Great article about Two Stroke Oils and Premixes

About Two Stroke Oils and Premixes


 One of the best “general” information articles about current two stroke oils is at the link below.  While this article was originally authored with a specific view towards two-stroke personal watercraft oils, we consider the info to be very current and well written … it is “highly recommended reading” for any vintage two-stroke owner.

For those not wanting to muddle through the entire text, we consider the two excerpts below to be far and away the most important with respect to vintage two strokes.

Castor Based Oils

Mr. Robert Verret wrote :  I mentioned a third category of base oils earlier, vegetable or Castor (not Castrol, that’s a manufacturer) bean oil. This oil is derived from pressing oil out of castor beans and distilling it. ‘Bean Oil’ as it is often referred to, has some very unique characteristics; some very good, others not so good. The good is that it is an excellent lubricant. It seeks out hot spots in the engine and clings to those hot surfaces much better than petroleum type oils. The bad is that it does not mix with gasoline easily and it burns ‘dirty’ (excessive carbon/varnish deposits). In the early 70s, before power valves were used, castor bean oil was very popular in racing 2-strokes. Now that power valves are common and we have improved petroleum and synthetic oils, castor bean oil is seldom used anymore. Several companies still market it in the form of a degummed castor oil for racing applications only.  It should be avoided for recreational use unless you enjoy tearing your engine down for a top end cleanup fairly often. Several manufacturers formulate their oil with castor bean oil as an additive (antiwear agent) rather than base oil. They blend it with their petroleum and synthetic base oils. When castor bean oil burns, it has an unmistakable ‘sweet’ smell. (end of excerpt)

Like the author of the above text (Mr. Robert Verret) we too have been involved with two-strokes for 30+ years, and we have also spoken with many well educated and well informed folks in the lubrication business.  All the experienced and reputable folks we have ever spoken to agreed on the basic qualities of Castor oils.  That is, castor bean based oils are not particularly clean burning, do not mix homogonously with virtually any fuel, and are very expensive to manufacture …. But they offer a film strength and lubrication quality that is not matched by any other oil…. Period.

Like Mr. Verret, we also understand that many folks get very emotional about the particular oil brand they use.  For us, choosing a 2-stroke oil is all about the science and results … emotion doesn’t matter.  For air cooled two-strokes being run at high rpms and high loads, there is no better choice than a castor based oil.  During our stint of running the DG Performance race team from 1975-1979, we ran ONLY 20:1 Castrol “R” in every Team DG machine raced out of our shop.  The sheer film strength of the bean oil allowed us to run tighter than normal piston clearances, and thereby netting better performance and long term piston life (as a result of less piston “rattling” in the bore).

Castrol “R” was certainly a bit dirty, but it was the best stuff of the day.   Of the castor bean oils currently available, we prefer Maxima 927.  While it is not “perfectly clean” we have found it to run cleaner than any other bean oil we have used (and we have used plenty).  In addition, the film strength of “927” is every bit as good as the Castrol “R” of the 1970’s, with a lot less mess.

In 2010, we built and road raced two 1970 Kawasaki 350 Bighorn Production Class enduros in the western AHRMA road race events.  We ran these machines on a 20:1 mix of “927” and 105 octane race gas.  While stock Bighorns redline at 6000rpm, ours turned 8500rpm and ran a bit over 100mph.  At the Willow Springs track in Southern California, we lapped at 1:52 (an average speed of 80mph).    At Willow, our Bighorns were constantly at peak rpm in 4th & 5th gear for every moment of each 15 minute practice session and race.  Our race weekend entailed 4 outings for each of 2 days … a total of 120 operating minutes at peak rpm in high gear (per machine) … with not one mechanical issue at all.  We respectfully submit that there isn’t any way to subject a vintage two stroke to any more intense abuse than this, and we feel that the lubrication abilities of the Maxima “927” were a fundamental contributor to our weekends of trouble free racing.

After our race season was done, we fitted our Bighorns with street-lighting, and ran the exact same machines on the 100 mile Hansen Dam classic motorcycle ride north of Los Angeles.  At this event, we ran the Bighorns on a 40:1 mix of “927” and 91 octane pump gas (most of the ride was done at 3000-5000rpm).  The bikes never put out any visible smoke (except at long stop lights) and never came close to fouling a spark plug.

Despite all this very positive experience with a Castor based oil, we openly acknowledge that Castor oils “ARE NOT” necessarily the best choice for all vintage two-stroke applications.  Robert Verret’s excerpt below explains that.

