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Discussion Starter #1 (Edited)
Hello gents,

I have a bad case of tinworm in my M5, which is not suprising given that the body has covered almost a quarter of a million miles in all weather (mostly wet here!). I have already cut out and welded in new jacking points on all four corners, and after removing the exhaust mufflers, I can see that there is a fair amount of corrosion to the rear end also. I've started to wire brush it back, and it looks like I have just caught it in time, but what concerns me the most is the fact that the rear subframe mounting points are starting to rust. They are still solid, but I want to clean all that surface rust back to raw metal, treat it, and re-underseal before it gets any worse. This job is of course only possible if I can get access to the mounting points, which will require the rear subframe to be removed!

So, I'm guessing;

1.Exhaust off
2.Rear calipers off
3.Unbolt propshaft from diff
4.Unbolt struts from hubs
5.Disconnect handbrake cables somehow (probably at the lever end), I see they are also suspended from the body on some hooks in places.
6.Remove 4 main big bolts, and lower whole rear end subframe assembly complete with diff/driveshafts/hubs down and away on a jack

Don't think I've missed anything? After all, the subframe would have been fitted on the assembly line at BMW as a complete assembly, so it should come off pretty easy!

I already have the rear bumper off and have the rear section of exhaust off (muffler delete). To be honest, I don't really want to remove the whole exhaust system and heat shield etc, but fear I might have to, to get to the handbrake!

If anyone has done this, any tips are much appreciated, especially as I'll be doing this job on my own, on my driveway.

Once I've sorted this rust business, she'll be ready to go for the half a million ;)

All the best, a rusty M5 owner!
 

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Remove the parking brake cables from the handle inside of the car (this is easy) and they will slide out of the interior. Sounds like you have the rest of it figured out.
 

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on a different note - wouldn't it be more practical to buy a new chassis (ok, not new, a used one in good shape, do all the preparatory work, i.e. paint it, protect etc) and swap everything inside?
 

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You will also need to disconnect wiring for ABS speed sensors, pad wear indicator and vertical headlight aim sensor. Well done for taking on this work, I'm also trying to preserve my M5 and 528iT for many winters to come, the amount of road salt used around here is unbelievable!

One other thought: you may be able to just drop the subframe by a couple of inches to get enough room to clean and repaint the mounting points. I did this when replacing the subframe bushes on the touring. There was some surface rust but it looked worse than it was as the steel is very thick there.
 

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Discussion Starter #6
After looking at a post on another forum about replacing subframe bushings, it looks like you can lower the subframe by about 3 inches at least by just removing the subframe bolts, which should be just enough room for me to get in there and address the rust.

apetrov1x5: Replacing the entire shell wouldn't be time or cost effective, I've already sorted all other areas for zero cost as I have all the equipment in my workshop. I am also a classic car owner, so I am no stranger to welding in new panels when it comes to rust!

I'll get a few photos...
 

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I'd love to see any pics you have of the sill and jack point repairs. I've got a couple of small patches to do on the sills and the bottom edge of the rear doors are rusting, perhaps from previously leaking vapor barrier. Not sure how to attack the folded edge of the door.
 

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Discussion Starter #8
I'd love to see any pics you have of the sill and jack point repairs. I've got a couple of small patches to do on the sills and the bottom edge of the rear doors are rusting, perhaps from previously leaking vapor barrier. Not sure how to attack the folded edge of the door.
I encourage you to action the jacking points sooner rather than later. Here's some photos of mine:

This, may not look so bad...



But by the time you have found good solid metal, you end up having to cut out quite a bit. Here's the patch, properly cut in and formed to the original shape before welding in...



And here it is after welding in, grinding back, undersealing and stone chipping...



For the bottom of your doors, get a wire brush in an angle grinder and be brave, it is ALWAYS worse than it looks on the surface! The only way to prevent it coming back 100% is to cut all the cancer out and weld in fresh!
 

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Very nice. Thanks for the advise. Do you happen to remember the thickness of the sheet metal you put in? It would seem to me that the sill and jack area would be thicker than say a door panel or fender.
 

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Discussion Starter #11
Very nice. Thanks for the advise. Do you happen to remember the thickness of the sheet metal you put in? It would seem to me that the sill and jack area would be thicker than say a door panel or fender.
The patch you see above was 18 gauge. The outer sill is only a cover, behind is a much thicker braced section which acts as the load bearing point, which is the case for all four points. If you are lucky, as I was, the inner seal was rust free with all the E coat still intact.
 
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I'm going in to do this this weekend. Anything different from the steps posted in your original post?

Doing subframe bushings, diff mouns, all 3 diff seals, and the Dinan subframe diff mount reinforcement plate (this one being the reason I'm dropping it, really).
 

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Ill be doing the same as obioban except im not using the dinan kit for my subframe reinforcement and ill be rebuilding the diff.
 
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the cables go into the aluminum hub where the wheel bearing resides. there's many ways to skin a cat but I didn't disconnect anything inside the cabin - just removed the wheel and caliper (which you have to do anyways), knocked off the rotor, and disconnected the cable from the "expanding lock" mechanism that operates the e-brake shoes. It then slides out the back, and the cables hang loose under the car while you drop the subframe.
 

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I want a lift :crying2:

That will make getting to that front upper arm a lot easier.
 
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