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Viton brake caliper O ring

John-G

Well Known Member
Please don't let this thread drift off topic as to why I'm not using an aviation brake fluid. What I'm using is working great for me in the RV-12 (and I'm not advocating others use the same brake fluid I'm using).

I've been using a synthetic Amsoil automatic transmission fluid as brake fluid for four years now because of the much higher flash point than standard aircraft brake fluid. The Amsoil flash point is 467°F ... a few degrees higher than my backup choice of Mobile 1 synthetic ATF (which can always be added if on the road since it is available at most any auto parts store). The higher flash point is way beyond the 250°F or so limit of the caliper piston's Buna N rubber O rings that come from Matco ... so was thinking of changing over to Viton O rings which are rated to around 400°F.

I was on the phone today with Matco ordering some spare parts for the Matco MHE51-B wheel/brake assemblies used on our RV-12's. While on the phone with Matco, asked if they offered Viton O rings for the caliper's piston .... nope, they only sell the Buna N rubber O rings.

So my question is ... have any RV-12 owners switched over to using Viton O rings on the Matco caliper piston? If so, who did you purchase the O rings from and what was the part number?

Happy flying,
 
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John,

The manufacturers don't install viton because it is harder at low temperature, may not seal as well. "Low" means arctic. Look up the specs and decide for yourself.
 
mcmaster carr

You can find viton orings at Mcmaster.com. Same "dash number" as the Matco parts diagrams show, just made out of viton.

I've been using them in my RV-10 brake calipers along with Royco 782 brake fluid (424F flash point).

Lenny

So my question is ... have any RV-12 owners switched over to using Viton O rings on the Matco caliper piston? If so, who did you purchase the O rings from and what was the part number?

Happy flying,
 
Dan – good point about the temperature … I see the softer Viton Fluoroelastomer 70A Durometer O-ring is only rated to 0° F, which on a cold winter day here in Ohio may be pushing it when landing after being at altitude.

Lenny – Thanks for pointing me towards Mc Master-Carr … Being in Florida, I’m guessing you are likely using the Vitron O-ring rated to 0° F.

I see where McMaster-Carr offers a chemical-resistant hard Viton Fluoroelastomer 90A Durometer O-ring rated down to –20° F (same as the softer 70A Buna N rings) … so suppose that would be the better choice for here in Ohio as long as the extra hardness does not create any issues with the caliper pistons. Knowing me, if the temps are much below 20° F, I won’t be getting near the airport except to shovel snow. The specs say these O-rings are suitable for static or dynamic sealing applications.

Is anybody using the harder Viton 90A Durometer O-rings in their RV's calipers? If so, please offer some feedback on your experience.

Edit: Just noticed there is also a brown Vitron Fluorelastomer 75A Durometer O-ring available which is rated -10° to 400° F. Probably the best compromise over the harder 90A O-ring. Would think that a temperature rating to -10° F should offer enough low temperature margin for a flight on a 20° F winter day.
 
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I'm using the 75 durometer orings, being in FL, I'm not concerned about the 0F rating.
The stiffer oring could make the caliper piston bind and not return as easily. I'd stick with the same or at least similar durometer as the original nitrile o rings.
 
Is brake fade the underlining reason for higher flash point fluid?

For me, it is just desiring a much better margin between how hot the brake components can actually get and the brake fluid's flash point.

The brake components can get quite hot after long stop and go taxi scenarios (think very long conga line to depart Oshkosh) or an emergency heavy braking after a landing because a deer just hopped onto the runway or a constantly dragging brake.
 
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I'm using the 75 durometer orings, being in FL, I'm not concerned about the 0F rating.
The stiffer oring could make the caliper piston bind and not return as easily. I'd stick with the same or at least similar durometer as the original nitrile o rings.

Thanks for the feedback Lenny. If I were up in Alaska this would be a non-starter ... but being a fair weather flier here in Ohio and not much of one to leave the house when it is really cold outside, I think the brown 75A Durometer O-rings will work out quite nicely for my purposes and they are rated down to -10? F.
 
I bought an entire kit of 75a O-rings from Amazon 5 years ago, good bargain at $60 or so shipped. Caliper sized ones are in there, as well as many other on my plane. No issues noted for a plane while located in the mid-Atlantic. We shall see about the Sierra's (haven't moved it yet).
 
I am happily using the Brown Viton FKM rings rated -20 to 400F (in FLorida). I bought them at https://www.oringsandmore.com/viton-fkm-brown-orings
As I recall they had better pricing than McMaster Carr.

Pete -Thanks for the link ... reasonably priced O-rings and nice that one does not have to purchase a bag of 10. Even after ordering 4 (so there are two spares on hand) the savings more or less pays the shipping.

Thanks to all that responded ... very informative.

Happy flying,
 
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