1998 Mercury 300 Pro max twins breaking lower mount studs

[air trap]

Hi I have a 30' Cat With twin Mercury 300 promax engines, I'm having a problem with my lower motor mount studs snapping. I broke on right at the shoulder on one engine and one in the threads under nut. all the studs were replaced within the last 2 years when the center section were rebuilt. Has anyone else had this problem?

Thank you to SNF for the post.

[frank jackson]

not all bolts are created equally ........ where are your replacement coming from ? rubber mounts ? worn out ?

[air trap]

Frank thanks for the reply they are solid mounts I only use mercury Marine studs washers and nuts. The Capri midsection pivot tube bushings all have been replaced within the last year.

[frank jackson]

tightening to the correct torque ? Checking for loosening ? Beyond that no clue ....Sorry

[NICE PAIR]

Frank thanks for the reply they are solid mounts I only use mercury Marine studs washers and nuts. The Capri midsection pivot tube bushings all have been replaced within the last year.

Are you sure it has solid uppers?

[JPEROG]

Contact Chaz here on the board. Scott Porta came up with a solution for this problem and Charlie has done a bunch of them since. He calls it the slop stop. His procedure actually allows you to put some preload on the bottom mounts which spreads the load.

Joe

[air trap]

Are you sure it has solid uppers?

yes solid mounts!

[vnemous]

[Chaz]

Thank you Joe and Vnemous !

Air Trap ,

Very first thing that comes to mind is the upper mount. I know you said you have solid mounts . I did the same thing years back when the double hump mount cover broke . Then I added a girdle over the top and then the three bolts that hold the cap down broke .
Mercury went to a solid ( no bolts no cap ) one piece mount in their 2001`and up engine adapter plate on 250 / 300 HP models .
Not sure what you have or it's condition , but it's worth a look and an upgrade if you are still running the early model parts .
So now that we have the tiller arm securely fastened to the engine the next weak link in line is the relation between the lower yoke and swivel pin . The factory uses a wave washer to take up excessive slack and pits a clip at the bottom and calls it a day.
For offshore race boats , I machine a 316L tube that runs the entire length of the swivel pin that is welded in place that has threads sticking out from the bottom of the pin that uses a cup and nut arrangement to secure that junction .
For single engine or river runners that don't have the need for a double walled swivel pin I make a shorty version that does the same thing with less machining and welding time as well as a few pounds less weight.
Both use a shim stack I cherry picked out of different models .
End result has been no more broken mid section parts . Longer lasting swivel pin bushings and better handling on single V hulls .
As Joe said , this is something that sensei Scott Porta' taught me a few years ago . I'd like to think that I took the pebble from his hand and improved on material selection and application since then. I just did a set that are going to him . I'm sure the teacher will be pleased ...

Chaz