Two stroke longevity

The subject of two stroke vs. four is highly debated, but there are some factual mechanical differences that greatly affect the longevity of the two designs.

Just so we are all on the same page let’s talk about the two motor designs for a moment. A four stroke circulates oil through its crankcase and over or through the crankshaft, then spatters the oil all over the cylinder walls and the bottom of the piston (this aids in cooling). As a result the inside of a four stroke is oil bathed as a result a four stroke is not prone to rusting.  The two stroke is a totally different animal. The crankcase of a two stroke is used to compress the air/fuel charge for the next power stroke of the motor, therefore it must be clear of bulk oil. This compressing action is like its own supercharger and is why the power per CC is so good in a two stroke motor.  All the lubrication in the two stroke comes from what oil you mix in the gas or what the injector pump squirts in; this oil is giving a fraction of a second to lube the top and bottom rod bearings as well as the main bearings and cylinder walls and piston rings, then it is passed through the transfer ports, compressed and ignited with the air/fuel mixture, then it passes out the exhaust port and trails out the pipe as the signature blue smoke. This all happens in less time than it takes to blink.  The amount of oil we are talking about is so small it is difficult to comprehend; if you mix CR 250 to 40:1 (as I do) that is forty parts gasoline to one part oil. Consider that in one cycle of the motor, the gas/oil mix used is less than one quarter of one teaspoon, of this only one part in forty is oil. Now make that do all the things that a quart or more of oil is used to do in a four stroke. It is truly amazing it works at all, but work it does and when it is all set just right a two strokes’ performance can be spectacular.

 A big factor is the vulnerability of the two stroke to internal rust.  This is a particular issue for personal watercraft, but it is a big deal for any two stroke motor in storage. Condensation from the humidity in the air will collect inside the crankcase and can cause microscopic rust pits in the lower end connecting rod bearing and from that point the crank shaft is doomed, in most cases the motor will last only minutes of running time. One of the best things you can do to preserve your two stroke is proper storage. A process called fogging is the best way to protect a two stroke in the off season. Fogging puts a coating of sticky oil all over the inside of a two stroke their by protecting the delicate needle bearings from rust.

In the event that a two stroke motor is filled with water, the motor must be restarted as soon as possible (NO, IT CAN’T WAIT UNTIL TOMORROW); this is an old sad story in the personal water craft biz. If you know what you are doing, pull the plugs and spin the water out and cycle fresh plugs through it until you get it running then put it back on the water and drive it until it is good and warmed up, then you can put it away and call it a night. Think about doing this on a holiday while everyone else is enjoying a day at the lake. Not to say that you dare leave water in your four stroke Waverunner motor but odds are you won’t have any damage from rust if you get the problem handled at first opportunity.

Four stroke motors are much more resilient and have much lower maintenance needs, for example a 125 two stroke motocross bikes’ performance will begin to fade within ten hours of replacing a piston and rings. The larger displacement two strokes will go some longer and the smaller some less, but in any case the piston is a wear item in a two stroke. A comparable four stroke will give years of service with very little performance loss with nothing but commendable service.

Sensitivity to temperature and altitude is another point of interest as a four stroke has a much more forgiving nature; little compensation is needed in most cases, unlike the two stroke counterpart that is highly sensitive.  The window of operation is very small in the case of a two stroke motor, too warm or too high and they foul spark plugs, too low or too cold they melt big nasty holes in themselves. Aluminum has a rather low melting point about 1220deg and the two stroke won’t clean up and run right until the exhaust gas temp is above 1150deg or on the ragged edge to start with, so any change either gets you a hole in a piston or at best a black worthless spark plug.  This is also where we run into the problems with the ethanol enriched fuels.  In an oxygenated fuel a portion of the liquid is not fuel but rather oxygen trapped within the compound, so some of the liquid passing through the jet is not fuel but trapped oxygen, this leans out the fuel mixture and raises the combustion temperatures closer to the melting point.

The long and short of this is that the virtues of four stroke include low maintenance, user friendly power delivery, and durability. Two stroke on the other hand have a power to displacement ratio that current four strokes cannot dream to mach.  So whatever you chose to ride by snow, water, dirt, make your riding more enjoyable by maintaining your machine properly, and understanding its strengths and weaknesses.