Laminar's M3 5.0 swap: Fünf-Null

[squashman702]

With a soft rubber bushing, as the diff moves, the head of the bolt in the bushing is allowed to move quite a bit. I don't know if this bracket is technically "double shear" but it would certainly reduce movement of the bolt.

This one is technically double shear, but apparently makes it very very difficult to pull the diff. I'd also be worried about haphazardly welding mild steel to cast iron - you need to use an arc welder and nickel rod to do it right.

I'm thinking of doing something like the AAW bracket but instead using poly sway bar bushings so that I don't have a solid connection from the diff to the subframe. That just seems like a recipe for snapping the brittle ear off of the diff under wheelhop.

I'd be a little nervous about welding to cast right next to the pinion bearing. That thread makes it seem like all you need to do is crank up your heat a little extra on the welder and you'll be good to go. In reality, stick welding with the correct rod is probably the way to go, along with pre and post heating so that the weld cools correctly. I would want to avoid all of this next to a bearing or seal.

I think your idea with the poly bushings is decent, but I wonder how much of the load it will actually transfer to the brace with the bushings added instead of it being solid mounted.

[Laminar]

I'd say it depends on the stiffness of the bushings on either side. Also, with an adjustable setup some preload could be applied to ensure that both sides absorb the force.

[jalopi]

i'd put money on the diff casing being made out of cast steel, not iron. just look at the dude in the picture, he seems to be using a regular mig welder to weld that plate to the diff. i don't think you can mig cast iron like that - cast steel will mig just fine though

[squashman702]

i'd put money on the diff casing being made out of cast steel, not iron. just look at the dude in the picture, he seems to be using a regular mig welder to weld that plate to the diff. i don't think you can mig cast iron like that - cast steel will mig just fine though

You can MIG to cast iron all you want, but doing it in the car like pictured is not the right technique.

[tptrsn]

You can MIG to cast iron all you want, but doing it in the car like pictured is not the right technique.

Care to elaborate? Just saying it isn't right without further explanation isn't terribly helpful.

[squashman702]

Care to elaborate? Just saying it isn't right without further explanation isn't terribly helpful.

I explained it a couple of posts ago, but you need to pre and post heat welds to cast, and use a nickel rod. Last time I helped my friend weld axle tubes to a housing, we preheated with a torch, then wrapped everything in a heavy blanket afterwards to let it cool as slowly as possible. Definitely not something I'd want to do in a housing that had bearings/seals, etc in it.

[tptrsn]

What rod number worked well for you?

[squashman702]

What rod number worked well for you?

I have no idea what rod my buddy used. I wasn't able to stick weld at the time, so I was manning the torch and the temp gun.

[Laminar]

In the interest of getting the M3 out of the garage so that my DD can have a cozy warm spot on these chilly mornings, I drilled and tapped for the 14mm bolt, no additional bracing.

Bolt is BMW# 07129908011, it's a suspension bolt from an X5. M14x1.5, 100mm long. Grade 10.9. $6.62. I looked at McMaster and Fastenal and couldn't find anything in this thread that was long and high grade. Anything close was at least twice the price. I ordered two so I can keep a spare in the car.


You vs. the guy she tells you not to worry about.

I took video of the whole process and may put a little something together for YouTube.

[jalopi]

You vs. the guy she tells you not to worry about

lolololololol

so wait, you're breaking 14mm bolts on the regular now?

[Laminar]

Stock is 12mm and I broke two of those. New is 14mm and we'll see how long it lasts.

[ponchiz318]

Stock is 12mm and I broke two of those. New is 14mm and we'll see how long it lasts.

Wait, your engine is stock, right? Any plans on purchasing or fabricating a bracket?

[Laminar]

Stock engine. No plans to purchase. I bought two of the 14mm bolts and will carry a spare with me. If this new bolt breaks, I'll put the spare in to get me home then worry about a bracket.

The other thing I realized this week - the first time the bolt broke I was on all stock bushings. Simply the act of driving forward was enough to push the driveshaft up into the tunnel and begin sawing through it. This time, if I was really easy on the gas I was still able to drive. I realized that when I upgraded the diff bushings to poly all around, that included the diff ears. The old rubber bushings easily deflected and let the diff hit the tunnel. The new bushings had just enough stiffness to let the car drive forward carefully so I was able to make it the 45 miles home without a tow.

I've found at least one person that put some stiff rubber on top of the diff case so that if the bolt ever does let go, the diff can't move up further than the rubber would let it. It's another thing I wanted to do but since I really wanted to get the M3 out of the garage I didn't take the time.

[ponchiz318]

My engine has been modified so it'll put more power down than yours. I know that in my car, the rtabs need to be replaced. I'm thinking I should do the entire rest end now with poly bushings now. Or maybe just make a bracket and see if that works.

