Had one of the pivot bolts break on drivers seat bottom. It broke as I was stepping over the windshield while loading back onto the trailer. Almost ate some windshield.

When I got home, found the bolt had sheared off where the shoulder and threads meet. Had great fun removing the piece still threaded into the seat frame. Removed the other pivot bolt to see what it was I needed. It was found loose. Suspect the the one that sheared had loosened itself first. Both screws had sharp edges on the threads.

Found a replacement shouldered screw on McMaster Carr. They have 3 stainless options. 18-8, 316 and 17-4. Which is the preferred option? I'm leaning towards 17-4.

FWIW, probably a good idea to check these things occasionally. Could have been ugly if it failed in rough water.

Here's the "good one".

I would use whichever one they say has the best strength. They are probably all corrosion resistant enough. I just looked at McMaster and it says that the 17-4 PH has twice the strength than the 18-8. Sounds like a winner.

Just noticed I typo'ed the 17-4 into 74-2 (a little distracted, now fixed). The 17-4 definitely has a higher tensile strength (130k/lbs VS 70k/lbs), but I know that high tensile strength means more hardness which can be brittle (breaks before it bends).

Not sure what the orig screws were made of. It feels very light weight in the hand but kinda hard to drill. They are totally non-magnetic.

Got 4 of the 17-4's on the way. Might as well do the front pass bolster while I'm at it. Think I'll start checking them at least once a season.

zz28zz,

If they are "totally" non-magnetic, they are 316. Your failure is the expectable outcome of a loose shoulder bolt; the stress was induced at the weakest point and the highest leverage. Higher tensile strength is not tantamount to higher hardness, and for sure not brittleness. Strength, hardness and toughness are separate qualities in metals, each with their benefits and drawbacks, depending on purpose directives. For what these bolts are doing, the higher tensile strength is desirable. A bit of removable Loc-Tite would definitely be in order. If you can get to the far side, and there's room for it, a nylock jam nut wouldn't be a bad idea, either.

Thanks. Brad.

That's what I thought abt 316 also, but the McMaster site says they may be slightly magnetic??

I agree the failure mode was due to fastener not being tight. When tight, the threaded portion is loaded in tensile and the shoulder is loaded in compression, both good things. When it loosens, the load on the threaded portion changes more towards shear, not good.

The threads having sharp edges tells me they have been loose for a long time. That's on me since it's been in my possession for 10+ years. I've never had to dig into the bolsters so I wasn't aware of this possible failure mode.

After spending a couple of hours laying on my back drilling for an easy-out and looking at how the seat bottom is supported, these fasteners support a significant load due to the leverage factor. Take the leverage factor and add the g-force's encountered, that's a lot of stress on those 5/16 -18 threads..

Will be using Loc-Tite for sure. Probably the blue. The head is 3/32" hex and will deform with excessive force. There's no threads protruding out the backside of the seat frame "nut" so can't use a jam/lock nut.

Generally, you get about 60% shear strength, of the bolts tensile strength. (Measured in lbs/sq in)

Grade 8 bolts have 150'000 lbs tensile. per square inch of shank surface area.

That brings up a good point. I was planning on using blue Loc-Tite to prevent this from happening again (loosening).

However, my experience with stainless fasteners threading into stainless structure is they do tend to experience galling. . Humm....

zz28zz,

For a semi-permanent install like this, I'd use Loc-Tite, not anti-seize. It's not like you're going to be taking them back out any time soon or on a regular basis. You don't want to have to keep tightening these up. As you stated, you only have a 3/32" hex socket to tighten them up with. That socket will strip out LONG before you see any galling or before you have enough tension on the threads to really keep them from backing out. Loc-Tite will hold them indefinitely if you give it enough time to set up.

One dick's opinion.

Thanks. Brad.

Spent all day on this today. Both of the new 17-4 S/S shoulder screws experienced severe galling. Almost didn't get them back out. They both left some threads inside the hole. Got both holes drilled and tapped again. Think I'm done with the SS fasteners. Need to see what other materials are available.

zz28zz,

Nothing wrong with stainless fasteners. Just need the threads to be clean and clear, which isn’t exclusive to stainless. For the record…. Loc-Tite serves as a decent lubricant until you get them in place and tight.

Thanks. Brad.