It didn't just break when he disassembled it or we'd see clean cast iron. It broke in service.

 

When you have two large bolts at 120 ft/lbs clamping the whole assembly that little ole M8 bolt isn't adding any structural rigidity to the assembly. Plus the fact there is only one on one end and the fact there is an alignment sleeve that also transfers any load in the spacer to the diff also proves it's not structural.

Also think about how the forces from the diff rising and pulling on the PPF would be transferred to the spacer, it's not even in a direction that would cause that crack

 

I would completely agree with you if i hadn't seen 2 of these crack in that spot while in service.

If they didn't see any load there they wouldn't crack right? That about lateral loads? Wouldn't that bolt be seeing those?

 

In order for rotation to cause the casting to flex it would need to bend and stretch the two PPF bolts and do that enough to flex the casting to the point of cracking. I dunno I just can't wrap my head around that little ol' bolt playing much of a role. I've personally ran a spacer without this ear or bolt with no ill affects as well.

What do you see happening without this bolt? How would the problem manifest itself?

 

I can't explain what forces are causing the piece to break but the fact is they're there. The proof is the photo in the first post. He didn't break that when he took it apart as you suggested. It was already broken. Knowing that they break in service was reason enough for me to replace mine with an undamaged piece.

​​​​​I'll agree that it's probably they aren't designed to see any load there. Maybe the ppf bolts weren't properly torqued. Maybe the ppf flexes in an odd way that the designer of the part didn't account for. The fact remains that they see forces that break them. In my mind that makes it structure and something worth replacing.

 

Different thermal expansion/contraction of the sleeve that protrudes into the spacer would be my guess.

 

I wasn't really expecting a forensic analysis but I'll throw my hat in the ring.
Rust is likely culprit #1, it is a euro fuji diff. Theres oxidation inside the crack so its been like this for a little while, it definitely broke in service.
Improper PPF adjustment is a close second. Ive never actually checked my PPF angle to be honest. Upon install, i slapped it back in place and called it a day.
Another possibility is a pothole. Roads here are not made for scampy sports cars and a land crater could be just enough impulse to crack an already weakened area.
I had no idea this was a common breakage point, so I'm going to replace it in the name of due diligence.

 

Both materials are made almost entirely of iron so I don't think there would be much difference in expansion, but that's definitely an idea I hadn't thought of.