Crank That Thing Up
Since your personal watercraft is a considerable investment, it makes sense to provide it with quality service and maintenance to ensure a long and trouble free life. Just taking the time to perform a quick inspection before and after each riding session can prevent tons of problems. Trust your senses if they tell you something's not right. Does your craft shake violently at low speeds and then smooth out as you speed up? Do you feel an unusual vibration at a certain rpm? Or, worse yet, are you afraid to run at full throttle because you think your engine may shake itself right out of the craft and wind up on the bottom of your ride spot?
Believe me, it happens. Especially in modified machines with the engine running at considerably high rpm. Usually because no one paid enough attention to the engine's balance.
Here's an example: During a routine inspection and part replacement you discover that the tapered end of your crank and flywheel have some nasty grooves, bumps, and humps on their surfaces. This is the first warning sign that things will get worse very soon unless something is done to correct the situation. There are several possible causes for this problem. One: the flywheel may be out of balance. Two: the crank has become out of round. Three: the driveshaft may be bent or the impeller out of balance. Any of these can contribute to engine damage, and stock machines are not immune to them. The difference is that a modified engine will blow up a little bit sooner.
There are a lot of powerplants that fit this description, engines whose owners are not aware of the silent killer inside the craft - the out of balance crank condition.
Not all cranks are created equal. Some will last 5-10 million cycles, while others may come apart in less than one season. I've seen cranks in brand-new machines or right out of the boxes that look like they've fallen out of a truck. Some cranks fit so tightly that you need a 40-ton press to separate them, and some will move 0.020" if you hit them with a soft five-pound hammer.
The problem is that you never know how tightly fitted your crank is until it comes apart. The only way to prevent a problem is to practice early and frequent engine inspection, paying special attention to the crank and flywheel. Realignment and balancing at operating rpm is a sure way to prevent problems.
If you're building a motor, I recommend you invest in a quality aftermarket crank from a reputable manufacturer. This can save you thousands of dollars in the long run.
Every time a moving piston stops and reverses its travel from BDC to TDC, it wants to drag the whole engine with it. The reason is simple. Since the energy of the moving piston can't be stopped or destroyed, it is exchanged between the piston, crank, and cases. These reversed impulses, which are delivered twice per crank revolution, are called Primary Reciprocating Imbalances. This imbalance is distributed by the connecting rod and it can't be counter-balanced.
Above: The counterweight which has moved forward 3:00 or 9:00 can be realigned easily, but if the crank has opened up like a sandwich from A to B, it's best to throw it out. Some shops will push the flywheels together and weld them up, but built-in stress at the lower wrist pin will cause fatigue sooner or later.
Rotating imbalance made up of the crank pin connecting rod's big end along with the big end's bearing and washers occurs when weight is placed on one side of a rotating part without adding equal weight at an equal distance on the other side. Fortunately, rotating imbalance can be overcome perfectly.
Suppose we overcome 100% of the rotating imbalance. The reciprocating imbalance is still there, made from the weight of the piston, rings, bearing, circlips, and the connecting rod's small end. This stuff is jerking the engine up and down a thousand times a minute. Let's call this 10 units of primary imbalance.
If we up the weight on the side opposite the lower pin by 10%, we find the engine jumps up and down with only nine units of imbalance. Raising that counterweight 30% cuts us down to seven units of imbalance and so on. If we balance at 100%, we eliminate the vertical imbalance, but the engine still jumps from side to side with 10 units of imbalance. What I recommend is balancing the vertical at about 50%. This cuts the reciprocating weight by half, reducing the load on the main bearings by about 50% as well. Street motorcycles are balanced at about 70-80%, which contributes to the rider's comfort by shifting the imbalance from the vertical to the horizontal. In this case, the horizontal imbalance is much less important. But watercraft engines have totally different requirements.
