you mean the bolt in this post?:
https://forums.m3cutters.co.uk/threads/ ... st-2261624
M14 front diff carrier hexbolt edit
no, these are also the rear bolts, so M12 but apparantly the hex head fits without problem?
Or these ones?
https://forums.m3cutters.co.uk/threads/ ... st-2261473
M12 rear carrier torx bolts
That first one is an upgrade (or at least a variation) on the stock bolt.
The stock bolt, and for sure the all threaded bolt shown in the 2nd link is a weaker bolt, as the threads run the complete lenght of the shaft.
Threading makes the core thinner. The stock bolt with partly unthreaded shaft is stronger, but the shaft isnt as thick as it could be.
Grade is 10.9
That first bolt (in the pic in the first link) is a DIN931 style bolt. If its the same 10.9 grade its much much stronger (think 30-60% depending on thickness and thread) as a completely threaded bolt. This is because the part of the shaft that isnt threaded is the full thickness of the size of the bolt (so ~12mm with an m12 bolt)
I made a similar upgrade, not because of a clunk but because I broke my rear diffbolt..... (the normal z4 has one long 14cm m14 bolt at the back and 2 smaller ones at the front). So the other way around compared to the z4m
This is a comparison between bolts that I made back then:
All bolts shown are 10.9
My original bolt broke at the point between the central rear bushing and where the diff sits against that bushing. Thats the point where it flexes and shears if force is put on the diff.
The bolt goes all the way through the bushing and through a lug in the diff cover and is held on with a nut on the other side
The lowest one you see in the pic above is the bolt of the stock Z4. It is threaded all the way
The middle bolt is from a 550i. This is basically the same boltstamping but they threaded it only for a part.
This is stronger because wherever you roll threads, you make the core of the bolt thinner by about half the thread depth (~half the thread rolls in and pushes half the thread out, if that makes sense).
The upper one is the din931 style bolt. Its shaft, the unthreaded part, is just as thick as the top of the threads, so about 1,5mm thicker than the 550i bolt and about 3mm thicker than the original bolt (the thread pich is 1,5 and thats roughly the height of the threads.
If you translate that into crosssection of the core, where the oem (full threaded) bolt is 47,5 mm², the DIN931 bolt is 77mm² (the outer mm take up the most area, and thus give the most strenght). If core area equals strenght in a linear way for the shear forces, the din931 bolt would be 62% stronger.
So thats quite an upgrade.
With the stock z4 the Din931 bolt fits fine with the powerflex bushing I use. It fits through the core. The bushing comes with a large ring (that sits between the bushing and diff) and I had to drill that a little bit bigger, just 0,5mm or so not a lot (or file with a round file)
But how things line up with the z4m with z4m powerflex or strongflex bushings, I don't know. But switching to din931 bolts will significantly make for a stronger construction all togheter. IF thats really necessary or not, I dont know. It was on my tuned car.
I've got shore80a also for the diff bushings. I wanted to minimize the risk of whine and still wanted a bushing that has a long lifecycle as on the normal z4 that rear bushging a known failure point as that bushing is a hydrobushing, and my car puts almost 60% more force on that than stock.
I don't really know what the benefit would be for a stiffer bushing; I don't have the feeling that the drivetrain feels sloppy, the jolts through the diff are I think not as strong as jolts coming from the suspension as that puts the car's weight into it. That doesnt happen with the diff.
But I dont think stiffer bushings might damage things or something. Its held in the subframe.