Thetford mount flange cracked

AlphaFloor

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Dec 17, 2022
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Fresno, CA
Trying to sort out waste tank leak. After removing 2 pounds of silicone sealer I discover the white flange the Thetford valve mounts to is crack at the bolt bosses. There were some old posts I flound about converting to a Valterra valve but mostly they just referenced the photos with a dead link. Anybody have an idea on how this can be done or how to fix/replace the flange?
 
Complete Thetford valves are available from Applied GMC and Amazon to name a couple.The new valve bolted right up to my 1978 black tank and fixed my problem.
The Applied website has a good picture of the backside of the valve and the oring seal.
 
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I can try and fix the broken ABS retainer plate but worried it will just break again. I am not sure if the retainer is removeable or not. I saw some old posts talking about how to convert to Valterra valve but they all said see my photos for how it was done and that was a dead link. Hate to but $425 tank to solve the problem.
 
I read that the photo site is down for a bit, I have had the bad mount issue for a few years and think I've seen most of the pics you were looking for as I have looked at options. The GMC upfit coaches used some poly something plastic tanks that are hard to attach fixes to. Coachmen used more repair friendly ABS tanks. If you are certain your issue is that joint, and you are about ready to pull the whole thing out, maybe a slathering of 3m 4200 sealant would get you a few years. when that leaks, you would need to replace the tank. I have slathering the 4200 and old, semi-setup lap sealant all around that joint for at least six years and stayed functional. Better? no. cheaper? yes
 
The first order of business is determining if you have an ABS tank or a Polypropylene tank. Make no assumptions here. I've done a ton of research and reading on this point, and have not found a cut-and-dry pattern as to which coaches have ABS tanks and which have PP. Some folks have said it was this upfitter or that upfitter, this year or that year, but I've found counterpoints in each case.

So, if you scrub ABS with a rag soaked with acetone, it'll start getting sticky on the surface and the rag will start to bite/grip if you know what I mean. This won't happen with PP. Or, you can glue a scrap of ABS to the tank with ABS cement. If it's PP, it'll peel right off after setting up. This is the "peel test" you might see referenced elsewhere.

As far as fixing the flange, I've repaired two this year with plastic welding. I did it a little different each time, but in both cases the results were good! Both RVs have been holding water without dripping.
 
For the '76 Palm Beach, the posts referring to the black tank repair are #216 and #232. There may be other mentions elsewhere too.

And on the '75 Glenbrook, the black tank repairs are in posts #222, #225 and #226.

I approached the repairs differently for each coach. On the Glenbrook, I cut the crossmember out of the way, then welded it back into place. I also retained the original thetford valve parts, and reassembled things with an o-ring I sourced in a random bin of Caterpillar parts at my local truck/tractor parts store. I could not find such an o-ring elsewhere. Ignore comments about PP being unweldable without an inert gas cover-shield. That isn't true if you get your plastic welding iron set to the right temperature.

On the Palm beach, I decided I wanted more readily available parts, so I switched to a Valterra valve. It has the wrong bolt-circle for the four mounting bolts on the flange. These mounting bosses were so busted up anyway, I decided to abandon them completely. So I got a Valterra flange, bolted four thread-inserts to them, and used the flange as a pattern to construct a new bolt-pattern on the tank. I buried the thread inserts in PP rod and bonded them to the original flange. I used lots of anti-seize during assembly, in hopes I won't over-stress the plastic during disassembly 20 years from now.
 
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Thanks Paul. I think my tank is ABS but will go wipe it with some acetone if the rain stops. My tank is solid but the mount ears on the flange are broken. I will take a look to see if I can get a meal plate behind them to bolt to or if I need to try and glue the flange back together. I believe the flange is ABS. Vintage trailer has a new flange for $12 but I can't think of a method to remove the old flange without damaging the tank. My sender also leaks, rod rusted off and screws stripped in the tank. Will need to patch over that as well. Plan B is to get some 3M 5200 and glue it back together and worry about tank change when that fails later on
 
Do you have pictures of your flange? If it looks like either of mine (in the links above), it'll be PP most likely.

If your tank is ABS, your flange will be ABS. If so, you have chemical options that are easier than plastic welding. You can make your own slurry with scraps of ABS dissolved in acetone. You can thicken it as desired, and apply it as needed. You could groove out contaminated cracks and fill them with fresh material. Of course, ABS can be plastic welded too, much like the PP. I've had to do that to the grille.

