Aggressive engine wobble out of nowhere

[Chuck Hawley]

Originally Posted by a64pilot

Be sure to replace the bolts with at least grade 5 bolts, ideally there should not be threads where it goes though the coupling.
Do not use hardware store bolts.
You tell bolt grading by head markings.
Do not use bolts with no head markings

Yes! Higher strength bolts do not cost THAT much more considering the critical nature of this application. I foolishly replaced drive shaft bolts with SS because of the salty wet environment in which they were located, and they broke very quickly. I would soak your new bolts in Boeshield or similar protective spray (I am sure that CRC makes something similar) and then remember to check them annually. This would also be a good fastener to torque correctly.

One other thought: could it be the vibration damper in your engine? Or could this be implicated, if not the sole cause? When I had vibration, I wondered if I should have replaced it. Not too expensive ($280 for my Yanmar) and not THAT difficult to replace.

Chuck

[hamburking]

Originally Posted by tommycook

....I checked the engine mounts and they do not appear to be cracked, but they are extremely hard to see.

For hard to see areas, like behind the engine, down in the bilge looking up, etc...stick your camera/phone in there and take a picture.

Sometimes, I would just sit quietly staring at the engine. Then touch every bolt, screw, and hose. My family thought I was crazy. But its surprising how you can find loose hoses, nuts, and tiny leaks when its all quiet and you have time to look. Although not glamorous, I also take lots of engine pics to review at home. Its amazing what you can see in a picture on a big computer screen.

[Chuck Hawley]

I agree with the general theme that you cannot tell if engine mounts are broken when there is the weight of the engine on them. Three of my four Yanmar engine mounts were in terrible shape, relying on gravity to keep the engine in place. Extremely hard to determine, even with the engine shaking. If in doubt, undo the flange, measure the engine mounting bolts so you can attempt to replace the engine in the same place, and jack up the engine so you can see if the mounts are broken. Very satisfying when you determine that they were, in fact, torn or broken.

[Cadence]

Originally Posted by tommycook

My wife and I are on our way down the ICW in Jupiter Florida. As we slowed to wait for a bridge opening we heard a clunk and the engine started shaking aggressively. It runs perfectly in neutral at all RPMs, but as soon as I put it into gear (forward and reverse) the engine starts shaking very hard.

We are now being towed (all systems off) and I can see in the engine bay that the engine is still shaking as the drive shaft turns while we're being towed. I tried to get a look at the prop off the back of the boat, and it did not appear to be fouled.

Any ideas for troubleshooting would be greatly appreciated!

Cheers!
Tom Cook

I don't think you have more than picking something up on your prop.. Take a swim

[charliehows]

Theres a lot to be said for learning the basics of general engine maintenance - aside from reducing costs, things like regular oilchanges, anodes, impellers, hose checks, tappets, checking for internal accumulation of residues, etc, etc, almost every time I start off doing something routine I stumble on something else that needs attention - case in point - Last time I changed the transmission oil I noticed that one of the driveshaft nuts was loose - on closer inspection 2 out of 4 were loose, probably only a matter of time before the other 2 worked their way loose. Locktited and tightened, will keep an eye on them but no problem caused.

[CassidyNZ]

Originally Posted by tkeithlu

Bolts: Grade 8 gives you about 25% more shear strength than Grade 5. Stainless steel gives you less shear strength than Grade 5, but how much is hard to say because there are lots of things called stainless steel. In general, it is gummy steel compared to real bolts. Just because it usually doesn't rust does not mean it's strong.

No reason to assume that stainless steel bolts are inferior. As you said, there are lots of things called “stainless”. Amongst them 8.8 graded or better SS bolts. They are “real” bolts and are way more preferential than zinc plated steel or worse, just ordinary steel.

How do ratings compare? The first digit relates to the ultimate strength of the material, whilst the second is the ratio of yield stress to ultimate strength. Grade 8.8 bolts have an ultimate strength of 800 N/mm2 and a ratio of yield/proof stress to ultimate strength of 80% (640N/mm2). So a grade 5.6 bolt will have 500N/mm2 and a yield/proof ratio of 60% (300N/mm2), nearly half.

But just because it’s stainless does not mean it’s weak.

[RLaird]

I believe it’s the rubber of mount separating from the flanges. Very hard to see without applying some lift. May need an assistant!
Shouldn’t be ant washers on the coupling bolts in my ‘limited ‘ experience?..

[dfelsent]

I had vibration. One loose nut on the stainless fully threaded stud on the flex coupler. It was whacking the transmission flange. Ouch but not terminal. Very fortunate.
Bought proper grade 8 steel studs. Used loctite. Torqued to spec. Ran an hour. Re-torqued. Ran ten hours. Checked torque again.
That plus realignment made a much smoother engine.
Check those studs and bolts.