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Rear Subframe Woes...


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So my I had a field day this weekend.  It all started with the *simple* desire to replace the bushings with poly units, and found that both spring perches on the trailing arms/control arms were rusted through!  And on top of that, there's a crud ton of rust chunks that poured out of the subframe/rear axle carrier itself.  Oh and did I mention - both output shaft seals on the diff and the diff cover gasket are leaking like they don't exist.  

 

So - here are my questions.

 

What to do about the subframe/axle carrier?  The outside looks okay - minimal surface rust, but I'm worried about the inside.  New one runs in excess of $900.

 

What about the diff?  Rebuild it myself (never done it before), send it off to get fixed, or buy a new unit - maybe an LSD?

 

 

I just bought some NOS trailing/control arms - so those are taken care of.

 

 

Thoughts?

 

Ben K.

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replace the subframe.  get a decent rear subframe from someone parting out a car.

 

diff does not need a rebuild.  the output seals and cover leak are easy to fix.....and cheap.

 

but if you want an lsd, this it the time to find one and put it it. of course, a used one might need new seals too.

 

on the NOS trailing arms...are they complete with stub axles and hubs?  if they are just bare arms, you will have to carefully measure for what size spacer shim is needed for each one.  this is important for correct tq on the bearings.  it is actually better to find a complete TA in nice condition than a bare NOS arm.

Edited by mlytle
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Where are you?

 

Parts some places are easy (around here)  and others (Ohio) are really hard.

 

I hate to differ with Marshall, but I have found that 95% of the time, hubs and stubs WILL move

between trailing arms just fine.  That said, I always check, but BMW got their manufacturing tolerances

pretty darned close by 1970.  Now, they were using 1950's methods, so they kept the spacers,

but IF you keep the spacers together with their shims (as you would) it almost always works.

 

t

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I would remove the subframe, have it bead blasted so you can inspect it properly.

If the rust has eaten too much metal it would be prudent to find a non- eaten

donor from our group here by placing a FREE ad in the WANTED/FOR SALE

section. Safe is better than sorry after all that work. Besides - you don't know

the condition of the current crusty subfram as related to 4-wheel alignment.

A tweeked subframe can cause  poor steering/handling/poor general

feeling driving this simple car. A lot of time, owners, miles, pot hole strikes

in those components.

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I talked to Korman this morning - they will more than likely install the stub axles for me and take care of the shims.  

 

As far as the subframe goes - a new one (genuine BMW) might no longer be an option.  I called around today and BMW in Germany is expecting new ones around 20 June, but was told not to hold my breath on that one.  The dealer said he more than likely can't even put an order in for one without it getting kicked back by BMW in Germany.  I will call BMW Germany tomorrow morning/late tonight and see what they say.

 

The subframe is already pulled and cleaned off on the exterior - bead blasting the crud out of it is an option, however I wonder how good of a job it'll do towards the center of the square tubing.  In addition, what about the boxed off section in the back with only the tiny hole.

 

So I thought about getting a competent metal shop to jig the subframe, then cut it in thirds, clean it up that way and inspect everything - if it too far gone, no loss per say.  Then place back in jig and reweld.  I'm not 100% sure if the jig can keep the subframe in alignment after the reweld.

 

I've also thought about getting just a few sections taken out of the subframe to do the inspection (towards the center).  That should prove worth while and I can probably do myself.  This also won't requite a jig.  Alignment should still be fine if care is taken with rewelding - a small section *shouldn't* cause any warping.

 

Last option would be send it off to a shop for a complete overhaul and convert it to house a different diff altogether, opening up options for LSDs....

 

Thanks for the reply's thus far.  :)

 

Ben K.

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Oh my!!  When I first saw the pile of rust sitting in front of the crossmember - I thought yeah - mine's that bad.  But then I saw where the rust had completely eaten through it.  WOW!

 

On a side note, 2002haus is powdercoating my one for a very good price and it will more than likely be shipped over to Korman - I'll be calling Carl tomorrow to confirm this and also to schedule the stud axle swap on the trailing/control arms.

 

Ben K.

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