All of us owners of Little British Cars have had, or will have, a problem with rubber. I’m not speaking of tires, though. I’m speaking of all the other rubber in your car; the strips under your windshield, radiator hoses, belts, suspension components, brake fluid seals, valve cover seals, and many others. This article is not an engineer’s design guide, but a brief glimpse into the factors that affect rubber products used in our old cars.

Age
Age is probably the chief culprit in the failure of rubber parts for us LBC owners. In a non-critical application such as window seals, who cares? The biggest problem is getting a wet lap in a downpour (hopefully the passenger’s lap). Suppose that you come across that “barn find” MGA that somehow starts up after 15 years in storage. Are you going to replace critical rubber parts in the brake system before you drive it? Hmmmm? When were the brake hoses and the wheel cylinder seals replaced last? Were they ever replaced? How long did that NOS (new old stock) brake hose sit on the shelf before you bought it?
Automotive rubber parts may start to degrade from age in as little as 5 years (from their cure date, not their installation in your car). You may not be able to see the changes, but parts can start to become brittle or otherwise lose their original material properties. Most automotive rubber parts will start a rapid decline at about 10 years. Again, it’s not a big deal if the vent window seal cracks, but what about those crispy hoses from your fuel pump to your carbs? Have you been smelling gas lately? What about those pretty stainless braided hoses? They have rubber underneath the braiding.

Rubber Compounds/Fluid Compatibility
This subject can be very complex, so I’ll restrict it to fluids usually used in our LBC’s. When a rubber compound is exposed to a fluid that is incompatible, it may degrade rapidly. It may harden and get brittle, it may swell up and get soft, or it may even dissolve. If you’re fixing your brake system and need an O-ring, don’t go to the hardware store! Just because it’s the same color as the one that you removed, it’s not necessarily the same! Order the correct part number!
For those of you who must replace rubber parts that are no longer available, see the rudimentary fluid compatibility chart below. Please don’t use this information to replace critical parts. If you do, you’re a “test pilot”. When selecting a replacement rubber part, note that rubber compounds are available in different hardnesses, usually on a “durometer” or “shore A” scale. Industrial suppliers such as McMaster-Carr Supply are a good source for rubber because they actually tell you what kind of rubber you’re buying (as opposed to your local hardware or auto parts store).

Mechanical Rubber Seals
Mechanical rubber seals are usually categorized as “static” or “dynamic”. Dynamic seals, such as O-rings, are those that are trapped into a cavity while another part slides across the seal. Dynamic seals are usually engineered precisely and are intolerant of incompatible fluids, poor metal surface finishes, and incorrect squeeze (the amount that the seal is compressed when installed). An example of a dynamic seal is the O-ring for a brake caliper piston. Static seals are usually trapped between two parts ( i.e. a valve cover gasket) and cannot move. Static seals are usually more forgiving and cause less problems.

Temperature
The rubber compounds listed in the chart will tolerate the temperature range of –40oF. to +225oF. Generally, rubber will become very stiff and possibly brittle at the lower end of the range and soften considerably at the high end.

Pressure
For rubber seals such as brake system O-rings that will be subjected to high pressure, use only correct replacement parts from a reputable supplier. The rubber parts are frequently manufactured to close tolerances and an incorrect replacement can fail under pressure.

Fluid Compatibility Chart

Fluid	Compatible Rubber Compound
Auto Brake Fluid	Ethylene Propylene (EPR or EPDM)
Gasoline (with or without Ethanol)	Buna-N (Nitrile)
Antifreeze (Ethylene Glycol)	Buna-N (Nitrile) Ethylene Propylene (EPR or EPDM) Neoprene Butyl
Automotive Oil, Petroleum Base)	Buna-N (Nitrile)
Grease, Petroleum Base	Buna-N (Nitrile)

~~ Dave Evans