API – TC Oils

Mr. Robert Verret wrote:  The API-TC standard was developed for Air-cooled, high rpm, high output 2-stroke engines operating under severe load conditions. Although this standard is no longer reviewed and updated since 1993 b the API, it still is in effect today.  This standard most accurately addresses the condition Rotax and 2-stroke racing motorcycles and snowmobiles operate under. Almost all these oils are formulated with synthetic or synthetic blend base oils and all use a low ash type detergent. If you walk into a store that handles a variety of 2-stroke oils, it is relatively easy to find TCW3 certified oil. The manufacturer proudly displays that certification on each container. API-TC certified oils, on the other hand, are very difficult to find. There are two main reasons for this. First, many small API-TC oil manufacturers can’t or won’t spend the money (about $75,000) for the testing and certification process. Second, most engines requiring API-TC oils are for racing applications (Motocrossers and Crosscountry motorcycles) and don’t offer warranties with their engines anyway. The owner/operators of these machines know what oils work and don’t work. They do not need the  API-TC certification on the bottle to help them decide what oil to use. (end of excerpt)

The truth is that API-TC oils are also top level lubricants that have excellent film strength, and are much easier to “live with” than Castor oils.  Besides being somewhat cleaner than bean oils, the API-TC oils will also mix “homogonously” with gasoline, and have little or no tendency to separate.  For vintage machines that spend very little time held at peak rpm (in the higher gears), API-TC oils are the better choice.  Among these oils, our favorites are Yamalube R, Kawasaki K2, and Maxima Super M.

About the Engines Lubrication “Needs” –  A two-stroke engine actually has two very different kinds of lubrication needs.  The lower end crank and rod bearings prefer a slow drip of a very heavy viscosity oil, while the top end parts wear best with a deluge of a very lightweight oil.  Since these are two very opposite lubrication needs, you have to choose whom you want to please.  For most two-stroke owners, it’s a no brainer.  You can buy 100 cranks and crank bearings that will all offer the same performance, but every owner wants to do whatever can be done to preserve a good running piston / cylinder set.  Given all this, heavier premixes of lighter viscosity oils are more desirable to most owners.

About Premix Ratios and “Oil Migration Time” –  Oil Premix ratios are another subject that some folks tend to get emotional about … and like oil brand choice, our choices are about science and results…not emotion.

The objective of the premix ratio is to maintain a certain level of “oil-presence” in the engine interior during it’s average “operating-use” cycle.  But how does one measure or assess the “oil-presence” … The most effective way has been with a radioactive additive.  We explain below.

Trying to keep it simple…here is how it works.  A test lab sets up an engine on a dyno stand, and begins feeding the engine a premix of an oil that has a specific level of mixed-in radioactive additive.  As the engine is run, a Geiger counter at the exhaust exit measures the amount of radioactive material being eliminated.  In this way, it is possible to factor the amount of radioactive material being put into the engine, verses the amount being sent out the exhaust.  The net result is the amount of “oil-presence” inside the engine.  In short, these tests showed that the oil-presence in the engine is a function of the operating rpm.  That is, the “oil-presence” inside a two stroke drops significantly as the operating rpms increase.  What this means is that an engine being run at 4000 rpm can maintain a very healthy and happy level of oil-presence with a 40:1 premix.  However that exact same engine being run at 8000rpm needs to have a 20:1 premix to maintain the exact same level of oil-presence inside.  This is why our 350 Bighorn road racers ran happily on the public roads on 40:1, but needed 20:1 for our sustained high rpm racing use.  It bears noting that in both the 40:1 street and 20:1 racing situations, our Bighorns made no visible exhaust smoke at all, except when they were held at idle speeds for a long time.

A further example of this is shown in the carbureted two-stroke 951cc SeaDoo watercrafts of the early 2000s.  In an effort to reduce the smoking during initial take-offs, SeaDoo engineers setup the oil injection systems to deliver no oil at all at idle speeds (and we mean zero oil).  The logic was that at idle speeds there is virtually no oil migration at all.  The high oil presence from the previous high speed runs was enough to allow the engine to run happily at idle for 10+ minutes with no oil at all being added…. And it worked great.

The lesson here is that your premix ratio should be a function of the average operating rpm that your vintage two stroke runs at.  If you are at peak rpm all the time, 20:1 is a good idea.  However for recreational level riders that don’t “scream” their engines constantly, leaner premixes will yield excellent long term wear.

