Setting Anchor, Shearing Motor Mount Bolts ?!

[mckinneyb]

I have a chronic problem with motor mount bolts (to the engine) on my Yanmar 3YM30. The engine is new (200 hours), I just sheared off the second set of bolts that connect to the motor...starboard side aft. It was aligned, and runs smooth when bolted. Any ideas??? Would setting the anchor cause this?

[mckinneyb]

I have a chronic problem with motor mount bolts (to the engine) on my Yanmar 3YM30. The engine is new (200 hours), I just sheared off the second set of bolts that connect to the motor...starboard side aft. It was aligned, and runs smooth when bolted. Would setting the anchor cause this? Any ideas???

[Me-and-Boo]

Torqued too tight is one thought (stretching the bolts). Wrong strength range on the bolts. Not torqued tight enough (the engine shifting forward and back). Incorrect mounting.

[Astrid]

Could be a matter of hull flexing underway. What kind of coupling do you use for the prop shaft?

[HopCar]

Are you using hardened bolts?

[Target9000]

Also what bolts are you using that are shearing off? SS? Grade 8? Something else? Is the engine bed itself secure? What are you bolting into?

And as Astrid said, are you using a flexible coupler?

[mckinneyb]

I was using number 8 bolts, I assume SS...no flexible coupler...maybe that's next.

[svcambria]

I just sheared off the second set of bolts that connect to the motor...starboard side aft.

Do you mean the bolts that connect the mount to your engine beds, or the bolt that connects the mount to your engine?

Anchoring could not do this; something in the water suddenly stopping the prop might...

Michael

[HopCar]

Grade 8 bolts are stronger than stainless steel so I wouldn't change that.
Bolt Depot - Bolt Grade Markings and Strength Chart

[Bash]

Originally Posted by svcambria

Anchoring could not do this; something in the water suddenly stopping the prop might...

...or failure to pause in neutral when shifting from forward to reverse.

[perchance]

If it is the same bolts on the same side I would suspect engine torque when powering up. Engine torque puts a lot of strain on the mounts and is literally trying to twist the engine opposite its' rotation

[sabray]

Without knowing exactly which bolts are shearing I am guessing that the bolts are not set with proper torque. Probably to loose unless your laying into them Neanderthal style when you install them.could be alignment bin conjunction with poorly torques bolts. The mounts should share the load if one set is failing they may be taking more then their share of the thrust and vibration.