Engine Mount Attaching Hardware

My basic question is:
What type of fastener to use to fasten engine mounts to stringers?

More details:
I'm going to be installing a new 53hp diesel engine(Yanmar) in place of an older different make 50hp diesel engine in a 40 sailboat. The basics of the original mounting scheme are not changing. Only the shimming required for the new engine. I'll be using the engine mounts that will be supplied with the new engine.

The old engine mounts had coarse threaded stainless bolts, of unknown type, welded to stainless plates inside the stringers. The heads were welded to the plates. The threads stuck up through the stringers, shims and mounts with regular stainless nuts and washers on top. The stringer material is all fiberglas, no wood or aluminum. I have 316 stainless flat stock available for backing plates and additional shimming as required. The old mount bolts and the plates they are welded to came out and I can fit in new backing plates and bolts up through. The old ones will not fit the new mounts.

It seems to me that welding the head of the bolt to the backing plates would weaken the bolt. I'm also concerned about the strength of stainless for an engine mount. Just because it has been that way for many years doesn't make it right.

 

Both welded on bolts or nuts are used. With nuts you have to drill a hole in the stringers, with bolts you don't have to do that.
The best solution is to use flexible mounts bolted to the plates. The central threaded hole allows for 1/2" adjustment length so no shims are normally required.

Welding may weaken a bolt, but the load on it is very modest, so don't worry.

 

More info

Im talking about attaching the flexible mounts to the stringers. Is there a boat building standard that specifies a fastener type or material for bolting the mount to the stringer. Is stainless ok/strong enough? Fine thread vs coarse thread?

It will be a new engine with new flex mounts attached (through bolted) to fiberglass stringers. What I could not find was a refence to proper fastening of the mounts to the stringers. The shims are to keep the hight of the mounts up so I don't have to adjust the engine too high up on the stud on the flex mount.

 

Think about the engine weight vs the torque.
I've made several test runs with the flexible mounts attached to 1/2" aluminum backing plates with two short M8 bolts and no fasteners to the stringers at all. The 80 hp diesels did wander a bit and after they had settled in what I presume to be their favorite position, I drilled holes in the stringers and used stainless wood screws with 13 mm (1/2") hex heads to fix the plates in position.

If I were to do it again I would use stainless backing plates because after removal of one engine last winter I saw considerable corrosion damage at the bottom side of the backing plates.