Dear Jack and Grumpy,

Many thanks for your two excellent replies; The scales literally fell from my eyes as I read them. Yes, of course - excessive crankcase pressure!
It makes so much sense now that you have suggested it, for several reasons...

Firstly, if we are talking about oily vapours under pressure (rather than oil in liquid form) it would be so much easier for them to defeat the O-Ring in question (still in perfect condition!) at the bottom of the Filler Tube...

Secondly, pressurised vapours would have no trouble making it up to the top of the Filler Tube and defeating the seal up there under the Cap (as evidenced by the photos)...

Thirdly, it would explain why the problem has persisted since I changed the oil 5hrs ago, even though I swung over at the time from SAE30 to 20W40 in the hope that the thicker product would cure the leak (in the event that the O-Ring was indeed the culprit). Vapours obey no viscosity laws, regardless of the rating of their parent oil...

Fourthly, it would account for why the thing's Plug has been consistently dark around the cylinder-end of the threaded section, since the motor was brand-new. Have a look at this photo that I took just this arve:

[Linked Image]

This is more or less how the Plug has looked since new. The electrodes actually used to be even darker than what they appear to be in the photo, but the recent change to the thicker oil seems to have agreed with them, which would be consistent with the "blow-by" theory...

Now, with regards to the cause of the said excessive pressure, it surely can't be a blocked Breather (surely?), because the problem has been there since the motor was new, and I've always been reliable with the oil-changes since then anyway. Nonetheless, does anyone know offhand whether they are serviceable? I had it off 5hrs ago and never gave it a second thought at the time, because I was distracted with "O-Ring"-type thinking...

At any rate, I like the "Chongqing-Crap-Manufacture" theory a lot, because something like a slightly "out-of-round" Bore would transcend any counter-arguments citing the motor's newness and lack-of-wear, and would readily explain the dark Plug colour since new. I actually took the Mower back under warranty only 6mths after purchase back in 2006 to complain about the Plug-colour (thinking that it could be indicative of excessive mix-richness), but the mower shop couldn't find anything amiss. I'm now wondering if the buggers bothered to check the compression? Well, it's a good excuse to buy myself a Compression Guage I suppose. That would surely be the best first step in determining whether the culprit is the Breather or Blow-By... Hmmm... Maybe I should also try putting 2-stroke oil in it...(Joke!) grin

Once again, Many Thanks Gentlemen, and if I don't speak to you again before Christmas - Best Wishes to you both!

Moby (but with an imagined "W" between the "O" and the "B") Mick...