Custom outboard setup, have some engineering questions

[bikerider]

A while ago I put together a 68 Evinrude 6hp outboard motor leg with a briggs and stratton 6.5hp pressure washer engine. In the pictures you will see the adapter plate I made to attach the two. On the engine side I used nylon lock nuts on the bolts to hold the engine in place. I was out on the water yesterday and two of the three bolts holding the briggs engine broke. The mounting system has always seemed solid so I was surprised when the the bolts broke. When I made the adapter plate I just used the run of the mill bolts and lock nuts from the hardware store. I am by no means an engineer, so now I am wondering if I need to replace all three of the bolts holding the engine with grade 8 bolts or if my design is flawed and this will happen again no matter what grade hardware I use.

 

[GA_Boater]

The gap between the plate and motor crankcase allows flexing of the three bolts. Grade 8 won't help cuz B&S motors are shakers.

Use a thicker plate or make a spacer adapter.

Doesn't the Briggs use 4 mounting bolts? It's been a while since I flipped a mower over.

 

[bikerider]

The gap between the plate and motor crankcase allows flexing of the three bolts. Grade 8 won't help cuz B&S motors are shakers.

Use a thicker plate or make a spacer adapter.

Doesn't the Briggs use 4 mounting bolts? It's been a while since I flipped a mower over.

I wish it did use four but this one only has three. By spacer adapter do you mean thick sleeves for the three mounting bolts?

 

[GA_Boater]

No, I mean a plate to support the bottom if the motor. Sleeves will just delay broken bolts.

You need the make the entire mount rigid.

 

[bikerider]

No, I mean a plate to support the bottom if the motor. Sleeves will just delay broken bolts.

You need the make the entire mount rigid.

If I understand you correctly you mean the engine needs to be mounted to the leg with no gaps whatsoever?

 

[GA_Boater]

Yes, pretty much.

 

[RaisedByWolves]

Bolts dont hold things in place like most people think.

Bolts squeeze two things together and friction is what holds them in place.

 

[bikerider]

Bolts dont hold things in place like most people think.

Bolts squeeze two things together and friction is what holds them in place.

So I learned the hard way.

 

[RaisedByWolves]

That’s the way of the world.

The important thing is that you tried and learned something useful.

Now your all set to go ahead and find your next mistake.

 

[bikerider]

That’s the way of the world.

The important thing is that you tried and learned something useful.

Now your all set to go ahead and find your next mistake.

Exactly. I'm always grateful for those more knowledgeable than myself.