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Old 11-28-2016, 08:29 AM   #1
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1968 20' Globetrotter
ANN ARBOR , THE GREAT LAKES
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1967/1968 Black Tank Modified Thetford to Valterra Adapter Adventure

Like many, I merrily dumped with a Thetford to Valterra Adapter. One day, realizing that a little dribble was actually a tiny crack in the attachment of the Thetford valve mounting flange to the ABS tank, and not a leaking Thetford valve or Valterra Adapter Bayonet, I removed the black tank to investigate.

I expected a polyethylene tank, but found that the original tank Jackson Center supplied in my Fall of 1967 built '68GT was insanely flimsy ABS. The tank's ABS wall is so thin that it is unimaginable, it's like a plastic 2 liter pop bottle.. I pencil sketch on paper thicker than that. Who in their right mind would simply glue a 3” PVC hub into it with no reinforcement, or a molded-in flange, or back-plate?? That is what Jackson Center AS specified... Ignorant and destined to fail.

Since there is no reinforcement, the slightest movement, as would occur in attaching a dump hose or road vibration, would cause the thin ABS to crack. Due to the coagulant nature of black tank contents, this crack may only weep a little intermittently and go unnoticed for years, rusting the supporting pan and saturating its cheap open-cell foam insulation with you know what...

The ecologist in me discouraged more plastic in the landfill and supporting additional plastic manufacturing. The Hillbilly in me said fix this tank with junk already laying around the shop. Other than the crack, the tank was wholly intact and serviceable, so I chose to mend the crack.

Who doesn't love JB Weld?? They make a kick-ass epoxy plastic adhesive which flawlessly sealed the crack after prep with plastic primer-cleaner (purple acetone I suspect). I cautiously spread the crack open, packed it with JB plastic epoxy, and gently clamped it closed. Reaching inside the 3' PVC flange I cleaned, primed, and layered JB around the flange inside the tank, more for stress distribution than leak prevention.

The long bayonet stack of black tank discharge componentry creates a tremendous moment arm working against this join, so I reinforced the flange to the tank with metal, stainless bolts, and JB plastic epoxy as illustrated. With the tank flange secured, next went on a new Thetford valve and then the original PVC hub which allows grey-water to bypass the black tank… All restored to original, but not meeting modern codes.… So, simply add another 3” gate valve (Valterra) downstream from the Thetford hub, open the Thetford, close the Valterra, and you have a Twenty gallon combined tank (That is the capacity that I measured the system filling with gallon jugs).

Airstream manuals of this era never disclosed the actual tank capacity, only stating that the tank holds “about a week's worth”. Coincidentally, it's the same twenty gallon black/grey combo tank capacity that AS furnishes in the 2017 Sport model, but the modified older configuration is actually superior, with the flexibility to discharge grey-water separately when “allowed”. I fitted a 12V tank heater 'cause old GTs never had forced air tank heat. So far, so good.

One of the remarkable features of this generation trailer is that the sewer discharge apparatus is well protected in the “bumper box”, so there is never a ground clearance issue with dump valves, and the height of the discharge port is advantageous to dumping kinetics. There is a door in the bumper box floor that opens to facilitate the handling of the hose. That means that all the effluent discharge attachments, ideally assembled, must fit into the bumper box.

With the Thetford to Valtera adapter, plus an additional Valterra Gate, plus a Sewer Solution or back-flushing elbow/s, things are a little tight.
So, the “Thetford to Valterra Adapter” not only adds a few inches of “Moment Arm”, but also makes for less handling room inside the bumper box. The “Adapter” also restricts the 3”D flow to 2-5/8”D flow. I've seen where some folks permanently affix the Thetford to Valterra Adapter with glue or screws.

I google searched thru ten pages of “Thetford to Valterra” and didn't find what I did next… I can't be the first to discover this… It's too simple...

Driven by intuition, I studied this idea quite a bit, because there is “no going back”. I measured everything more than twice, considered that a thicker O-ring might be needed, and/or the Thetford hub-face might need face milling?? Reaming OD-ID interference?? Nope, none of that. The little ear-tab alignment/centering appendages on the Thetford hub ears, which I'll call “Valterra Preventers” can simply be sawed/filed off, and that's it.. “We don' need no stinking adapter!!” I can't say for other years, but '67/'68 for sure.

And it's stronger… The Thetford bayonet only has two hooks to engage the Thetford hub which has four “ears”. Both Thetford and Valterra pipes have the same 3”ID. The original Thetford hub face mates to the Valterra O-ring with proper compression and no leakage. The four Valterra hooks grip the four modified Thetford ears firmly, so now there are four points of clamping instead of only two. Additionally, if you break one Thetford hook, you're done for if you don't carry a spare. If you break off one Valterra hook, you still have three clamps. The industry has trended toward Valterra. Valterra is inexpensive and very available. Thetford is expensive and difficult to source quickly. The Valterra valve seats securely onto the modified Thetford hub, and I found no need to set it with a screw, or silicone the O-ring.

