Tip: Liquid Shim
 

I adopted a technique long used on the combat aircraft production line that served my homebuilding purposes very well. During the construction process, even military aircraft frequently have situations in which gaps between mating parts occur and must be shimmed prior to permanently fastening together. Particularily problematic is a "tapering gap" condition. Frequently, mating the wheel pants to the brackets exposes this condition and I'll address that in a moment.

We've all seen RV's with puckers in the fuselage skin, usually at an intersection of longeron and bulkhead or stringer. This is caused by riveting the gap closed rather than taking the time to shim out the open void between the parts. On the production line, a worker can easily spends hours and hours with a piece of aluminum shim stock and sanding disc in an attempt to fashion a shim, tapered or otherwise to fit acceptably within the confines of the gap. In an effort to reduce manhour costs and enhance quality, a space age material was developed to address this tedius process. I don't know what the actual material was, we production types simply referred to it as "liquid shim." While the exotic material we used at McDonnell-Douglas is not generally available, I often made up an ersatz home brew which consisted of nothing more than West System epoxy with lots of cotton flox mixed in (roughly to the consistency of peanut butter) to make it structural. The process goes far faster than it sounds. After masking off the local area with bits of tape, the mixture is forced into the gap between the parts (i.e. bulkhead and skin) until some of it oozes from around the parts and out the rivet hole. Wearing latex gloves helps. The liquid shim will find its way throughout the void and fill in the gap. Using a plastic scraper, clean up the excess ooze-out from around the general area to avoid excessive sanding later. After allowing the mixture to cure overnight, the hole is then reopened by passing a drill bit through it, the countersink retouched and finally the rivet shot. There you have it...a perfectly pucker free exterior skin.

Among the most important applications for liquid shim I found was precisely fitting the wheel pants to their mating metal brackets. On my RV, virtually all of the brackets met the interior of the wheel pants with a very noticeable and often tapering gap. It is tempting to simply torque down the screws mating the pants to the naturally flexible brackets but I believe all this really does is "preload" the brackets with an undo and constant stress and doing so is quite likely to be a major reason why so many builders have reported cracking of the brackets over time. To address this common problem, an after-market stainless steel type wheel pant bracket became available. By shimming the brackets to the pants precisely in the first place, I believe the problem would go away. That's only my opinion though.

Some builders have used a variation of this technique to precisely "float" hinge halves into place particularily on the flaps and cowling. In fact, I long used a similiar technique to help insure interchangeability between avionics doors on the T-45 nose cone. It was required you had to be able to remove the piano wire holding the hinge halves together with slight finger pressure alone, no needle nose pliers or vice grips allowed, and demonstrate this with any random door pulled from stock. Believe me, it's not as easy as you might suppose. Just imagine removing a flap from your RV and having it fit the RV parked next to you! Liquid shim proved to be the only realistic way to consistently maintain random door interchangeability in part by liquid shimming its mating hinge half to the structure on the nose cone door thus assuring repeatability.

There you have it....an easy building technique I'm sure you can find any number of situations to use it in.

Rick Galati RV-6A "Darla"

Thanks Rick! I really like this idea and will use it to smooth out a couple of lingering dents in my fuse.

liquid shim
 

Liquid Shim = Hysol EA 9394

http://www.loctiteaero.com/Images/Da...ol_EA_9394.pdf

Also is an excellent paste adhesive with good high temp strength.

Thank you Brice!

That link really jogged my memory. Hysol is EXACTLY the material we used in production. Imagine mating a very large skin....the size of a couple of ping-pong tables onto a structure containing machined ribs, spars, intercostals......you name it. It is a virtual certainty that gaps will occur in any number of random and not so random places. Hysol....."liquid shim"......was applied in virtually the same manner as proseal but the intent is to fill in all voids and gaps between the mating surfaces. It cures very hard and generates A LOT of dust particles when sanded, respiratory protection is mandatory.

I am constantly amazed that the Internet has truly democratized the sharing of information for the benefit of all. Thanks again Brice for sharing the link and reacquainting me with an exotic material I worked with countless times yet had forgotten about years ago.

Rick,

Is that liquid shim considered to be 'structural'? I'll probably have to do a little shimming between nose ribs and joint plates to make a nice transition between the wing leading edge and fuel tanks. Hysol sounds better than working down a piece of aluminum, but I'd be riveting through it.

Anybody have an idea where you could find this stuff? I didn't find much useful info on Google.

Thanks,

Dave

Liquid Salvation!
 

Amen to all of the above. I know first-hand of 2 major airframers where life would be a lot more complicated without liquid shim. Also useful is the "combination shim" approach: getting a solid shim fitted so that it's close, then using the liquid shim to fine tune the fit. A further refinement of the technique is to allow the shim to cure, then apply a bead of sealant along the edges. Reason being that liquid shim can become brittle, crack, and (in some cases) fall out. The extra weight of the sealant bead is justified by the fact that it traps the shim, so it stays where you want it.
I've used good old J B Weld with great success as a liquid shim at home.

Dave,

The real deal (Hysol) is normally drilled through after cure. There are some variations of Hysol and I cannot say for certain if 9394 is the precise mix I used. Reading the data sheet, it sure seems like it though. Interestingly, it is even referred to as liquid shim.....I thought that term was "shop slang." Like many items we used on the production floor, I tend to doubt it is even available for the retail trade. Note that it is mixed 100 parts to 17 parts by weight.

Using West system epoxy with cotton flox as I mentioned has worked very well as a "liquid shim" substitute for me. After cure, it drills very well. Using a 3X rivet gun seems to have no effect at all even when setting AD4 rivets. I have noticed no degradation anywhere I have applied it. Inside the fuselage, it is Akzo primed like everything else and you wouldn't even know it is there. Bonded to the interior of the wheel pants, a location where it is (arguably) subject to the most stress, it looks as good today as the day I applied it...approximately 300 landings and a tire rotation later. But prudence dictates I must qualify my personal anecdotal experience with a familiar caveat...YMMV.

I have used Hysol products for alot of things and it is a great product, shelf life is not that long and you usually have to buy a fair quantity depending on the product.


My source has been,

E.V. Roberts
8500 Steller Dr
Culver City, Ca 90232
310-204-6159

EA 9394
 

This is an adhesive. The irony of riveting through it is that many are using this for bonded structure to eliminate mechinical fastners. Your milage may vary based on surface prep, mixing, and cure conditions. I've never seen anyone rivet through it, but I can ask around.

1 year shelf life
 

Actually, the shelf life is not too bad - it's one year at 77F according to the data sheet listed previously...

http://www.loctiteaero.com/Images/Da...ol_EA_9394.pdf

Do you know what is the smallest package and cost is from EV Roberts?

gil in Tucson - but I used to live a mile or so away from EV Roberts...