I had a cracked rear spring the other day so I DIY'd it. I've taken suspension apart before on other E46 cars and it's always been quite easy - but on the M3 I found it a lot more difficult than I remembered. Fortunately, after an hour of swearing, I discovered it was all about technique, even when I was doing it alone. However, it would be easier still if there were two of you.
Firstly, the genuine BMW rear springs are ludicrously expensive for what they are - I think they are £150 each and it is recommended that you change them both. The genuine springs don't last much more than about 30k miles before they crack again, and can go a lot sooner. Make no mistake though - they WILL go again. They all break on the last couple of coils because the spring goes noticeably thinner as it reaches the end of coil and goes into the pigtail - this appears to be the weak spot and it sheared presumably due to metal fatigue.
The Sachs ones from ECP/CP4LESS that many people have fitted on M3Cutters are NOT the correct ones and are the equivalent of the pink ones fitted to the convertible. BMW have a service area on their website where you can type in your chassis number and it will tell you which spring you need, as although most coupes will have the green springs, some, by virtue of their options list, will have pink springs, and some will have yellow.
Anyway, mine needed green ones so I had a look around and no one does them! ECS Tuning recommend Suplex springs as an alternative to the Genuine BMW ones and are manufactured in Germany. They appear to be high quality, robust and excellent value for money at £86 for the pair.
The way to do it is below:
- Jack car up, remove wheel.
- Remove 18mm lower shock bolt and remove 13mm bolt on the ARB link.
- Push down hard (you/your friend may need to bounce on it and then you manoeuvre the spring out every time you bounce.
- There should be two spring pads - the one with the pointy bit goes on the bottom, the other goes on the top. Fix them to your new spring (pay attention which way round it goes in - there should be an arrow saying TOP on the spring).
- Now the trick here is to get the top of the spring and the pad seated sort of into position. Again, bounce down on the trailing arm until the top is nearly in place - it'll obviously still be at an angle until you get the bottom pad in place.
- Get a prybar (45cm or longer makes it easier) and wedge it under the bottom of the spring/spring pad which will cause the spring to compress AND the lower arm to push down when you lever the pry bar giving you some room to get it in place. You have to use quite a lot of force to get the bottom of the spring over the bit the spring pad sits on, so don't be gentle. Then just keep trying to slowly manoeuvre it into place and eventually the bottom spring pad will seat correctly onto the upper arm.
- Jack up the upper spring arm until the ARB link comes level again then put the bolt back in an tighten up with two 13mm combi spanners.
- Raise/lower the spring arm until the lower shock hole lines up and put the 18mm bolt back in.
Things to avoid/forget:
1) Don't try and put to the bottom of the spring in first, then lever the top in - it just doesn't work.
2) Forget spring compressors etc. They are unnecessary and they simply will not fit into the spring area - it's tight. However, they can be good to take some of the height out of the spring before you put it in. During my struggle, I tried to use spring compressors before discovering that they wouldn't fit, so took them off. I then noticed that they had shrunk the spring a bit, which was helpful.
3) Using a jack in the wheel well to push down on the lower arm to give you more room - this is just going to lead to you putting a hole in your wing or something and it simply won't work.
So there you have it, all you need really is a decent length pry bar and to wedge it under the lower spring pad.