then it must be "push" but one way will allow the pin to drop out and then it is a full clutch casing removal job to recover the pin. I stand corrected!
If you really want to test it without running the bike you could remove the sensor and place it in a shallow tray and add boiling water to the tray and test continuity across the terminals with a continuity tester, remove from the water and as it cools the thermostat should switch and break your continuity circuit.
After all the messing about with the plug (heating it and flooding it with fuel) have you replaced it as it could be breaking down. It could be that simple and after all the problems you have had you are looking to deep.
Before you order the water pump seal, remove the impeller and check for corrosion. You may need to replace the casing as the magnesium casings do fail, the new casing will be alli so no more problems. The cost should be about
Should be the the same as the Rev, drain coolant (or lay the bike down, you will loose some coolant) remove the the water pump cover and the the circlip retaining the impeller. Remove the old seal (this can be a little tricky but the seal will come out with a little persistence) DO NOT PULL THE IMPELLER SHAFT as you can pull it out of the drive gear behind the water pump casing. After removing the seal inspect the shaft for wear if all ok grease the new seal with water proof grease and push the new seal into place with a socket that is very slightly smaller than the the seal. Reassemble the impeller and circlip replace the warerpump cover and add coolant. Job done. Flush your gearbox oil two or three times and all should be well.
Kombivan, I have pulled out a very old montesa cota 247 service manual and full parts diagram / numbers manual. It is probably useless to you but the head nuts are 14.5 ft-lbs to 18 ft-lbs the cylinder nuts are 22 ft-lbs to 25.5 ft-lbs
Harsh words, I have two lads that live and breath trials, along with the other members of the youth academy.
You will always have people that come and go, but to lump it on all youth is simply not fair and also will alienate the youth from the sport. Come on get behind them, my lads quite often talk about older riders that have given them time to explain a different line or just talked about trials and the bikes in the queue.
or iPad 
Water Pump Seal
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Glad to hear it all went well, nice clean oil now?