What material O-ring to use?

Bonnewagon

Geezer
Sep 18, 2009
9,019
113
Queens, NY
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.
 
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RanMan

Not-quite-so-new-guy
Jan 10, 2023
15
3
Annapolis, MD
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.
 
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81cutlass

Comic Book Super Hero
Feb 16, 2009
4,362
113
Western MN
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

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.
 
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Bonnewagon

Geezer
Thread starter
Sep 18, 2009
9,019
113
Queens, NY
Great info! But what about high pressure? The hydraulic rams run around 300-350 psi.
 

81cutlass

Comic Book Super Hero
Feb 16, 2009
4,362
113
Western MN
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.
 
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Bonnewagon

Geezer
Thread starter
Sep 18, 2009
9,019
113
Queens, NY
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.
 
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