We notice that the engine is quite stationary trough wide RPM range when transmission not engaged. When moving under power, the engine/c-drive assembly vibration/oscillation seems to be in the neighborhood of 1/8". The motion is visible, but no vibration transmits to the vessel. Is this typical, or an indication of poor alignment of the engine/c-drive? Or other cause. Or is the well balance power train perfectly stationary when motoring?
THE REST OF THE STORY
As a follow up to this posting we did purchase new engine mounts from Jamestown Distributors in Bristol, RI. $168 ea. identical to the original VETUS-MITSTEUN. The installation went well following the aggregated advice of many on the BB. Pat (Shenanigans) provided a 4x4 to span between the cockpit coamings and a come-along made the engine lift easy. We found we had to unbolt the engine from the frame (4 bolts) to slide it aft far enough (1/2") to disconnect the bolts between the coupling and the transmission output shaft flange. Then raised the engine about 3".
After disconnecting the 4 bolts supporting the C-drive to its frame, the entire frame assembly could be easily lifted off the 4 bad engine mounts for removal. We had no bolts seized or difficult to remove. No wires were disconnected and the only hose requiring removal was the seawater feeding the transmission heat exchanger. (that hose was replaced with black exhaust hose 1-1/4" which fit perfectly on both ends. Our marina indicated that the OEM plastic hose is no longer rated for below waterline.)
There seemed to be little opportunity in the reassembly process for re-alignment horizontally. the only slotted holes being in those in the engine mounts and the amount was minimal, a couple mm. Vertically we attempted to place the leveling nuts on the replacement mounts exactly where there were on the old measuring from the top down as the old were now variably slightly shorter (perhaps compression over time).
The bushings on the Vetus coupling were completely worn through on one side resulting in a metal to metal contact and a bit of metal wear. We inserted new bushings.
Everything was reassembled all bolts/nuts slightly loose. The engine slid forward and after loosely bolting to the coupling, alignment looked pretty good, not technically measured. The coupling bolts were tightened. With all other bolts loose, we started the engine and engaged the transmission briefly forward and aft under the theory with the bolts loose that the assembly would "settle in". And it did seem to. Again the alignment looked OK. At that point all bolts were tightened. We did not do a precision feeler gauge test nor did we adjust the leveling nuts. Total job about 6 hours with 2 of us.
Anyway the new mounts and coupling bushings took care of our knocking sound as well as a generally reduced vibration and engine noise.