Why castor oil provides superior protection



While not perfect due to eventual varnish issues, castor based oil is superior.

HERE IS WHY: (long read but worth it)

For any fluid to act as a lubricant, it must first be "polar" enough to wet the moving surfaces. Next, it must have a high resistance to surface boiling and vaporization at the temperatures encountered. Ideally, the fluid should have "oiliness," which is difficult to measure but generally requires a rather large molecular structure. Even water can be a good lubricant - under the right conditions!

Castor oil meets all these simple requirements in an engine, with only one really severe drawback in that it is thermally "unstable." Thermal instability is not really a drawback but a hidden benefit and here's why:

This unusual "instability" is the thing that lets castor oil lubricate at temperatures well beyond those at which most synthetics will work! Castor oil is roughly 87% triglyceride ricinoleic acid, which is unique because there is a double bond in the 9th position and a hydroxyl in the 11th position. As the temperature goes up, it loses one molecule of water and becomes a "drying" oil. Castor oil has excellent storage stability at room temperatures, but it polymerizes rapidly as the temperature goes up. As it polymerizes, it forms heavier "oils" rich in esters. These esters don't even begin to decompose until the temperature reaches about 650°. Castor oil forms huge molecular structures at elevated temperatures - in other words, as the temperature goes up, the oil exposed to these temperatures responds by becoming an even better lubricant!
Unfortunately, the by-product of this process is what we refer to as "varnish." So, you can't have everything, but you can come close by running a mixture of castor oil with a polyalkylene glycol like UCON. The mix seems to have some synergistic properties, or better properties than either product had alone. As an interesting sidelight, castor oil can be stabilized to a degree by the addition of Vitamin E (Tocopherol) in small quantities, but if you made it too stable, it would no longer offer the unusual high temperature protection that it did before.

Castor oil is not normally soluble in ordinary petroleum oils, but if you polymerize it for several hours at 300°F, the polymerized oil becomes soluble. Hydrogenation achieves somewhat the same effect.

Castor oil has other unique properties. It is highly polar, or has a great affinity for metal surfaces. It has a flash point of only 445°F, but its fire point is about 840°F! This is very unusual, if you consider that polyalkylene glycols flash at about 350-400°F and have a fire point of only 550°F or slightly higher. Most common synthetics that we now use burn in the combustion chamber if you get off too lean. Castor oil does not, because it is busily forming more and more complex polymers as the temperature goes up.
Most synthetics boil on the cylinder at temperatures slightly above their flash point. Synthetics also have another interesting feature - they would like to return to the materials from which they were made. These are usually things like ethylene oxide, complex alcohols, or other less suitable lubricants. This happens very rapidly when a critical temperature is reached. We call this phenomena "unzippering" for obvious reasons. So, you have a choice. Run too lean and the engine gets too hot. The synthetic burns or simply vaporizes. The castor oil decomposes into a soft varnish and a series of ester groups that still have powerful lubricity.

So, what oil do you think is best in an aircooled engine that has a tendency to get too hot? If you are wondering what is killing bushing bearings, its running at full throttle where engine is not able to fully dissipate the heat load causing synthetic oils to vaporize and quit working.
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it was a long read. i even understood some of it. i've always been intrigued but in the dark about castor oil.

tell you what...i bet more people would be willing to give it a shot if they simply had a layman's formula...you know, "use this" something of average cost & quality, readily available, then just tell me how much of what to mix with what?

no disrespect was intended by that, but sometimes you can get way over my head...can you simplify it for me?

"average quality castor-based pre-mix formula for a broken-in happy-time" = ?


I don't mix castor with synthetic and buy blended oils such as Maxima 927 and Klotz Super Techniplate that are 80% synthetic and 20% castor oil based.

If you do mix, than 1 oz of castor added to 4 oz of synthetic oil gives a 5 oz blend that should be added to each gallon. If you go this route, it is VERY important to use a quality castor oil that is degummed and preferably a first press oil.

If it doesn't say degummed, don't use it or you'll get too much varnish build up. The minor varnish that will build up is actually a nice lube that keeps things running better and longer. Get a little varnish build up on your bushing bearing, and it should never wear out!

DON'T USE CASTOR OIL FROM A PHARMACY. It contains sugar to make it easier to swallow and we know what happens when sugar is added to fuel and burned in an engine.


Good reading....Just curious tho...If varnish is a big issue (is it?...what harm can it cause other than possible carb plugging issues?...seems to read that the varnish gives added protection)...Could one simply use the castor oil for a little while (until a slight varnish builds up), then stop using for a bit to let the varnish clear up a bit then go back to using it (if one can keep a light coat of varnish according to how I read it, the engine should be optimally protected?).
Thanks for the info!

PS - Pharmacy is where I though of getting the Castor...good thing you said NOT to get from there....where is the best place to find such a thing I mean other than buying it pre blended in the 2 oils mentioned above (I don;t think those oils are sold here locally)


A bit off topic but interesting...

Rotary engines such as the Bentley and licence built copy by Le Clerget used in 1st world war aircraft used a total loss lubrication system.

The oil of choice was castor oil - one rather unfortunate problem however (other than what happens if you used up the oil at 5000ft) is the fact that Castor oil is a laxative... and as the engine doesnt burn it all... you get the idea..

just a little light relief... lol

Jemma xx


The varnish isn't gonna occur in the carb but in the engine. With 20% castor and 80% synthetic blend...it will probably never be a problem. The issue with varnish is if you use a high % of castor such as 100% castor. It may cause rings to stick, may build up on piston, and may clog up bearing races.

Jim H

Thanks RC, very informative...I had questions about this some time back and now they are answered.


V informative if I can get my hands on some maxima 927 I'll use it next spring.