New hold down bolts

[HiJoeSilver]

Thought I'd throw this out there to see what others thought or if anyone had bullets to shoot through my idea. It's already done but still at a stage I can undo it fairly easily.
When I removed the shell I obviously had to take out the old hold down bolts. Like most I'm sure found they were badly corroded, many with enough corroded that there was often a significant thickness (strength) of the bolts gone. Original bolts were 1/4 inch. Looking at them as they came out my engineering brain noted that most of the corrosion was on the bolts where it passed through the wood, and were often in better condition above and below that section. I surmised that the wood which will always retain some moisture accelerated the corrosion where the bolts were encased in the wood. Many of the bolts had about half their thickness corroded away.
When I put this back together I don't want to worry about getting to the bolts for a good 30 years again, so solution?
I put my engineering degree to work and decided to replace all the hold down bolts with stainless steel bolts. since the stainless steel is slightly weaker than steel I increased the size of the bolts by drilling them out slightly larger to replace the hold down bolts with 5/16" vs the original 1/4". It wasn't just a what's the next size up, I pulled out the old calculations for the root diameter of the bolts to determine the effective diameter of the bolts and used yield/tensile strength of the stainless (65/100 ksi). Compared this with regular steel Grade 2 (57/74 ksi) and grade 5 bolts (92/120 ksi). Went to work and calculated the bolt strengths. I also got way into the weeds and calculated what the tear through would be for the bolt to tear itself through the steel of the outrigger. Lots of number crunching.
With the size increase and stainless steel they came out stronger than grade 2 grade 5 or grade 8 bolts in 1/4 inch.
Torqued the bolts to 130 in-lb with nylock stainless nuts and stainless washers on both sides.
Overall it cost a little more but I think it will be well worth the small difference.

[judflor]

Sounds good to me. I also am an engineer. You can get some high strength 1/4-20 stainless steel bolts - 17-4 SST 135,000 psi min tensile strength, A286 SST 130,000 psi min tensile strength and Bumax 88 SST 116,000 min tensile strength (see McMaster-Carr catalog); not as high as Grade 8 steel bolts (150,000 psi min tensile) but still plenty strong. I don't know what what grade bolts were originally used Airstreams but I bet you these stainless bolts would do the trick. I am thinking of replacing my corroded steel bolts with stainless steel bolts.

[sailormom]

Thanks for this info on strength. I'm a big fan of SS despite the cost.

[55AS]

Agree. Permanent fix. I got lucky when I pulled up my flooring and found original bolts in very good condition despite being 61 years old. AZ weather probably helped that.

[perryg114]

I used stainless bolts on everything and I put 1x1 angle iron stiffeners on top of the C-channel to eliminate pull through and reduce point loads. The factory really botched the rear of my trailer and some of the bolts were at the very edge of the C-channel which weakens it considerably. The bolts that were used originally were pretty weak carriage bolts. I doubt you could go wrong with even cheap 18-8 stainless bolts.



Perry

[Melody Ranch]

If Wally would have had this available he would have used it. New deck screws are self tapping and strong. They are designed and made for flooring (deck) applications. No need to overthink the old carriage bolt replacement. (only my opinion.) http://www.grainger.com/product/SPAX-Deck-Screw-6PU93?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/6PU82_AS01?$smthumb$

[perryg114]

Most of those deck screws won't thread through metal. In fact, most of the screws designed for sheet metal won't thread through steel even with a pilot hole. I found some stainless sheet metal screws that were hardened and they work well for holding on belly skins etc but they have a hex head that is not flat. Deck screws work fine for wood to wood connections.

Perry

[HiJoeSilver]

Thanks all, gives me a little more confidence that others have gone this route as well. I tend to overthink things sometimes. Well a lot of times. Try to think and plan everything down to the nitnoid detail taking in as many possibilities and options and come up with what will work best. Beat into me by the military, and always want to do things "right", not half a'd like a lot of things in the AS's seem to have been done. A "little" more thought and effort can go a very long way. Still seems true in the new AS's.
Brainstorming how to redesign the forced air to the tanks situation, and replacing the old fresh water gravity fill. Definitely didn't like how either was done in the first place. Been brainstorming all winter think I'm starting to finalize those. Probably post those ideas later.

