What material O-ring to use?

[Bonnewagon]

I am trying to restore convertible top cylinders. There is an O-ring at the top and both are leaking. I extricated them but it is impossible to tell what material they are made of. I figured something like VITON or BUNA-N would be applicable. It must seal against ATF and fairly high pressure. Someone suggested the green HNBR stuff they use on AC systems. I have used those for different things but I am not sure they are ATF resistant.

 

[RanMan]

I am trying to restore convertible top cylinders. There is an O-ring at the top and both are leaking. I extricated them but it is impossible to tell what material they are made of. I figured something like VITON or BUNA-N would be applicable. It must seal against ATF and fairly high pressure. Someone suggested the green HNBR stuff they use on AC systems. I have used those for different things but I am not sure they are ATF resistant.

It's hard to go wrong with Viton. Check me on this but I think they resist just about every type of petroleum fluid. They are usually a light brown color IIRC.

 

[Clone TIE Pilot]

There are several different grades and types of Viton with some being better.

https://www.viton.com/en/products/product-selection

 

[81cutlass]

Viton and Buna are basically going to be your main options. There are some more exotic materials but unless you want a custom shot of 20,000 and 6 month leadtime it's probably going to not provide noticeable improvement.

If you REALLY want to get into things, I suggest checking out the good old Parker 5712 catalog

https://www.parker.com/content/dam/Parker-com/Literature/O-Ring-Division-Literature/ORD-5712-Parker-O-Ring-Material-Offering-Guide.pdf

If you don't design hydraulic parts for a living , you just want a simple answer. The simple answer is both are ATF safe. Viton is better for high temp applications, Buna is better for low temp. I would go Buna. Viton is good if you have a hydraulic system that gets very hot and is not expected to operate at low temps. Buna is better for standard operating temps and is seen to seal better at extreme low temps.

If it was under hood where it is exposed to 180-220F+ regularly, I might bias towards Viton or HNBR. At work we usually say Buna is good to ~160F before it will start to degrade at long life (2000-5000hr type thing) Viton is solid up to 200F at long life. HNBR even above that.

But as it's going to be in a car cabin where it will see -20F storage to 120F operating, Buna is right in it's sweet spot. If it stores under pressure or if you operate at -20F (which you won't, it's a vert top) you MIGHT see some weaping with Viton as it doesn't perform at very low temps as well as Buna.

This is a case where the cheap option (Buna) is the best option.

 

[Bonnewagon]

Great info! But what about high pressure? The hydraulic rams run around 300-350 psi.

 

[Built6spdMCSS]

VITON would be my choice.

 

[81cutlass]

Great info! But what about high pressure? The hydraulic rams run around 300-350 psi.

As long as the sealing geometry is correct the material won't have any impact.

Buna and viton are regularly run to 3000-5000 psi in hose fitting o ring applications. Typically 1.5x overpressure tested so 7500psi is a good target.

 

[Bonnewagon]

I just went back and re-read all that pamphlet again. Impressive! I saved that for future reference. The detailed explanations like those are the reason why I chose the RACOR water separating fuel filter made by Parker for my Whaler. As for the O-rings I guess the VITON and BUNA are easily available and not so expensive that I can't try both. I can try one of each type in either cylinder and test them. Being winter I can actually test under freezing conditions. I modified the hinge bolts that secure the hydraulic cylinders for easy replacement. Normally the outer bolt is impossible to get out of the way and makes extraction extremely difficult. It forces you to unbolt the entire male hinge assembly and lower it enough to get the outer bolt clear. By grinding off 1/16" from each now I can remove/replace the entire cylinder in a matter of minutes. The car is stripped so I can test and change parts easily. Funny thing, on the 1st Gen site the guys that figured out these O-rings are indeed serviceable had no idea what type to use. Some stated that ACE hardware grade was fine, one guy sourced from a hydraulic supplier, another used the HNBR AC rings. Leave it to the G-body guys to have the best information, as usual.

 

[Bonnewagon]

Here is an update: I used the BUNA-N O-rings on both cylinders. They worked great!!! I polished the ram pushrods so as not to damage the seals. The top went up and down very nicely. That is until one of the hoses blew out of the cylinder. The threads just ripped right out of the aluminum cylinder. Weak 55 year old metal? Suddenly high cylinder pressure from the new seals? I was careful not to cross-thread anything. Oh well, so I grabbed some cheapola chinesium cylinders off Fleabay just so I could continue working on the top. Later when all the bugs are fixed I will buy some good quality American made rams. But the fix is valid- just replace the O-rings and the cylinders work leak-free again.