[Laminar]

Here's my upgrade to the 14mm bolt:



Snow thrash video coming soon.

[ponchiz318]

Great stuff man. Seems easy enough to do.

[SLR]

I ran a pro drift car for years with just a M16x1.5 bolt. The aluminum center in the forward bushing is quite large so it easily drills out with a 5/8" drill bit and leaves plenty of meat, which was a perfect fit for the M16 bolt. IF the factory rubber is intact I wouldn't change topoly since they are known for working their way out.

This took a beating with sticky tires and a Tilton triple plate. The Tilton is a brutal clutch, basically on-off, and it sheared 3 of the OEM bolts .


Another big contributor to the failures is the deflection allowed by rubber bushings. if you go solid aluminum on the whole diff, it isn't able to get the deflection required to break the bolt in the first place. 14mm still will give peace of mind even with the aluminummounts, and like I said, stock rubber here with a 16mm bolt, over 400rwtq hasn't broken even with abuse from the most brutal clutch around.

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I'd be a little nervous about welding to cast right next to the pinion bearing. That thread makes it seem like all you need to do is crank up your heat a little extra on the welder and you'll be good to go. In reality, stick welding with the correct rod is probably the way to go, along with pre and post heating so that the weld cools correctly. I would want to avoid all of this next to a bearing or seal.

I think your idea with the poly bushings is decent, but I wonder how much of the load it will actually transfer to the brace with the bushings added instead of it being solid mounted.

That weld-on diff brace is a terrible idea and execution.

The diff case is definitely cast iron, and so to even weld properly requires a pre-heat and good penetration with a high-nickel rod. Meaning the diff needs to come out of the car and be completely disassembled.

Doing it in the car means damaging the front seal on the pinion, and also most likely coking the oil in the pinion bearing. So the diff also needs to be drained, flushed with gasoline, drained again and ventilated to evaporate the fumes, and then commence the work, then replace front seal, retorque flange with new crush washer. And even then, I can't see how you remove the diff that that brace on there other than maybe installing at 45⁰ angle and fighting with it for 45min.


The big bolt upgrade is so easy and works like a champ!

[squashman702]

That weld-on diff brace is a terrible idea and execution.

The diff case is definitely cast iron, and so to even weld properly requires a pre-heat and good penetration with a high-nickel rod. Meaning the diff needs to come out of the car and be completely disassembled.

Doing it in the car means damaging the front seal on the pinion, and also most likely coking the oil in the pinion bearing. So the diff also needs to be drained, flushed with gasoline, drained again and ventilated to evaporate the fumes, and then commence the work, then replace front seal, retorque flange with new crush washer. And even then, I can't see how you remove the diff that that brace on there other than maybe installing at 45⁰ angle and fighting with it for 45min.


The big bolt upgrade is so easy and works like a champ!

Fully agree about that design, was just trying to be nicer about it initially haha. Gear pattern should be checked again as well if you're pulling the pinion gear. Even with changing the crush sleeve, I'd want to check preload.

Kind of interesting how different the attitudes are in the BMW world versus Jeep (where I have a lot of my mechanical background) in regards to gear setup. BMW guys as a whole seem pretty uninterested in properly setting up differentials versus the Jeep crowd who mostly seem to think it's a black art.

[Laminar]

I ran a pro drift car for years with just a M16x1.5 bolt. The aluminum center in the forward bushing is quite large so it easily drills out with a 5/8" drill bit and leaves plenty of meat, which was a perfect fit for the M16 bolt. IF the factory rubber is intact I wouldn't change to poly since they are known for working their way out.

Tell me more about them working out - shouldn't the bolt tension hold it in place?



I assume it pulled out after the bolt snapped.

[Laminar]

Installed my Christmas present.

[Laminar]

I went for a drive in the snow.

[Laminar]

The whole project can be knocked out in an hour for around $100.

I used this set of wheel studs: Amazon

And this set of long open-end lug nuts: Amazon

[jalopi]

Any idea what grade they are? Seems kind of cheap compared to what most other companies are charging

[tptrsn]

Is there any advantage to the bolts? Also, what quality/grade are those studs? I probably need to do this for quick pit stops during drifting, even though I'm actually a pretty fast one man pit crew with the bolts. I guess I've gotten used to them haha...

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Whoops, I didn't see jalopi asking one of the same questions just ahead of me...

[Laminar]

Is there any advantage to the bolts?

I haven't been able to think of any. I'm sure ze Germans thought they had a good reason.

Also, what quality/grade are those studs?

Not sure. I asked and I'm waiting to hear back from the vendor.