[55AS]

Originally Posted by perryg114

Most of those deck screws won't thread through metal. In fact, most of the screws designed for sheet metal won't thread through steel even with a pilot hole. I found some stainless sheet metal screws that were hardened and they work well for holding on belly skins etc but they have a hex head that is not flat. Deck screws work fine for wood to wood connections.

Perry

I agree deck screws are best for wood not metal. I have, however, used stainless steel sheet metal screws to attach a new belly pan with good success. Drilling the right size pilot hole in the steel frame is the key.

[cwf]

I like the approach with the thru bolt and nylock in stainless. I replaced many over the years with this upgrade where I could improve bolt strength without damaging surrounding material. If you hold and don't crush the wood, that should hold the shell nicely.

I never used the swap where there was electricity passing. Not for grounds or interconnected parts where electrolysis can increase.

I also like the idea of spreader plates like the angle iron. But I might have tried aluminum instead.

[perryg114]

I coated the steel with POR15 to insulate and protect it. I did this on the fly and already had the 1x1 angle. You can also coat aluminum with POR15.

Perry

[Hillbilly75]

Since I didn't have access to under the frame/hold down plate for through bolts, this is what I used. Hardware man said it was grade 5. Also stepped up to 3/8 and added several extra bolts. They tapped the steel nicely and tightened up great.


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[Melody Ranch]

I just got back to this...Deck screws are made for self tapping metal applications. Thats why they are used.

[perryg114]

You need to have through bolts on the hold down plate and on the corners where the larger bolts go into the frame. Sheet metal screws won't cut it for shell to frame connections. All those will do is hold the floor to the c-channel. Remove lower skins to do it right. No free rides or short cuts here.

Perry

[Hillbilly75]

Originally Posted by perryg114

Sheet metal screws won't cut it for shell to frame connections. All those will do is hold the floor to the c-channel.

Perry

A pry bar under the hold down plate confirms this. All I had done was connect the channel and floor to the plate. A connection has to be made to the cross member below as well. I added a half dozen grade 8 bolts from below, with aluminum shims, washers, and nuts on top. Going to add some steel shims under 3/8 bolts on the corners, including two extra bolts through added angles outside the frame, per InlandAndy's instructions. Big thanks to you, Perry, for making me scratch my head a little bit more. Probably saved me from a lot of trouble later!

[perryg114]

Your welcome. At least you did the right thing and fixed it. Good job.

Perry

[JCWDCW]

Has anybody thought about the galvanic corrosion that might occur with Stainless on the steel frame?

[perryg114]

No way to get around it other than coat everything with POR15 or similar. You can also dip bolts in paint and wet install. You bolts won't rot at least.

Perry

[HiJoeSilver]

Thanks (or damn you) JCWDCW, I had very slightly but your prompt made me think about it even more. Figured there would be very little electrolyte to cause corrosion, but who are we kidding these AS's leak and could pool in the c channel. Lots of research over the last 24 hours. So after researching much more I'm not sure I'm comfortable with the galvanic potential between the Steel and stainless steel, as well as the aluminum. The difference is even bigger between the SS and aluminum which worried me more than the SS to steel. Found several ref's that said SS bolts through steel probably wouldn't be a major factor, but SS to aluminum could. So I'll be replacing the bolts again. This time I'm getting some Grade 8 Ultracoat/ArmorCoat (http://www.kimballmidwest.com/catalo...at%20Flyer.pdf) bolts from McMaster-Carr. Should have dome more research in the beginning and done this first. No issues between the bolts and the steel or aluminum. The coating is actually a zinc and aluminum coating, which sounds perfect.
I'll use bolt with similar coated washers and nuts and then use a backing nut to lock the first nut in place as they recommend not using anything on the threads for lubrication, Loctite may be fine but why chance it when I can just lock with backing nut.

[perryg114]

Use a hot dipped zinc plated washer and stainless steel bolt and that will take care of the corrosion. Zinc is anodic to all of them.

Perry