O Ring compatabilty

[Barry]

Many of us may have purchased generic O ring kits at very reasonable prices. I have a metric kit and Imperial kit with around 750 O rings in total which seems to meet most requirements. Although it is possible to buy kits in other rubber types all the common ones seem to be NBR (Nitrile Butadiene Rubber).

I did some research which suggests NBR is acceptable for use with Oil, grease and petrol with a rider that Viton is superior for petrol. The Viton kits if you can find them cost 3 or 4 times as much as NBR.   As far as I can make out even fuel hose is made of NBR which suggest it should be OK to use NBR O rings in the carbs for example.  

Anybody else have experience with using NBR O rings or do we have a materials expert who can comment on the differences between NBR and Viton or know what material the various OEM O rings are made of.
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[Bob_Roller]

About the only problem you may encounter, is with fuel .

Not so much that it will leak, as the o-ring will swell and be unusable after you disassemble the component .

Working on Boeing 737's, there are only 4 fluids on the aircraft, synthetic turbine engine oil, synthetic hydraulic fluid, jet fuel (kerosene), dino oil based hydraulic fluid for landing gear struts .

Only jet fuel seems to be the problematic fluid, if the incorrect compound rubber o-ring is used, it won't last for more than a week or two before it starts to leak .

All of the fuel system components have viton rubber seals .

[Barry]

No Question Viton is the superior material and aircraft deserve nothing less.

NBR is rated to work with gasoline though. I've been using some in the carbs over the last 3 or 4 years without problems and I didn't do that without checking the compatibility first in what seems to be an industry reference "Parkers O ring handbook" http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf
The difference seems to be the amount by which the rubber swells in contact with gasoline. And they all swell to some extent. It was interesting to see that our old enemy ethanol when added to gasoline causes progressively more swelling as 5%  10% and 15% ethanol is add to straight unleaded. NBR O rings used in a carb are not likely to cause a leak if they swell a bit but as Bob says they may not be reusable next time the carb is stripped down. No big deal when you have 20 of them in the kit. Anyway oem O rings don't last for ever or we wouldn't have to replace them would we.  I'd still like to know what type of rubber they are though.
 
The other thing I learnt was the sizes available in these O ring kits seem to follow a standard. There is a large range of diameters available but only a limited number of sizes in cross section which is often the most critical dimension when the O ring has to be a good fit in a groove.

The metric series O rings typically come in 2.0mm 2.5mm 3mm and 4mm cross section.

The Imperial kits (with dimensions in mm for comparison) have 1.78mm 2.62mm 3.53mm and 5.33mm cross section O rings.

Between the 2 it is often possible to find a substitute size for an oem O ring.

[Dizerens5]

Bob you are an aircraft specialist maybe you can explain something that has puzzled me for years: aircraft landing gear always seems to include a scissor-shaped linkage, facing backwards, connecting the sprung and unsprung components. What is its function? It doesn't seem to add anything at first sight!

[Bob_Roller]

The 'torque links' as they are called in the industry, allow the inner part of the landing gear to extend and compress under weight like a shock absorber.

It also allows the nose gear to be moved for steering .

If you disconnect the links, you can rotate the part of the gear with the tires attached in a full circle, the links are the only thing that keeps the tires pointed in the correct direction .

There's nothing inside of the landing gear except for a restrictor tube and a large needle .

In theory, it's no different than the front fork tubes on a motorcycle .