So, that's the way it worked out for this Hillbilly's 1968GT. Pretty much a low cost, win-win, fixing the leak, rendering the Thetford to Valterra Adapter unnecessary, and adding a waste-water discharge option.
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Old 11-28-2016, 08:58 AM   #2
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1967 22' Safari
MILAN , Illinois
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Rework of Thetford to Valterra waste drain parts

Nice job on this project. I have been thinking about doing this to my '67 Safari. I may have to ask you for guidance when I work on mine next spring. Thanks for posting all the pics! Ed
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Old 11-28-2016, 09:30 AM   #3
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1967 26' Overlander
Bugtussel , Oklahoma
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Thank you very much! I may try this with the original tank on my 67 Overlander.
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Old 11-28-2016, 11:00 AM   #4
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1968 20' Globetrotter
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Thanks ED,

While I was going through the re-do of the bath floor (the PO had “replaced” the bath floor without regard to structure for his eBay sale photos), I mocked up the possibility of a grey tank.

I'm certain that on the '67/'68 one could snake a 1-1/2” drain alongside the existing black tank, and stay within the support pan.
I took pictures to study, shouldn't be hard to use the empty bay ahead for grey-water tank. The frame cross-member isn't in the way, and it already has a huge opening in it.

There are grey-water plumbing runs accessible above floor and within the main frame rails, you'd just have to decide where you wanted to tie into them and add a 1-1/2 gate going into the Thetford hub's 1-1/2” inlet. Looks totally do-able. The new tank could vent to the shower vent pipe inside the pocket door bulkhead… Honestly, it looks kinda easy...


The black tank, grey-water drains and vents are configured to fit the bath's unique floor-plan. So I'll respect the original design, feeling that too many of these vintage '67/'68 baths get ripped out and discarded for the sake of more tank capacity.

This might not be necessary for the smaller single axle sizes, but for the tandem axle, family sized trailers, it looks like a good plan to increase tank flexibility without major redesign.

The '67/ '68 bath, I find well planned, comfortable for this six-footer, and highly functional. Mine is in nearly mint original condition, so I look for ways to work around and preserve it. It has a very open feeling, and I do enjoy the contemplative view out the huge back window, while sitting on the can.
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Old 11-28-2016, 12:35 PM   #5
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1968 26' Overlander
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Excellent posts. I had no idea how fragile the black hole is or that there could be room for whole other galaxy around the outside of it.
I am sending you all about a week's worth of Karma.
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Old 11-28-2016, 03:24 PM   #6
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1968 20' Globetrotter
ANN ARBOR , THE GREAT LAKES
Join Date: Apr 2011
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I have internet images of 1968 black tanks that are polyethylene. Thetford made the Polyethylene tanks. That's why I thought my GT would be that way. Why my GT is ABS, I can only guess that it was cheaper at first, and later proved to be a bad idea, or maybe the polyethylene tanks were in the California models. The GT Land Yacht trim has some other cost “savings”, so maybe it's just a GT issue.

Below is a photo of the crack before repair. The top of the ABS tank is flat, about 1/4” thick and has a lip around its edge which sorta locates it in the galvanized pan. The thin ABS tank walls and bottom, seemingly vacuum formed, is glued to the top. The foam surround supports the walls and bottom, otherwise it would blow out. I see no logic in its design. Especially that there is no reinforcement where the PVC hub adheres to the thin ABS. It's just stuck into the hole with some glue smeared around it.

Here's a picture of the frame bay ahead of the black tank where you can see plenty of room and passage thru the cross-member. You might fit a 20 gallon grey tank in there. That yellow-orange stuff?? 1968 was the only year that AS sprayed the under-floor with closed cell foam instead of fiberglass batting. I found the foam to be well adhered to the floor, and there was no off-dusting found in the bellypan. There was no evidence of critter infestation or gnawing anywhere in the foam. I think the foam was a good idea as long as the plywood is dry.. I submerged a piece in my pond for a week, and it absorbed no water.
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Old 11-28-2016, 05:07 PM   #7
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1968 24' Tradewind
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Brings back memories of all the issues with my Trade Wind. You are correct about the thin factory tank. I bit the bullet and bought a new fiberglass replacement from Inland. It was expensive but very well built and it saved me a lot to time so for me it was worth the money. Looks like you did a fine job.
I had to replace my bathroom floor too. I put a couple of layers of fiberglass over the wood between the frame rails and all the way to to the door. It did two things. First was it really stiffened up the floor in front of the toilet. Second, it made the area around the flange waterproof in case of future leaks and stiffened the floor around where the flange is attached.
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Old 11-28-2016, 09:50 PM   #8
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1967 22' Safari
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Black tank

I think the reason for the thin ABS tank was in consideration of weight not necessarily to cheap out on the tank. When you consider the thought and design that went into these trailers back in the day it wouldn't surprise anyone that the tank would prove both viable to hold the weight of the liquids and solids and still be weight distribution effective. Especially when design did not have to take into consideration Gray water waste back then. That stuff all just went to a Gopher Hole or directly onto the ground. Much nicer now to install a Gray Tank to handle this usage of water/waste. Ed
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