GAS in the MAgneto

As long as the motor is running good do what Venice said. When the time comes, and it will, to replace the seal, usually when the motor starts running rough, here is the only source I've found for a replacement > Motorized Bicycles Gas Bike Engines Gas Powered Bikes Motorized Bicycle Parts
You'll need to remove the magneto rotor (magnet) by removing the nut that holds it on, prying the rotor off using two large screwdrivers and 'wiggling' the rotor off the crankshaft. There is a woodruff key that you don't want to loose. Be careful removing the magnet. The crankshaft is easily bent if you apply too much force so go slow and be easy. Use a sharp tool to pry out the old seal, clean the bore before installing the new one. Find a socket that is the same diameter of the seal and use it to drive it in making sure you seat it completely into the recessed area (seal bore) where it is supposed to go. Use some oil on the shaft and the inner surfaces of the new seal when installing. The most common reason for seal failure is the original not being driven into the bore completely and the magnet rubs against it causing friction which destroys the rubber. Hope this helps. We're here if you have problems.
Tom

DonnnN said:

What causes these seal's to fail over time?

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In a perfect world, and in theory, the seals should last a very long time but in reality they simply wear out. Microscopic solids that move with the oil/fuel work on the close tolerances in a shaft seal eventually wearing down the 'lip' or sealing surface. In our application heat from combustion also promotes deteriation of the rubber seal.
Sometimes, as I stated above the cause of seal failure, especially premature failure is due to the seal not being installed correctly at the factory. If it is not driven completely into the recessed area designed for it the magneto rotor will rub against the outer lip quickly wearing it out and causing failure. Another reason was also given here. Any scratch or surface imperfection on the crankshaft will quickly damage a seal.
Interesting side note here is the unavalability of replacement seals. The factorys that produce the 2 stroke engines apparently have them made especially for these engines. I did extensive searching for a replacement and the closest I could get was a place that would custom make them but they would only make them in quanity. $300.00 for the first 5 seals, and you had to order 300. You could replace your engine much cheaper than that. One of our sponsors is the only source that I've located for shaft seals for the 2 stoke engines. Motorized Bicycles Gas Bike Engines Gas Powered Bikes Motorized Bicycle Parts If anyone knows of another supplier please let us know.
Tom