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Jul 29, 2004, 09:59 PM
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W'rkncacnter's Avatar

Castor or synthetic oil?

I have several older engines, OS Max .35, OS Max .10, K&B .61, K&B .40, Super Tiger .20, and Thunder Tiger .15, and I read that the .61 at least needs castor oil to run properly. After looking around the forums for a while and I am left wondering why some older engines Need castor oil in their fuel.

Where exactly can I find fuel with castor oil in it any ways?
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Jul 30, 2004, 03:30 AM
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Tony Oliver's Avatar
I can't help with the supply of castor based fuel in the US, but there is a reason for older engines using castor.

When the older engines were run there were few, if any, synthetic oils around which were suitable for mixing with the other ingredients. As a result, almost all were castor based. It mixed well, didn't break down under extreme heat and pressure, and lubricated well. One by-product was the gunk which came out of the exhaust and also coated the inside surfaces with the shellac type of lacquer. Later use of a synthetic oil seemed to cause problems - my OS30 is a good example of this. I could run a couple of tanks of synthetic oil fuel through it and then it would overheat and lose power. The only recovery was to revert to castor. Improvement was immediate.

I don't know the reason for this but that was the case. Certainly there was much discussion about it at the time (1970s) and advice seemed to be if you use a new engine, it didn't matter whether you had castor or synthetic. An older engine should be run on whatever you started with. The odd thing was that castor could be used after synthetic with no apparent ill effects but not vice-versa.

The amounts of oil depended on the use and type of engine - ball raced engines needed less apparently but I played safe and used 20%. Castrol MSSR was the one I used most (still got some). Instead of the brown/black deposits coming out of the exhaust, it was clear. It still coated the model and was slightly easier to remove but it smelled awful!

Jul 30, 2004, 11:56 AM
Pro Bro #1442
W'rkncacnter's Avatar
Thats good news if I read it right, all but the OS .35 haven't even been started. So as long as I break them in with synthetic they run fine with it?
Jul 30, 2004, 12:05 PM
It flies? I like it!
Morgan Fuel's Omega, the pink stuff available at a lot of hobby shops, is a synthetic/castor blend. Castor is more tolerant of high temperatures than methanol soluble synthetics so a lean run on castor may be more survivable than one on straight synthetic. Castor does form varnish and is "slimier" but it also helps protect from corrosion. I never had rusted bearings until I attempted a switch to straight synthetic years ago. I have since gone back to Omega. After-run oiling is a good idea regardless of what you use and Rust-Lick or Marvel Air Tool oil are good choices.
Jul 30, 2004, 12:20 PM
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Tony Oliver's Avatar
The problem with fuels is that everyone seems to have a favourite. I rarely use synthetic oil based fuels (which is why I still have some) and run my OS and Super Tigre engines on castor fuel. However Lomcevak's suggestion sounds a good compromise as it has both types.

Castor fuels are readily available in the UK but I don't know what it's like in the US.

This thread is likely to get some strong opinions from users of both (and now the combined?) oils.

The early pioneers chose castor as it stayed oily and passed through the engine unchanged. They seem to have got it right - as usual - and getting a synthetic modern replacement hasn't exactly been obvious. That may be because of the relative cheapness and availability of castor.

Anyone got any opinions or facts to support or knock down this?

Jul 30, 2004, 09:17 PM
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downunder's Avatar
All engines with a cast iron piston definitely need castor if you want a long life (I'll add too that any bushed engine needs castor). The amount of castor can vary between engines though. A Fox needs around 29% but most others are happy with 25%. Some time ago I was test running an old OS Max II .29 and happened to be monitoring the head temp. It shot up to 190C so I knew something was wrong. Brain fade, I was using 20% all castor so I swapped to 25% all castor and the head temp dropped to about 135C. The brown colour you see on a piston that's been run with castor is where it's filled any pores and is actually a lubricant, not carbon.

With any engine that doesn't use an iron piston I'd be quite happy with a blend of castor/synthetic but at around 50/50. The only synthetic I'd ever use by itself is one of the jet oils that mix with methanol (Mobil Jet Oil 2, BP 2380 etc) although I'd certainly try tonyo's MSSR, which I believe is a synthetic gear oil, and a Castrol tech rep said it's even better than Castrol M castor for our engines.

For a good read on castor by a lubricant scientist (as against urban myths ) have a look at
Aug 01, 2004, 09:41 PM
Registered User

Sig still carries all-castor fuel, and they also stock both pints and quarts of pure castor oil that you can add to your existing fuel just to be safe.

All my older engines run cooler on all castor.

Aug 01, 2004, 11:09 PM
Cognitive Liberty
mwkid19's Avatar
Castor Oil
Aug 06, 2004, 06:27 PM
Registered User
Any older engine that uses a steel or iron piston running in a steel cylinder in our sizes needs a large percentage of castor. Castor has a higher film strength and will still hold up during lean runs where any of the synthetics burn or flash off. I had a nice experience with my first, and only, gallon of synthetic fuel. Two different engines, different manufacturers, both older baffled engines, got so hot that they kept running even with the needles out of the spray bar. One survived, slightly reduced compression, the other was ruined. Since I still use iron pistoned engines in R/C, CL and FF, I always add at least 4 ounces of castor to my fuel, and never use straight synthetic on anything even my ABC engines. Most fuels are a bit low on oil anyway. Castor may gum up, but it still provides protection to interior surfaces and bearings. Had a ball-bearing engine lost in an Oak tree one fall for several weeks. Oak trees leach out some very acidic juices. When I got the engine back, it was full of a very black sticky liquid. Some of the metal parts on the plane were pretty etched from the acid. The inside of the engine was still shiny with no damage. Used the engine for nearly another 1000 flights..
Aug 17, 2004, 12:02 PM
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alangorham's Avatar
Can all of you who have used synthetic oil, but are strongly recommending castor just clear one point up for me please?

Have all your bad experiences with synthetic oils come in engines that have ALREADY been run on castor?

I think that it is the varnish that has been left behind by the castor that causes the problem. I have run several engines on synthetic from their very first run (and I am not just talking about ultra-modern ABC, ballraced engines) without ill-effect or a perceptible shortening of engine life.

I agree that modern synthetics are much better that those available 25-30 years ago and I would also add that many people seem to think that moving to a synthetic oil is synonymous with a lowering of oil content. It's not!

/flame shield on/
Aug 18, 2004, 04:25 AM

Castor and engines

It is true that most old technology engines run better with castor based lube packages. It is not true that an engine broken in on castor ALWAYS requires castor. I have been flying for 40 years and have several engines that have gone from castor to synthetic lubes with no problems. The key is using a fuel with the proper amounts of lube.

Any ABX engine will run just fine on synthetic lubes as long as they are the right viscosity and amount in the fuel. Some fuel makers have tried to short change the oil package and that is where problems occur. I tested some very popular fuels 10-15 years ago and fond some as low as 12% total oil. There are NO commercial fuels that use oils capable of properly protecting your engine with oil volumes that low.

My favorite all time fuel was Excalibur. I used their 20/20 in my YS for sport flying, their 5% in my two strokes and 40/20 for warbird racing. All of these were 100% synthetic lubes. I also used Powermaster for many years with no wear or overheating problems.

Paul McIntosh

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