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DIY: Sedan M3 Rear Conversion

86K views 107 replies 17 participants last post by  cd330 
#1 · (Edited)
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!

Materials needed:

OEM m3 rear bumper w/ diffuser
Prefacelift bumper brackets
4-6 self tapping screws
Dremel
Drill
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!
 
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#2 ·
when i did it i took off my stock bumper shocks and replaced them with the m3 carbon kevlar bar, i then took off my rear bumper sliders, driller holes in the rear quarters and in the bumper and zip tied the bumper on and pushed the bumper sides in, i pulled my fenders out a bit to get it to sit more flush but didnt do much..heres some pics
 

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#6 ·
I started tonight, will finish up tomorrow and give you guys info as I find it out. There are many ways I have heard to do this, so I am going to try them out. I have the oem Kevlar bar, extra bumper brackets and will shorten the bumper shocks if necessary



 
#8 ·
Thank bro, I just need to know what you did with the side brackets. Take pics if you can. I know I Have to shorten the bumper shocks which I'm gonna have the exhaust shop do Tommorow. But I just need to have the bumper fitted correctly before I go in. Just let me know how you modified the side brackets and how you re mounted them. Thanks bro

Btw our cars are gonna be twins. Same wheels same rear end same amount of doors just diff colors. Except you'll have pdc and I won't :(
 
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#7 ·
I had to cut the tips and use my 330 rear bumpers tips and kevlar'd the back side and cleaned up the front. The M3 is quite a bit wider afaik, the quarter panels come out about an inch and a half further than the coupes. not sure about sedans. I used all factory mounting materials though, bumpers, impact shocks, side brackets, etc. That's OEM, i dunno about replica's from our sponsors though.
 
#11 ·
yea those sliders that are connected to the car i took off completely...i actually shortened one slider and put it on but the other slider broke while in the process on shortening it..
 
#16 ·
Well when I pulled my stock bumper off, on the driver side bracket the piece that holds the metal retainer which attaches to the fender liner was broken off. So now I can't attach the fender liner on that side. Than I tried to shave down the passenger bracket and I ended up cutting it too short. It ended up pulling the bumper too far in. I also could not find a good mounting spot with it. I ended up running out of daylight so I called it a night ( garage is filled with all my sisters furniture). I'm just gonna go get 2 new brackets Tommorow. I'll play around with the broken ones first and see what I have to do. Than attack the new ones. Im pretty sure that were going to Have to shave te 2 Studs that hang down from the body that mount the brackets. They are in the way for mounting the new modified brackets
 
#17 ·
Yeah, I had a problem getting my bumper off today as well. One of the filister screws was seized in to the top of the reinforcement bar. It ended up shearing off. I ended up getting the screw out, but it was a total nightmare and will have to be addressed tomorrow.

Looks like I am going to have fun with those mounting brackets as well :rofl:

How are you going to shorten your bumper shocks? cut and re-weld? back into a wall? lol....
 
#18 ·
My passenger t50 bolt seized on me as well and ended up stripping. I actually removed the bumper cover by taking out all the push clips haha. Gonna have a shop get that bolt out for me tomorrow as well lol.

Yeah I'm gonna have the exhaust shop cut and reweld. Safer that way. Too many negative variables that can happen with backing into a wall lol.
 
#23 · (Edited)
This is how the wheel, fender, and fender liner look with the wheel on:


Wheel well:


I cut the fender liner with a dremel. This I what it looks like now:


I totally removed the metal fender clip and screw. I shaved the screw off with a dremel and then used a self-tapping screw from the bumper cover to the body. The bumper cover will actually be pulled in enough to where you don't need the self-tapping screw, but I did it just to make sure nothing moves. :



This is a close up of the outside. You can see that the screw I used to mount the bumper cover to the body it is not visible.


I hope these pictures help. :thumbup:
 
#28 ·
yeah same here bro, took me 4 hours to get the bolt out of the reo bar. But i finally did it, after about a dozen broken dremel attachments a couple bent drill bits and a few broken flat heads haha. I did some damage to my tools trying to get that sucker out. I ended up just drilling it out, and somehow managed to keep some threads on the end:thumbsup:

I did get one side finished, the passenger side. And that happened about 30 minutes ago haha. Ill finish it up tommorow. Im definitly going to need to shorten the bumper shocks as the bumper is 1.25 inches short :banghead:

Ill post some better pics




Heres one of my feeble attempts to get that bolt out. Made it a flat head and attached a bit to my socket, the bit ended up braking haha.


And here's to show you that on a sedan not only do u need to modify the brackets. You need to shave down the metal flat right in front as well. IT WILL GET IN THE WAY and cause a nasty gap




 
#31 ·
No those were pics to show people what the original bracket looks like and the metal flap in front that needs to be shaved down. I'm goin to start working on it again now. Here's my pics of the fitment. It pokes a little but I wanted it that way so when I shorten my bumper shocks it will sit completely flush.




 
#32 ·
No those were pics to show people what the original bracket looks like and the metal flap in front that needs to be shaved down. I'm goin to start working on it again now. Here's my pics of the fitment. It pokes a little but I wanted it that way so when I shorten my bumper shocks it will sit completely flush.
wow, that looks really good. When I mounted the brackets to the original screws and mounting points, my bumper sat out about an inch from the body. I will need to shave the screws off from the body and remount the brack closer in to the body. Then push the bumper shocks in. I heard an inch is good. I know you measured about an 1.25" short. Let me know how that works out. :thumbsup:
 
#30 · (Edited)
id love to get the m3 rear bumper but with headers and no cats and the 2nd resonator removed my car will wake the dead if i take the muffler off to route the exhaust to line up...its as loud as an m3 right now with the stock muffler and sounds like one too. ill just wait until i have the s54 in here to put the m3 rear on.
 
#34 ·
So you didn't shave the brackets down? You just pushed them in as far up to the side body as possible and remounted them?

Good alternative to the bumper bolts. :thumbsup:

Also, in re: to the bumper shocks... I've heard two things without having to reweld.

1.) Take the shock off and look down the back side of the tube. There is a plastic cover inside the tube. You could drill a hole in that to relieve the pressure, push the shock in, then put a screw in the hole and cover it up to prevent leaking.

2.) E30 bumper mod. Slowly back into a wall (using a 2x4 between the wall and the shock). It will slowly compress like a 5 mph collision, but should not set off the airbags.
 
#35 ·
Hmm I might do #1

But yeah I did shave down the bracket. You only need to shave the corner tho where the farthest hole is.

Heres what I did to the drivers side flap.





Reasoning for this has nothing to do with the bracket but for the bumper. Since the bumper is pulled in, the sliders on the bumper hit this flap and don't allow the bumper so sit right at the fender. By cutting this out it allows the bumper to travel as u can see here.


Here's what u have to cut on the brackets



 
#46 ·
They aren't, the M3's are wider and the fender flares sit out more. Hence, modifying the bumper brackets and pulling them in closer to the body on the sedans. I should have taken a pic yesterday to show you.

Pretty much if you mounted the brackets on the sedan in their original mounting points, the M3 rear sits about an inch out from the body on the sides and 1.25" short at where it meets the wheel well.
 
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