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Re: [Shop-talk] Brush trimmer tales of woe

To: "'Shop Talk List'" <>
Subject: Re: [Shop-talk] Brush trimmer tales of woe
From: "Randall" <>
Date: Tue, 4 Aug 2009 11:47:47 -0700
> And the issue with my Echo engine was a scored cylinder - the piston crown
> was just fine rather than eaten up as you'd expect from a typical low
> octane preignition issue.

Somewhere I've got a TR3 piston that looks nearly perfect, until you turn it
to see the broken ring lands.  Almost certainly from detonation (aka
pre-ignition) caused by excessive oil burning (which dramatically lowers
octane) in that cylinder.  (Valve guides were badly worn, and one of the
stem seals I had added to band-aid the problem came apart.)

I'm not saying your problem was caused by low octane fuel; just that
detonation damage takes many forms.  I've read that combustion pressure gets
behind the top ring to force it harder against the cylinder wall, so maybe
extremely high pressures (like from detonation) could cause too much
pressure and subsequent galling?

> It seems odd for a 2-cycle garden implement, especially since they're not
> all that high in compression and should be intended for less-than-optimal
> operating conditions.

Seems odd to me too; but I have no idea what the dynamic compression would
be.  High power in a small, light package would certainly seem to imply
reasonably high compression, and I assume it burns a fuel/oil mixture, which
would lower the octane.

Still, I can't blame them for saying "this is what is required" ... if you
don't like it, don't buy their product.

-- Randall 
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