GrepAwk

Sorry for the cross post, but just didn't see any answers after several days....

ABC warning finally showed up at 68kmiles on an '03 S55 AMG. This is the hydraulic suspension leveling system. Pentosin CHF11S is the synthetic hydraulic fluid in the ABC and Power steering systems.

Time to clean out valve blocks and replace O-Rings. I have read most of the threads looking for info about the type of O-Ring materials required. Normal rubber BUNA rings dissolve.

Can somebody give the definitive " here is what o-ring material to buy that is required to work with Pentosin CHF11s and meets Mercedes requirements...won't corrode/break/crumble/disappear..."

I see Nutrile, buns, viton, Teflon .... Which ones do I buy?


Read more: https://mbworld.org/forums/s55-amg-s...#ixzz32GJGLiJa

George993

Old thread, but Internet still lacks a clear answer to this question. So let's look at 3 popular o-ring types which can potentially work with Pentosin CHF11S

1) BUNA-N = Nitrile = NBR

2) VITON = FKM = Fluoropolymer

3) PTFFE = Teflon

If those are not "equal", please correct. I have seen various posts that Nitrile (which I also used), Viton and PTFE work. I also saw contradicting posts that Buna dissolves.

Can anyone post links to actual manufacturer recommendations, lab results, multi year experience results etc which confirm or deny which of those 3 are acceptable for a use in use cases like piston seals (o-rings, U-shaped seals) in car suspension and convertible tops?

A second question is how to rate them from the best to the worst for the above use cases?

Thank you!!

George993

Klaus from TopHydraulics (whom I consider an expert if not THE expert on the topic) lists on his website that he uses Viton, Teflon Hybrids and Enhanced Polyurethane. No Nitrile. Obviously he has spent years perfecting the seals so may have proprietary formulas and different materials optimized for different seal shapes even used on a single piston etc and for obvious business reasons he is not revealing the details. So my questions apply to the off-the-shelf materials.

MikeJ65

Pentosin CHF11S is 1-decene, dimer, hydrotreated. The broader classification would be an alkene. Parker rates nitrile as a '2' for acceptability meaning 'Fair, usually OK for static seal'. They rate Viton as a '1' 'Satisfactory'. Their o-ring compound recommendation for alkenes is 75 durometer Viton. Backup rings should be PTFE.

George993

Thanks @MikeJ65. This is exactly the kind of information I'm looking for. So is Parker saying that PTFE is stronger in holding pressure but is less flexible/has less sealing capability so there should be two rings - Viton on the high pressure sealing side and a backing/support PTFE ring behind it on the low pressure side? But I don't see backup rings in Mercedes hydraulics (unless I misunderstand something). There are o-rings and cup seals. Cup seal is on the high pressure side, and a separate NOT directly backing it o-ring on the low/no pressure side. Which one, if any, should be PTFE?

Also is "satisfactory" Viton BETTER than "fair" nitrile? That's what I assume.

George993

If the above is true, the guys selling nitrile o-rings for Mercedes top etc rebuilds are causing double trouble: o-ring is a bad replacement for a cup seal and nitrile is an inferior substitute to Viton.

MikeJ65

George,

The OP was about ABC, not top hydraulics, and that was what I was referring to . I believe that the early valves used nitrile o-rings and PTFE backing rings. Later valves saved a few pennies by replacing the PTFE backers with nitrile backing rings. The issues normally seen are deterioration of the nitrile backup rings where used and compression set in the o-rings. The PTFE backup rings are almost always in pristine condition.

With regards to the top hydraulics, I don't think that anyone is suggesting that the o-ring fix for the front latch cylinder is the optimal solution. However, it is a much easier repair than pulling out the cylinder and sending it in or removing the clevis end to replace the cup seal in place. If you keep an eye out for leaks, you can get several years out of the o-ring repair and then pop another one in down the road if necessary. Viton would be a safer choice, not knowing exactly what the system will be refilled with. Parker recommends nitrile for petroleum based hydraulic fluids (Viton is just as good, nitrile is less expensive) and Viton for synthetic based hydraulic fluids.

Urethane is fine for u-cups on the top cylinders. Oil resistance, wear resistance, and low coefficient of friction all make it good for that application. Urethane is also easy to cast in rod form and machine into custom seals, which is useful for repairs where proper size seals might not be available from stock.

As far as the Parker ratings, 1 is the highest and should probably be described as 'excellent' as 'satisfactory' seems like a less than enthusiastic recommendation.

George993

Thank you, now it's clear. I still think buying replacement cup seals is a better solution than o-rings and disassembling the pistons is not that much additional work (I put them in a vice between two thick copper plates and unscrew). The front cylinder in R230 does not even need to be removed from the car for that, just the hydraulic lines need to be loosen up to pull the cylinder out of the window still on the line for easy access to remove the piston. And that should be a "lifetime" fix with proper seals.

But if putting an o-ring is easier for some people to get a few years out of it, that's fine too. It just sounds like the o-ring will last longer in Viton than Nitrile. Thanks.