For carb o-rings: why Buna-N over Viton?

[Guest]

I've read in a few places here and on I think bwinger's site and Robert Barr's that Buna-N nitrile is the best choice for Carb internal o-rings and it got me to wondering....so I looked up some specs:

In this case a lower number is better so it would seem that Viton it better than buna-N in most cases and equal where it isn't superior. Is there some real world fact that the figures don't show making buna-N really the better choice?

1 = Recommended 2 = Satisfactory 3 = Poor 4 = Marginal 5 = Not Recommended

Compression Set
V 1
BN 2
Wear/Abrasion
V 2
BN 2
Vacuum
V 1
BN 2
Chemicals/SOLVENTS
V 2
BN 4
Peroleum Oil
V 1
BN 1
Gasoline/Fuels
V 2
BN 3
Dynamic Applications
V 1
BN 2

/\/\ac

[bwringer]

This may be all wrong, but Buna-N is normally a wee bit softer than Viton. I forget the exact durometer numbers, but that has a lot to do with it. The o-ring seals in carbs are fairly tight fits, so a harder o-ring would make some of the seals impossible to assemble. And with the small o-rings around the pilot screws, you need a bit of flexibility. A harder o-ring wouldn't seal.

That's my theory, anyway.

The carbs are not subject to higher temps, so you don't really need the temperature resistance of Viton.

[Macmatic]

This may be all wrong, but Buna-N is normally a wee bit softer than Viton. I forget the exact durometer numbers, but that has a lot to do with it. The o-ring seals in carbs are fairly tight fits, so a harder o-ring would make some of the seals impossible to assemble. And with the small o-rings around the pilot screws, you need a bit of flexibility. A harder o-ring wouldn't seal.

That's my theory, anyway.

The carbs are not subject to higher temps, so you don't really need the temperature resistance of Viton.

Yeah, I wasn't really taking temp into consideration, only the other factors where Viton is better... fuel exposure and vacuum most of all.

I see where you're coming from with the hardness but I'm not sure I can agree with you on that. Buna-N is Shore A 70 and Viton is 75 and having just done two sets of these carbs today (with R Barr's great kits) and I have a hard time accepting that a slightly harder compound would have been a problem. Maybe on the air screw....

Ehh. Maybe I'll test them out on the next project bike but that'll have to wait until I get to Atlanta. In the meantime R. Barr has my GS750 starting in about 1/2 a revolution.

/\/\ac

[Ghostgs1]

Where I work we use viton 75 and 90 durometer o-rings almost entirely. 75 in static and 90 in dynamic installations. They have far greater service life. These are based upon recomendations of General Electric and Seimens for steam and gas turbine systems and auxilliaries. There are some fluids that would not allow you to use viton, but for petroleum based lubes they work very well.

[robertbarr]

Re: For carb o-rings: why Buna-N over Viton?

Is there some real world fact that the figures don't show making buna-N really the better choice?

/\ac

Yep... Cost.

Viton is a very tough material, and it's ideal for some applications, but when it's not needed, it's a waste.

The carbs are insulated (thermally) from the head by the intake boots, to keep the temperatures under control. In terms of efficiency, then, the ideal material for these particular O-rings is Nitrile.

Keeping in mind that on many bikes, the originals are still in service after 25+ years of exposure to fuel & heat/cooling cycles, and I'd say the Buna is a pretty robust & remarkable material!

It's a common material simply because it gets the job done at a fair cost. For the kits I assemble, the cost for Viton would be almost insane, because I'd need to have some of the sizes custom-molded for me. $$$! You could move the decimal point, at least.

[jimcor]

Hijack:

Robert, your dog is a ham!