Oil In Strut

[Bruce]

I'm replacing the front shocks with Bilstein HD shocks. There was a lot of oil in the strut and the manual says to add oil when replacing the shocks. I wasn't going to, but since I don't know the purpose of the oil I thought I had better ask before the fact. Any insight appreciated. Thanks.

[Techboy]

The oil is used with a hydraulic (oil filled) strut to facilitate cooling.

[1st 5er]

And I always thought it was a noise eliminating cushion.

[Bruce]

I emptied the oil and installed the shock and torqued the nut to 90 ft/lbs (the manual stated 87 - 110 ft/lbs and it helped to have the Bilstein gland nut tool for this). I was a little concerned that the collar nut only screwed in enough to cover the threads but didn't seat in completely. I assume that the shock bottomed out inside the strut tube. Thanks again.

[Peter Florance]

You were smart to empty the oil.
The oil should never be used for Bilsteins as they are upside down and can suck oil into the cavity at the bottom, locking the shock from hydrostatic force.

[1st 5er]

The oil should never be used for Bilsteins as they are upside down and can suck oil into the cavity at the bottom, locking the shock from hydrostatic force.

Wow, it's true, you can learn something new everyday! I just did, and early too. It's gonna be a great day.

[MikeB7]

Peter, what is the source of that tidbit? Documented cases or someone's theory? I must say I've never heard that one before, and the notion that the bottom end of a Bilstein would effectively be open to atmosphere at some point (in order to suck up fluid) doesn't seem right.

I can't say I've ever taken apart a set of struts and didn't find either oil or antifreeze. I'd been given two reasons for it - one was the aforementioned heat conduction and the other was corrosion prevention.

The latter always made sense, considering how tight the tolerances are - even a hint of corrosion in there and you'd never get that strut out. I was never entirely convinced on the heat thing though. Sure it helps, but given the tight metal-metal tolerances to start with, hard to imagine it being critical. I remember the tail back in the day of the Porsche on the autobahn having a shock/strut lock up solid due to overheating, but for us average Joe US drivers, that sort of scenario didn't seem like it was gonna happen. But still the manual said to do it and the thought of getting a rusty strut out of a rusty tube was frightening, so I've always done it, too.

[Peter Florance]

local shop owner first told me; he had to re-do a car to get the oil out.

The oil is really for cooling; it won't really help the bilsteins because the shock is in the top. I think a light spray or coating of oil is fine (on the sides) but not on the bottom.

I've gotten water in my shock tube when the locking ring seal failed. When I removed the shock and pushed down on it, water sprayed out sideways from the bottom.

[MikeB7]

True about the heat - look at a set of used bilsteins and you can see exactly where the heat builds up, and it's not down on the shock body.