yes I had a CMB 90 pre grand prix motor. Remember those? steel rod with needle bearings and a non stacked crank.(mechanical overkill on construction but indestructable) That thing shook so bad that it would not rev past 18000. we threw 20 degrees more exhaust timing at it and it literally did nothing cept piss us off. LOL It popped plugs and did all kinds of wierd shit. we literally had to take the flywheel and remove almost a half ounce from it and index it to the crank to get it to run right. Once that was done with no other changes bam 26000.

I will tell you exactly the method we used to balance at that time.(crude but it was effective, glad the crank was solid as it had to be bending alot) This was before we reversed the engineering to determine what we considered good balance numbers. In those days balance was closely held and anyone with any experience just would not talk about it. It was a don't ask deal.

we mounted the engine in rubber mounts like it was installed in the boat. we cut two 28 inch pieces of .020 piano wire and mounted one to the engine in the horizontal plane and one in the verticle plane mounted at the engine mounting lugs of the case. we spooled the engine up close to the 16000 rpm window and watched the wires. the one in the horizontal plane was shaking like crazy. like 3 harmonics down its length with an amplitude of 3 inches. so we just started drilling holes in the flywheel on the crankpin side until the wires vibrated evenly. crude but effective. Their was no way I could add a half ounce to the counterweight or remove that much from the piston. we had already gone there.

what prompted all of this was we noticed that an engine would run good in a boat with a solid one piece mount but when you hung it in the new technology rubber mounts the engine was a turd.

Multiple engine boats twins and triples were a entirely new nightmare. Compounded vibration. When the harmonics line up watch out lucy. My friends triple 45 hydro would literally shake the engine mounting lugs right off the engine case in 4 runs. he had picco engine cases stacked like cordwood

There can be alot of force generated