A body in free space has six degrees of freedom. I have been wondering about, and checking actually, engine alignment in my 270 DA. I have the following conjecture, comments are welcome, especially in pointing out any errors.
While I have never pulled the engine, it is my understanding the the rear of the engine mounts to the transom gimbal housing. It does so with two mounts, one on each side of the rear of the engine. These two mounts force alignment of the engine and outdrive at these two points. They are not adjustable.
Three of the degrees of freedom are translational; forward/back, up/down, port/stbd. The rear engine mounting bolts constrain all three of these at the rear of the engine. None can be wrong unless something is bent or broken.
Three of the degrees of freedom are rotational, pitch, roll, and yaw. The two rear mounting bolts constrain roll and yaw, they also cannot be wrong.
So, the only degree of freedom left that can be wrong is pitch. So, when we check engine alignment (which I read should be done every year), we really are checking the pitch of the engine. And since the rear mounting is constrained, we really are checking whether the engine is nose up or nose down.
While my outdrive has always slid into place quite easily, the engine alignment tool thinks there is a problem. It does not slide in all the way by hand. It should.
In reality, there is another possible source of problems. The engine drive coupler could also be bent, so that the output shaft is not perfectly in line with the engine drive shaft. This would cause the center axis of the coupler to wobble on its axis as the engine turns. So you should not check engine alignment at just one engine crankshaft position. You should check it four times in quarter turn increments. Then you are also checking the coupler.
So after rotating my engine 180 degrees, I can insert the alignment tool all the way in. It is still snug, but will go in. So I have a slight amount of wobble in the coupler in addition to the engine being tilted up or down. I really am more worried about the engine tilt at this point, and how determine how to correct it.
While I have never pulled the engine, it is my understanding the the rear of the engine mounts to the transom gimbal housing. It does so with two mounts, one on each side of the rear of the engine. These two mounts force alignment of the engine and outdrive at these two points. They are not adjustable.
Three of the degrees of freedom are translational; forward/back, up/down, port/stbd. The rear engine mounting bolts constrain all three of these at the rear of the engine. None can be wrong unless something is bent or broken.
Three of the degrees of freedom are rotational, pitch, roll, and yaw. The two rear mounting bolts constrain roll and yaw, they also cannot be wrong.
So, the only degree of freedom left that can be wrong is pitch. So, when we check engine alignment (which I read should be done every year), we really are checking the pitch of the engine. And since the rear mounting is constrained, we really are checking whether the engine is nose up or nose down.
While my outdrive has always slid into place quite easily, the engine alignment tool thinks there is a problem. It does not slide in all the way by hand. It should.
In reality, there is another possible source of problems. The engine drive coupler could also be bent, so that the output shaft is not perfectly in line with the engine drive shaft. This would cause the center axis of the coupler to wobble on its axis as the engine turns. So you should not check engine alignment at just one engine crankshaft position. You should check it four times in quarter turn increments. Then you are also checking the coupler.
So after rotating my engine 180 degrees, I can insert the alignment tool all the way in. It is still snug, but will go in. So I have a slight amount of wobble in the coupler in addition to the engine being tilted up or down. I really am more worried about the engine tilt at this point, and how determine how to correct it.