If you have PP, don't count on 3M 5200 doing much to seal things off. I plastic welded a patch harvested from a thick trash can over the sending unit hole on the Palm Beach. Each screw hole had a crack propagating outwards. The damage had grown quite a bit. I grooved out each crack, welded it closed, added the patch on top, and reinforced it with mesh. Hopefully that keeps the cracks from growing back.
 
Went out tonight, pulled the valve off Mr. Stinky and scraped of five pounds of silicone goo off.
Bad news, the tank, valve and flange don’t mind acetone at all. None of it got sticky. Tank seemed thin and rigid, not thick like poly. I took a picture while I was underneath and will try and attach it.
Three out of the four “mounts” the valve attaches to are no longer attached to the flange. The last one is only cracked halfway through. I think somebody tried to overtorque it to stop the leak from the split o-ring. Access sucks for getting at the upper mounts. I would like to find a way to polish the turd (pun intended) and get this to work for now. I am planning to pull the interior out to fix damaged spots next year so that is a better time to replace a black tank if I have to.
 

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Yep, that sure looks like the PP tank I fixed in my Palm Beach. Mine was a little worse if it makes you feel any better. The sealing face of the output flange was even cracked, so the o-ring didn't stand a chance. Not to mention where the tank itself was bashed in and cracked all over... Probably a 6" affected area at the bottom corner of the tank. With multiple cracks and flaps that had to be brought back together. It was a real mess.

But I have access to a lift so I could repair it standing up, already owned an airless plastic welder, and didn't want to tear the interior apart or pay for a new tank. It took several hours to repair, but it was kinda fun and satisfying.
 
you had a welder from Polyvance? Which one was it? I have never welder plastic before. Hope not to spend more money on tools than the tank would cost. Harbour Freight has plastic welders too, not sure what to look for though.
It isn’t a bad time to change it as my water tank is out at the moment and back wall is off the bathroom. Just worry if I take the interior out next year I will wind up making it leak when I reassemble it.
 
I've got the Polyvance 5700HT. I'm pretty sure it was a lot cheaper when I bought it than it is now. It does work well. There are a lot of knock-offs out there that look similar, but I can't vouch for them personally.

If your water tank is out and the back wall is off, that's one step in the right direction. If you can gain access to the other tank connections too, maybe changing it out won't be so bad.
 
Trying to sort out waste tank leak. After removing 2 pounds of silicone sealer I discover the white flange the Thetford valve mounts to is crack at the bolt bosses. There were some old posts I flound about converting to a Valterra valve but mostly they just referenced the photos with a dead link. Anybody have an idea on how this can be done or how to fix/replace the flange?
 
Thanks for all the tips and advise. I am thinking I will just change the tank. In it’s favour is interior somewhat apart so that job is easier. If I want to weld the plastic flange mounts up I think I will have to remove the crossmember immediately behind the tank and that isn’t s good idea right now. Currently the rear subframe is out of the coach to get repaired. Several people who couldn’t weld have “welded” trailer hitches on. I have the coach supported on jackstands at the crossmember behind the tank. Worried if I take that out things might change shape in a bad way
 
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That all makes sense. Sorry you have to buy a new tank and install it--it's never a fun thing to admit to oneself. I maybe should have bought a new one, but I'm resistant to spending money, already had the plastic welding tools, had a plasma cutter and welder on hand, and the coach was up on a lift. Those were a lot of convenient factors that made the repairs more doable. Even so, I had a fair number of hours sunk into three separate repairs on that tank. I also had two coaches to fix, and I really didn't want to buy two tanks and dissect two interiors.

Once it's said and done, you'll probably be pretty happy to have a new tank.
 
I would have to buy the welder and it’s about $240 now. I can pick the tank up at Applied and save the fortune shipping would cost. I am now told the black tank is my Christmas present but nobody seems me to want to wrap it up and put it under the tree 😳
 
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I dropped the old tank yesterday. Not a bad experience. I drove around with it partially filled with water and a lot of dishwasher detergent. No stink at all left.
Good I got a new tank, all three plastic fittings in the top of the tank were split where the fittings screwed in.
 
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