Around-Oz: Living the Dream!

Fixing a Stuck Blade on Thetford Cassettes

We have owned three Thetford cassette toilets in the last ten years and have had zero problems up until now that is! The valve plug mechanism kept getting out of synch with the operating lever after a little over 12 months intermittent use. This meant that it wasn't fully closed and of course we got lots of nasty smells inside the motorhome. We emailed Thetford (Hayman Reece) using their enquiry form and got an almost instant acknowledgment. Sadly despite sending follow up emails we are still waiting on instructions on how to fix the problem. Most disappointing from such a long standing company. Initially we found that you could force the failing piece back into the correct position, however it would fail again as soon as the blade was opened fully.


Thetford cassettes are well made, but being plastic you do need to be careful otherwise you will end up breaking something. Replacing the main seal is very easy. Replacing the release valve seals a bit harder and replacing the seals on the actuator valve very hard! We came across a fantastic article in the August 2004 (page 213) English mag Motorcaravan Motorhome Monthly (MMM). You can buy back issues on their website. Thetford provided them with a breakaway model to do a wonderful set of instructions for repairing a cassette. And you guessed it - nothing whatsoever about our problem! Maybe it is a tooooooo hard basket job! The biggest problem is that the body is made in two parts and then plastic welded - a beautiful piece of work! However, the bits we need to work on are screwed in before welding - not impossible to work on, but a bit of a nightmare. The easy way out is to buy a new one, but at $160 not really a viable option. Now the following photos look pretty gangrenous, but we can assure you it is quite hygienic and not the "brown stuff"! It is actually sediment build up (14 months) caused by the mineralised water found in most of South Australia. We eventually diagnosed the problem as the arm holding the plug seal was locking on the lifting guide slot. We felt this was happening because the arm was travelling too far. Easy we thought - fit a stop and it will be fixed! Wrong guess! We made a lovely stainless steel stop, but still no joy. The problem with this job is you couldn't actually see the mechanism working. Eventually we concluded that the arm was flexing slightly due to far too much clearance on the attachment point. So to get at the bits, first up gently pry off the two covers shown in the photo above left. This allows you to remove the eight stainless steel self tappers. Remove the rubber seal - make sure that you make a note of which side is the top as you will have to clean off any sediment.


The photo above left shows the plug valve working correctly. You can just see a spigot in a channel at the left of the disc. This is where the thing jams! You need to wiggle the arm until you can remove the plug. The photo above right shows the valve removed. The engineering on this bit appears to be on the weak side. The pin on the valve that lifts it up to seal is very small and must wear.


The photo above left shows an inspection mirror looking up at the screw holding the arm. If you look carefully you can just make out the Philips head screw. This is the root of the problem. It needs a flat large washer to stop flexing, but to work on this you would need to make a special Philips head tool (steel arm and silver solder in a short bit as normal offset drivers are too high). We could see this being a nightmare job and Valium inducing, so we opted for a very simple fix. We noticed that the arm was "leaning" just before it jammed. We put a 3/16W X 5/8 stainless steel round head machine screw in the arm, as shown in the photo above right. The round head slides on the main arm and should last the life of the unit. Assembly is fairly straight forward. The plug just drops in - nothing holds it in place as it can't go anywhere. Place the seal in position and screw on the flange. It is good practice to turn screws anticlockwise until you feel a slight click, then screw clockwise. This stops the screws starting a new thread. Finally push in the slide. It all worked beautifully and we tried the ultimate "destruction test" - got the grand children to use it! Armed with the info on this page you should be able to fix your cassette in under an hour for a cost of under 50 cents! By the way, we have never had to replace a main seal. We always put a capful of olive oil in the bowl after each empty. We find it rather odd that there is nothing on the Australian Website on maintenance and no info in the manual. Perhaps Thetford could look at this in the future?


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Last updated: March 30, 2006
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