About Oil “Film Strength” and Piston Seizures –  Many people believe that piston seizures occur when engine heat causes the piston to expand larger than the size of the cylinder bore…. but surprisingly this is not true.

If you could freeze your engine “in motion” in the middle of a long full throttle pass, and disassemble it for micrometer measurement, you would find the piston to measure at a .0005″ to .001″ press fit into the bore. That’s right, a slight press fit! The reason that it doesn’t seize is because the premix oil has such a terrific film strength that it acts as an irremovable buffer between the piston and the cylinder. That is, the bare metal surface of the piston never actually touches the bare metal surface of the cylinder because the oil stays between them. Many mechanics have experienced this phenomenon while cleaning a freshly bored cylinder in a solvent tank. Completely dry without any cleaning solvent, the piston moves through the bore with difficulty.  However while rinsing a cylinder bore with cleaning solvent, the piston glides all the way through with no resistance at all. This is because the solvent acts as a film between the piston and cylinder.

A piston seizure can only occur when something burns or scrapes away the oil film that exists between the piston and the cylinder wall. Understanding this, it’s not hard to see why oils with exceptionally high film strengths are very desirable, and why maintaining a proper oil-presence is also desirable.   Good quality oils can provide a film that stands up to the most intense heat and the pressure loads of a modern high output engine.   Here again, we mention that “absolutely no oil” has a stronger film strength than castor based oils.

About 100:1 Premix Oils –  Since the 70’s there have been repeated advertising claims made by various oil brands claiming that their oils can offer suitable lubrication for competition two-stokes on a 100:1 or 80:1 premix.  In many cases, these claims are “supported” by testimonials from various users.  Despite the oil makers claims, and the sincerity of the testimonials … it’s not so.  Running a 100:1 premix would be much like running only 2 quarts of oil in your truck, expecting that the quality of the oil can make up for the quantity …. It cannot….. no matter how good the film strength is “claimed” to be.  The truth is, many two-stroke engines can “operate” on a 100:1 pre-mix so long as rpms are kept very low (as might be the case on a novice class open bike).  But the first time that 100:1 engine makes an extended higher rpm run … it will lose the mathematics of oil migration, and score a piston.  There is just no way around the math.

About Oil Injection –  A majority of vintage street and recreational based 2strokes were manufactured with an oil injection system that eliminated the need for pre-mixing fuel.  Oils made for oil injection use have a considerably thinner viscosity than oils intended for premix.  It is unwise to use a premix type oil in an injector because a premix type oil can have difficulty passing through the small orifices in many oil-injection systems.  On the same tack, it is a very bad idea to use an “oil-injection” oil in a premix application because the reduced viscosity will not offer the lubrication needed in a pre-mix situation

Within oil-injection systems, there are two types.  The first is a simple “fixed feed” oil injection pump that is driven entirely by the varying rpms of the engine.  At  low rpms, the pump rate is slow, and at higher rpms, the pump delivers more oil simply because it is being spun faster.  Unfortunately, pumps like this generally deliver much more oil than is needed at idling speeds.  This is a particular problem for street bikes that often sit at stop lights in street traffic.

To relieve the excessive low speed over oiling of a fixed feed pump, manufacturers fitted, “cable operated” oil injection pumps whose oil feed was varied by a cable-driven valve connected to the throttle.  The cable operated pumps have a much broader range of oil output ability.  The average cable operated pump will feed an 80:1 – 100:1 oil ratio at idle speeds, and a 32:1 -50:1 ratio (depending on that particular engine’s peak rpm) at full throttle.  At the same time, many of these cable-driven oil injection systems had “divided feed” lines that delivered some of the oil to the inlet port near the carburetor, and other lines that fed oil directly to lower end bearings.  With this, split feed system, less total oil could be injected because less oil was being lost via atomization with the fuel/air mixture.

In truth, many of the cable driven injection systems did a great job of delivering adequate lubrication, along with very minimal smoking.  But sadly, these injectors had several significant down sides. Those are (in order of importance):

1)   No warning system if the pump failed or stopped delivering oil

2)   No warning system if the cable connection malfunctions or becomes disconnected

3)   Air Leak risk at fitting bolts that become loose

4)    Reduced oil delivery caused by a leaking fitting

5)   Oil delivery blockage at one of the many small orifices


For these reasons (and more) many owners opted to remove their problematic oil injection system and simply pre-mix the fuel.  This worked fine for Enduro/off-road applications where long idling and low speed smoking are not issues.  However for street machinery, the problem of low speed smoking was still a problem that had to be dealt with.


