Replacing the Thetford 31688 Seal Kit Without Losing Your Mind
The Thetford 31688 is the standard seal/rebuild kit for most Thetford cassette toilet models, specifically the 400 and 450 series. If you are dealing with a leaking toilet or a flapper that will not seal properly, this is the part you need. It contains the rubber seal, the spring, and the mounting hardware required to rebuild the valve assembly. I have gone through roughly six of these kits over the years across three different campers, so I have a few thoughts on the process. When you open the box, you get a rubber sealing ring, a compression spring, a metal retainer plate, and a small instruction sheet that is practically useless. The parts diagram for the 31688 shows how these components stack together inside the toilet body. The key thing most people miss is the orientation of the seal. The ribbed side faces down toward the valve seat, and the smooth side faces up. Flip it the wrong way and you will have a leak within a day, which is exactly what happened to me on a road trip through Nevada. I spent two hours trying to figure out why the seal kept failing before I just pulled it apart and noticed the seal was reversed. It was a cheap mistake, but it taught me to slow down on reassembly. The metal retainer plate goes on top of the spring, and the whole assembly drops into the valve housing. The trick is keeping everything aligned while you work from underneath the toilet. I usually recommend doing this with the toilet removed from the vehicle rather than trying to contort yourself under a cabinet. It takes about twenty minutes with the toilet off versus forty-five to an hour wrestling in a tight space, and you are less likely to drop parts into the floor cavity.
Download Resources and Sourcing
You can find the official Thetford 31688 parts diagram as a PDF directly on the Thetford website by searching their support section, or you can grab it through most RV parts retailers like Camping World, Lippert, or etrailer. The diagram is useful mainly for confirming you have the right kit for your specific toilet model, since Thetford has produced variations over the years. I keep a copy saved on my phone because the paper version in the manual is always somewhere between "lost" and "washed." A few online forums and RV owner communities also host downloadable versions that are easier to read than the official print. When ordering, make sure you are getting the correct kit for your model. The 31688 fits the classic Thetford 400 and 450 series cassette toilets, but if you have a Nova or a Terra model from later production years, you may need the 5562 or 5282 kit instead. Wrong seal kit means wrong dimensions, which means leaks, which means you are buying parts twice. I learned that one the hard way during a swap at a friend's place when I grabbed a kit off a shelf without checking the model number first.
Installation Walkthrough
Remove the toilet bowl from the base by undoing the four bolts around the perimeter. Set it aside carefully. Turn the valve mechanism on the bottom side and pull out the old seal assembly. The spring compresses, and the old rubber seal comes out with it. Clean the inside of the valve housing with a rag and some mild solvent. Do not use anything abrasive or acidic. A little rubbing alcohol on a cloth is enough to remove the buildup of minerals and waste residue that accumulates in there. Insert the new seal from the 31688 kit with the ribbed side facing down. Place the spring over it, then the metal retainer plate, and press everything into place. The spring should sit flush and the retainer should be seated evenly. Rotate the valve mechanism back into position and reinstall the toilet to the base. Tighten the bolts in a cross pattern, not all the way on the first pass. Get them snug, then go back and finish tightening. Over-tightening these bolts is a common mistake and it cracks the plastic housing over time. You do not need excessive force. After reassembly, pump the foot pedal several times to prime the seal. This seats the rubber properly against the valve. Then do a water test by filling the cassette and checking for leaks around the base. If you see moisture seeping out, the seal is likely not fully seated or is misaligned. Drain and redo it. It happens more often than you would expect, especially if you are working by yourself for the first time.
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Common Pitfalls and Reality Check
The 31688 kit is adequate for most situations, but it is not a permanent solution. The rubber degrades over time, especially in extreme heat or cold. In very cold climates, the seal can become brittle and crack within a year. In hot climates, it softens and loses its shape. I replace these kits every two to three years as maintenance regardless of whether I see a leak. The cost of the kit is low, but the cost of finding a leak inside your floor structure is high. A single leak under a cassette toilet can damage the subfloor and require a full section replacement that runs a couple thousand dollars depending on the camper. Another issue worth noting is that the Thetford valve mechanism itself can wear out. The plastic cam that the seal presses against develops grooves over time, and no amount of seal replacement will fix a worn cam. If you install a new 31688 kit and still get leaks after proper seating, the valve housing itself is likely worn and needs to be replaced. There is no workaround for that. The whole valve assembly becomes a separate purchase, usually in the $40 to $60 range depending on where you buy it. Sometimes the problem is not the seal at all but the toilet body cracking. Inspect the housing around the valve opening before you install the new kit. Hairline fractures are common in older units and they will let water escape regardless of how good your seal is. I found a crack on a ten-year-old unit that was only visible when I shined a flashlight at an angle. Replacing the entire toilet body is the only fix in that case, and it is significantly more expensive than a seal kit.
Bottom Line
The Thetford 31688 parts diagram is straightforward, and the replacement process is manageable for anyone with basic tools. The main value is knowing what you are looking at before you start, and having a realistic expectation of how long these seals last in your particular climate. The kit itself costs around fifteen to twenty dollars at most RV supply stores. Budget an afternoon if it is your first time, or about thirty minutes if you have done it before. Skip the panic-buying at a dealership markup, order ahead, and do the job yourself. The savings are real and the process is not difficult once you have seen it done a couple of times.
