Alright after much trial and error and no official DIY on the OEM m3 rear bumper conversion i decided to give it a go. Here is a detailed DIY that i have put together to help out my fellow fanatics.
By no means am i a mechanic and any work done i did on my own free will and at my own risk. I will not be responsible for anything that happens to your car during your installation. I am just showing you the procedures that i did to convert my non m3 sedan rear end into an M3 rear bumper. Work on your car at your own risk!
OEM m3 rear bumper w/ diffuser
Prefacelift bumper brackets
4-6 self tapping screws
assortment of sockets and tools
heres a couple pics of the brackets you must use
These next few pics will show you what the bracket looks like in its original position. Notice the metal flap the is in front of the bracket and keep that in mind.
Those are 10 mm bolts holding the bracket in place. Remove these bolts and pull the bracket out and put it off to the side. You will need these later. Now you will see 2 studs hanging down from the rear quarter panel. Heres where the cutting starts.
Take your dremel, and remove the 4 studs (2 on each side) as they will get in the way of the new modified bracket.
Next the metal flap that sits in front of the bracket needs to be cut like this. This has nothing to do with the bracket itself but has to do with the bumper. These flaps will not allow the bumper to travel the distance it needs to and cause a nasty gap at the fender as u can see. Here is what u have to cut.
here you can see what i mean by the flap blocking the bumper, in this pic u can see where the bumper is now able to travel.
Now that you have that squared away, its time to modify the brackets. Reasoning for this is because the brackets need to be placed closer to the body of the car so that the bumper could be pulled in to line up with the fenders.
This is what you have to cut on the brackets.
This is what the new modified bracket should look like.
Now comes the tricky part. Place the bumper on the bumper shocks, take your new modified bracket, place it in the bumper sliders and position the bumper in the correct place. Make sure everything is lined up the way you want it and mark where the bracket is sitting. Remove bumper.
Now take your bracket and a self tapping screw, and mount your new bracket in your marked position. After you have 1 screw in place, put your bumper back on check and make sure the fitment is how you like. If fitment is off, remove bracket and reposition. If its in the right spot, than you can put your second self tapping screw in place.
Now that you have your brackets mounted you can place your bumper on and check fitment on both sides. What you are going to notice (especially on sedans) is that your bumper will be 1.25 inches short of your fenders as shown.
Well this bothered me too much, so before you bolt up your bumper pull it off one more time.
Now get into your trunk and remove the 2 side plastic trays and the long plastic sheild along the backside of your trunk. Than disconnect the negative terminal on your battery, u dont want your airbags going off now do u? Underneath you will notice 3 13mm bolts on each side. These hold in your bumper shocks as shown.
After those are removed you can just pull out the entire bumper shock assembly.
Next pull off the black plastic cap on the rear end of the shock, underneath you will see a white plastic plug.
Now measure 1 inch on the shock, from where it enters the housing. All u need is 1 inch even tho i measured 1.25 it is 1 INCH!
Now heres the fun part. You will need to drill into the white plastic plug into the shock to relieve the pressure
*****PLEASE WEAR PROPER PPE (PERSONAL PROTECTIVE EQUIPMENT)*******
Goggles should be fine and a long sleeve tshirt. Reasoning behind this is because when you penetrate the shock, hydraulic fluid will fly everywhere and i mean everywhere.
Once you have penetrated the shock, push the shock in to your desired length (1 inch) More fluid will gush out and this is OK!. Once you have your desired length, take a screw and screw it into the hole that you just created. This is to seal the shock and keep the pressure that is still in the shock.
Do that to both sides, and reinstall the shocks in reverse order.
Now that your shocks are shortened and installed, its time to put your bumper on for final fitting!. If you still short, remove the shocks and screws and push it in another .25 inches at most. If not than bolt up your bumper and your good to go!
If eveything went well you should have something that looks like this!
If you have any questions feel free to PM me whenever ill be glad to help!