August 23, 2010

Fixing a leaking differential...with a straw!


A mechanic will replace parts that are broken, a truly great mechanic will tell you why that part is breaking.

Let me explain and it's a bit of a long story: at Watkins Glen last week, I once again had a problem with oil leaking out of my car's differential. This has been a nagging issue due to the driver's side stub axle "popping" out from the diff case. The stub could be pushed back in by hand but it would inevitably come out again and I was black flagged, I think 4 times on my first day with Group52.


Consensus seemed to point to a weakened or bent "c" clip, a 29 cent part which on my BMW M Coupe sits at the end of the stub axle assembly. General advice was to replace the whole left and right side units. By the afternoon of day one, I was ready to call it a day and drive back to NY having done only one clean session. This is precisely what happened last two times I went to then Glen and you'll understand I was not a happy camper.


But then I asked Spencer Cox for help. Spencer runs Speedsport Tuning which you will find in our "Axis recommends" section, he takes care of CG and Stee's race cars, he's the top as far as Porsche club racing prep. and support. "Tell you what, you take out the stub and I'll see what I can do about about the 'c' clip." Spencer told me so, what the heck, I jack the car and start wrenching.

I'd been told getting the stub axles out would be a big hassle involving removal of the exhaust and swaybar and various hours of labor but, as it turned out, none of that was necessary. The e30 derived trailing arms on the M Coupe droop far enough to allow for a pretty easy job. Once the stub was out some opined it might actually be what was worn and would need replacing.

Spencer looked at the part,looked at the retaining clip and said something else was the problem, that clip is not what's holding the stub in anyway. We had looked at ride height before jacking the car and it's not so low as to cause an issue but now as I pushed the stub back in, Spencer says "why is your axle spring loaded"?

"What, that's not how it's supposed to be?"
"Oh... "
"Pass me those needle nose pliers."


With that Spencer loosens the clamp around the outside CV joint boot, a clearly audible sucking sound is heard and the whole axle moves about a half inch towards the center of the car!
Tuns out a vacuum had formed in that boot strong enough to pull the axle sufficiently far for it to pop out of the diff case. Spencer said this is actually quite common in hard run cars and that what they do with race cars is precisely what he did to mine: he inserted a spray can straw under the boot allowing for pressure to be equalized.


Well, I could not believe it and I had to wait for the next morning to find out but not a single drop of oil came out of that case all day.

Morals of the story: look for why things are braking, don't just replace parts and good mechanics are hard to find, truly great mechanics almost impossible to find so please support and treat these gentlemen well. Their expertise will save your day. Spencer, you're the man!

Another big thanks goes to Sander Lee, one of Speedsport's clients who had brought Mr. Cox to the Glen. Without him both Adil (who had electronic gremlins act up on his GT3) and I would have gone home early. Cheers!



  1. Exactly people like Spencer Cox never cease to amaze me and lets me keep faith in humanity :P.

  2. But can he explain why a vacuum was forming in the first place? Seriously, I'm curious. Interesting story, and lots of respect for anyone who excels at what they do.

  3. I wish I could explain, but I would only be guessing. INstinctively I would have expected the great to pressurize the boot if anything... maybe someone out there knows.


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