Most engines use flexible mounts to reduce the vibration factor, but there is also the natural resonance frequency of the surrounding structure. Combined at a certain point, the vibration and the resonance may cause serious damage to the crank, flywheel, chassis, or even the external parts of the engine like the carburetor. When dealing with the twin parallel cylinders of a watercraft engine, firing 180 degrees apart, you might think that the up-and-down motions would cancel each other out. It does, but there is a new element introduced, the engine rocking from side to side, producing a gyroscopic motion. This combined rocking and orbital motion adds unwanted additional stress.
This movement can be decreased considerably by moving the cylinders closer together. But this creates a new problem because it reduces the size of the transfer ports which must have a generous area for the effective flow of mixture to produce serious power.
When it comes to a three-cylinder motor where firing occurs at 120 degrees, the cylinders do self-balance with respect to primary imbalance, but the "rocking couple" remain, even stronger and more violent due to the increased length of the crank. Quite often the engine will self-destruct for no apparent reason, however, closer examination may reveal that as a result of the cylinder boring and installation of heavy pistons we've crossed over to a dangerous out of balance point. An increased weight of 12 grams can result in nearly 500 pounds of additional force at the connecting rod at 7500 rpm. Couple that with increased combustion area and you begin to realize the potential problem. Please note that the same situation exists when pistons are on the light side, the difference is that the main bearings will receive a serious pounding in the opposite direction. If, by chance, the flywheel is also out of balance, expect rapid failure of the moving parts. Bear in mind that the faster or bigger motors will fail much sooner than their slower or smaller counterparts.
It's possible to spot and prevent impending problems. After splitting the lower cases you may notice friction markings on the outer surfaces of the bearings. It's mandatory to rebalance such a unit.
Crank balancing requires precision work and instruments, and when done correctly it pays generous rewards.
Quite often, the crank's counterweight is made of materials which may differ in density and toughness. Combine this with sloppy tolerances during the forging process and you end up with parts that don't match. If the counterbalances vary from one side to the other in the same cylinder, one may move toward 3:00 or 9:00 due to an uneven force applied by rotation. In such a case no amount of pin welding is going to save that crank or flywheel.
When building a performance machine, it's a wise move to buy a quality crank built by a reputable shop using the best possible tools, steel, connecting rods, pins, and bearings. It's a considerable cash investment, but it's much less expensive than a blown-up engine. In my opinion, if an engine's being rebuilt and the crank is found to be out of tolerance, twisted, or fatigued, it's best to get rid of it. Putting in a new unit almost always proves to be the best move in such situations.
It is possible to rebuild an old crank. It's done routinely to motorcycle race engines by using slightly oversized parts and precision inspection. The key word, as always, is quality. If the shop you use is up to the task, go ahead, but it might not be a good idea to be the guinea pig.
In a two-stroke engine there are many stress points reflected as mechanical and thermal loads in addition to the gas load on the piston. Not to mention a very substantial difference between the inner wall of the liner and the outer temperature, which leads to serious thermo-mechanical loads. A significant stress is also applied by the cylinder head bolts and the gasket assembly. A tangent point of the main stress is reflected at the lower connecting rod pin, the retainer holes in the counter-balanced flywheel, and the main bearings. The piston reaches its highest velocity at 80-90 degrees of the crank's rotation. At this point, the combined weight of the piston, connecting rod, bearings, and wrist pin exerts the highest pressure on the crank, trying to split it apart. That's why it's most important to inspect the crank frequently, especially in a high-output engine.
One of the best systems for crank inspection is to record runouts at the face of each counterweight. When they start to open up 180 degrees opposite the lower connecting rod pin, you can rest assured that the crank won't last much longer. There's nothing you can do to stop this destruction except replace the bad unit. Please note that before this happens the counterweight will usually move toward 3:00 or 9:00. This movement can be controlled by realigning the crank frequently. Also, welding the lower pins helps somewhat to extend the crank's life, but that's only a small fix and no amount of welding will save a bad crank. As you can see, there are hidden complexities which may escape the notice of an untrained or slacking eye. So as always, before you buy or go for service, know your supplier and don't be afraid to check references. And if you find somebody who does a good job, stick with them.
Remember, prevention is the key word for success and for eliminating the impending problem.
George Grabowski HPT Sport USA