O-ring Fix

Last updated: August 20, 2002

Fixing a Mazda Rotary with Damaged O-rings

By Tom Jelly, procedure designed by Paul Yaw

One seemingly fatal malady that will occasionally strike even a well cared for wankel is a leaking o-ring condition, where a small leak occurs in the o-rings that seal the flat side housings to the rotor housings. The characteristics of such a leak are continuous ejection of coolant from the radiator cap overflow tube, and white smoke while running or heavy white smoke during startup (usually accompanied by bogging during the first few seconds of runtime until the coolant is out of the combustion chamber) Usually the car will run well otherwise, but consume coolant.

Until now the only way to repair such a condition was disassembly and a rebuild - very costly, and a real bummer on a low mileage or recent rebuild.

My original engine has over 90k miles on it, and I had been planning on a rebuild, even to the point of buying a core engine to rebuild so I would have one to go in when the time came - I figured mine would eventually just puke an apex seal or something because of the high mileage. At the beginning of the '01 driver school season I was on my way to Gingerman and stopped at a gas station in MI for a final fill up before getting to the track. The car blew tons of white smoke and bogged at startup- I had developed a classic o-ring leak. Not wanting to believe it, I just kept the engine full of coolant at the track, and noticed another telltale sign of the condition- a thermostat housing mounted coolant temp gauge will read very erratically between 140 and 180 deg when you KNOW the car should be at a solid 180-205F. This is because of the airspace in the thermostat housing resulting from the ejected coolant.


After the run home I left the car in the garage for a week pondering what to do. I have been planning a rebuild for a while, but it was the beginning of the season and I really wanted to postpone the inevitable to the winter if possible so I started looking around for answers. I found out that Paul Yaw had done a procedure that had fixed a few of these problems over the years and one of them was still working fine after 2 years! I decided to give it a try.

You will need 3 bottles of CRC Block Weld available at Pep Boys. This stuff has copper powder of different sizes in a clear silicate fluid, and the bottle says you cannot over treat with the stuff. I looked after treatment and it really looks like the copper stays in suspension even after treatment, and the silicate stuff seems to get into areas of leakage where capillary action is the means of leakage-there was evidence of the stuff where I had installed a coolant flush kit in the lower heater hose coming off of the water pump. It was ONLY where the hose overlapped the nipple. This indicates to me that there really is no way the stuff could clog your radiator.

You will also need a gallon of purple power degreaser, again available at Pep boys.


  1. Turn the heat control on and fan to 2, car cold and off.

  2. Drain the radiator from the radiator drain plug under the car. Use a really big Phillips screwdriver, because a regular #2 Phillips will strip the plug, and you will be doing a lot of turning of that plug for this procedure!

  3. Fill the cooling system with plain water and drain again. I temporarily installed a prestone cooling system flush kit in the heater hose WITHOUT cutting the factory hose by using a 2” length of heater hose on the water pump nipple to the prestone tee, and the factory hose side to the other end of the tee. You may be able to get away without installing the flush tee, but you have to get all traces of antifreeze out of the system for the CRC to work.

  4. Run the car with the garden hose on, flushing the system for at least 2 mins, then drain it again. Now pour 1/3 of the purple power degreaser into the system, and fill the balance with water, and run the car up to temp. let cool.

  5. If your leakage is bad, you may have to remove the lower sparkplugs and turn over the engine after it cools to get the water out to be able to start the car, I only had a minor leak so this was not required.

  6. Drain.

  7. Repeat 3 times, with a cold flush in between each degrease cycle, then do a clean rinse. Make sure the heater is on the whole time, and make sure the rinse runs clear with no bubbles from the degreaser- you have to get all the degreaser out of the system!.

  8. You should do at least one hot runup after degreasing with plain water to help remove the degreaser before using the blockweld.

  9. Pour in 2 bottles of Block weld and top off with water. Drive conservatively 30 mins.


    This will allow residual cooling system pressure to force blockweld toward the combustion chamber thru the leak point, keeping positive pressure on the leak until the stuff has a chance to work and cool.

  10. Following day, drive the car for another 30 mins and leave for long enough to cool COMPLETELY. You should have no white smoke on startup at this point. If you do, the fix didn’t work and you should add another bottle of blockweld and drive for 30 mins and let cool overnight. If there is no white smoke or coolant ejection, the treatment worked!

  11. Drain off a bit of coolant and add antifreeze, or drain the whole system, add the desired amount of antifreeze, and add the drained water/blockweld mixture back to top off the system.

    We have not yet determined if it makes a difference if you leave the block weld in the system; I plan on leaving it in there till the end of the season and then running distilled water and 25% prestone.

    (This comment was added later after Tom had some time with the fix in place): I would suggest draining and flushing out the blockweld mix if you left it in and the car was fixed - my waterpump seal just went ( the car has over 90k on the original, so that might not be what caused it) as a precaution. No big deal, the pump was an EASY replacement and cost $100 from pepboys. I'm still happy w/the fix, BTW.

  12. Do not follow the directions on the Blockweld bottle- it didn’t work for Paul that way. It may be easier to run thru this with a ¼” OD clear polyurethane hose in place of the coolant overflow hose. This will make it easy to see when the radiator cap pressure relief opens, also, running this hose out of the car nose will prevent you from sucking antifreeze from your overflow tank back into the system and contaminating your degreasing procedure. Use a 2” piece of ¼”ID rubber hose to attach the poly hose to the water pump overflow cap. I did this on a car with no AST, BTW.

This procedure will probably be scoffed at by some on the list as a band aid hack job, but the fact that it solved the problem speaks for itself. At least 3 others did it while I was doing it, all successfully to my knowledge. My radiator remains unclogged. Car runs great, temps same as before. I figured I would give it a shot because I’d be rebuilding anyway, but now I may run the thing as long as possible just to see how it holds up.

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