Age old problem...loose pylons

So help me understand something here guys. How long have the 14/16 been around? Going on 40 years? And over a period of time, and depending on use, the pylon to casting loosens up? And the best fix we have to date is "fill it with epoxy" or shim with beer cans? I know the epoxy method will only last two or three seasons before it loosens up again. What a bugger to pull that apart only to do it again? Can we do better than that?

This AL on AL wear can be expected. What is the root cause? Does the bolt have any effect on the wear? I think to a certain extent it does. This, over time, becomes a pivot point, assuming originally you have a close to snug clearance fit between the pylon and the corner casting. There is initially a small clearance that progressively becomes enlarged due to fore/aft and side to side flextion. Theoretically this "enlargement" will occur above and below the pivot point (the bolt)?

Why not drill and tap a set of holes in the corner castings say in four places as far above and as far below the pivot point as possible and install a set screw to "lock" the pylon within the casting? Over a period of time, they would become loose and need tightening. I suppose after a long period of time they may cause some serious wear on the pylon, but do epoxied, beer canned or new boats stay tight forever?

What do you engineering gurus think?

Why not 2 or 3 smaller bolts?

Tapping holes in the lightweight aluminum castings is a really bad plan. There is currently not enough material in the wall of the casting to create an effective thread. All you will do is strip out the threads the first or second time you crank down on the set screws.

Think of what you mentioned about the age of the boats... 40 years. That's 40 years of high stress repetative motion. Even steel will wear out over that time.

Shimming (in my opinion) is the best solution. It allows you to make a custom fit, is easily removed for repair or adjustment, and is easy and quick to do.

We also have to keep in mind the tolerances of the aluminu extrusions and the castings. The tolerance stack up has to allow for assembly of the components when they are at the worst case possible.

Using multiple bolts will only delay the issue. Then you will have more than one elongated hole.

Just to support my statments, I design and build high volume custom automated equipment for a living.

I was just thinking that if you had more points holding the casting you would have less movement between the parts, but the tolerances might just be a problem after all. I would not suggest trying to thread the Al either.

Sam

No matter how many bolts or set screws you put on this joint, it will move, thus wear. Movement, to a certain degree is neccessary in this dynamic machine. If you stop the movement in these joints the loads will be transferred to the aluminum beams and castings which will not hold up to periodic loading very well. Aluminum is not good in fatigue situations.
The best solution seems to be the method that allows, but limits, the movement and can be easily and cheaply replaced periodically. Soda can shims seems a pretty elegant solution to me.

Have them TIG welded. That should stop the flex for awhile.

And welding it will make it a little difficult to disassemble if you ever need to replace a hull.