Steering pipe to rack O-ring size

[Marc_986]

Has anyone figured out the size of the O-rings on the connection of the hydraulic lines into the rack?

Especially the return line has a single simple O-ring. This should cost cents, not $20. So I was hoping someone already looked at alternatives

[Gilles]

Mark, I just bought the samme two small o-rings for my Volvo V70 wagon and they charged me $18.70 each...!

BTW, these are metric (?) viton o-rings, but I didn't wanted to disasemble the car and then having to go searching for the right size..

[Marc_986]

Yeah, as I have a bit more time, I might as well give it a try. I studied O-ring sealing a bit today and that was quite informative. Seems the best way is to measure the groove (diameter and width) and bore diameter and calculate the right O-ring from there.

Tomorrow I will need to remeasure the steering rack connection diameter, as today, in the dark, I didn't get reasonable numbers

In my other thread I had shown a few available adapters. One of them with double O-ring. From what I have learned now, that is actually worse than the OEM setup which has a backup / support ring and the actual sealing O-ring. The function of the backup ring is to prevent the O-ring from deforming under pressure, being pushed into the space between steering rack and adapter.

Part of the selection is also on material used. Basically either Viton (as OEM) or NBR. In my application I see no reason that NBR is insufficient. Real strength of Viton seem at high temperature, but with the electric pump I should never get there (doesn't flow that much as the engine drive pump does at high RPM and no longer in the hot engine compartment).

To be continued.

[Marc_986]

Getting back to this, I am quite sure that Porsche used:
12.5 x 1.5mm for the pressure side
13.5 x 1.5mm for the return side
Once you know the size of the O-ring groove and the steering rack hole you can use one of the online O-ring calculators and try different available O-rings. It will give you whether this O-ring would be ok for those dimensions, very convenient.

Based on that, I ended up using a 12.1x1.6mm ring for the pressure side. Should seal a bit better than the Porsche size (but I based it on the final groove dimension on my adapters). In the picture you can see the original O-rings in blue and the ones I bought in green text. Always hard to judge on a picture due to shadow, lens distortion etc, but when I put them on top of eachother, they seem to match really well.

Material wise there is roughly the choice of:
- Viton
- NBR 70 hardness
- NBR 90 hardness

Main benefit of Viton would be the temperature range. It should also have a greater lifetime, but as the rack has a static seal, not a dynamic one, I don't see that as a major selling point. On NBR, would be good to get the higher rating as it is less likely to be squeezed between rack and adapter under high pressure. I have chosen NBR90 as I don't need the temperature range.

Finally, the pressure side also has a Teflon backup ring / support ring. This ring will not seal, but is intended to keep the O-ring in place (not let it be pushed between steering rack and adapter). I didn't find this ring and re-used my old one.