About Oil injection Oils and Smoking –  It’s important to understand that there is no such thing as a “straight petroleum” oil, nor a “synthetic” oil…. those are just industry “buzz” words.  With the exception of castor bean oils, all 2stroke oils are a mixture of the chemical components needed to do the respective job at hand …. and it has been that way for many decades.  In the 70’s a very common component of most 2stroke oils was “bright stock”.  Bright stock was very inexpensive, and offered decent lubrication qualities, but it contributed greatly to excessive smoking, no matter how lean the oil ratios got.


  Nowadays, most “quality” two stroke oils have long since replaced bright-stock with Poly-Butane.  Poly-butane is roughly triple the cost of bright-stock, but it smokes much less, and still offers great lubrication qualities.  All that said, there are still several 2stroke oil makers that use bright-stock instead of Poly-butane … simply to cut costs.  The bummer is that there is no labeling that allows you to know which is being used, and very few oil makers that will give you an honest answer if you ask them.  This same scenario applies for several other primary components of current 2stroke oils.  But There’s more…..


  99% of all two-stroke oils containers say “smokeless” or “low smoke”.  The truth is that there is no engineering nor industry standards for varying levels of “smoke-free-ness”.  The oil makers can print anything they want on the bottles without ever having to meet or comply with any industry standards or limits.

  As a result of all this, we use exclusively Maxima Oils products, because the folks at Maxima are very open and up-front about what their oils are made of.  In the genre of Maxima 2cycle oils, their 927 castor oil is by far the best for high-temperature competition engines.  However if you try to use this castor oil in a street (premix) application, you can experience excessive smoking in engines that do not run particularly hot.  The most smoke-free oil that Maxima makes is their “Super M” which comes in a premix and oil-injection viscosity.  Super M contains no bright-stock, and instead uses the much more smoke free poly-butane.  Maxima also offers a purpose made “scooter” oil that is slightly more smoke free than Super M, however this scooter oils doesn’t offer the lubrication needed for a high temperature air-cooled vintage engine.

Piston Seizure Vs Piston Scoring –   Piston “seizure” and piston “scoring” are two different stages of the same problem. When the oil film on a cylinder is momentarily burned or brushed away, the bare metal surfaces of the piston and the cylinder wall will actually touch. When this happens, there is a sort of scraping that takes place between them.   If the oil film is quickly resumed, the marks from this scraping will often remain on the piston and (or) the cylinder wall.  A momentary scraping or “scoring” seldom causes any permanent or performance robbing damage. In a momentary scoring event like this, no significant damage takes place because the oil film is resumed before the piston and cylinder have a chance to start exchanging material onto one another.

Scoring is commonly seen on the piston face directly below the piston ring end gaps. The blast of combustion can get between the large end gap of a worn out ring and burn the oil off the piston and cylinder in that area…Hence the surface scoring. In most cases, score marks can simply be sanded off of the piston and cylinder. However when ever you see scoring, it’s a good idea to find the source so that it doesn’t develop into a full blown seizure.

Piston seizure is a case of scoring where the oil film does not immediately return. After a few moments of constant scoring, the piston and cylinder will scratch each other hard enough to remove material from each other. This floating material grinds itself into the piston and the cylinder as it continues to grow in size. As this snowballing material grows, it will drive the opposite side of the piston against the cylinder wall with a pressure so terrific that scoring on the other side of the piston begins to take place. While all this is going on, your engine is still running wonderfully at full throttle.   However if there is a momentary lifting of the throttle, the force of this scoring can cause the piston to “seize” in the bore.  In a “light seizure” event like this, the rings often escape any damage, and the engine will easily re-fire as soon as the engine cools down slightly.

The death blow of a “power-on” seizure comes when the mass of material between the piston and the cylinder wall finds it’s way to the piston rings. This nearly molten mixture of aluminum and iron can lock the ring in it’s groove. This ring locking, not the piston surface scoring, is what actually causes your engine to quit when you experience a “throttle-on” seizure.  When the piston ring becomes locked back in it’s groove, it’s incapable of providing compression sealing against the cylinder wall. This instant loss of compression, while the engine is at speed, causes a dramatic loss of power. That power loss, along with the added drag of the badly scoring piston, makes the engine quit or lock up in a nanosecond. In fact this entire seizure process, from the first scoring scratch to the piston locked solid, takes less than a second at full rpm. In such a failure, a single cylinder engine will seldom restart.