Has anyone had problem with dirt coming into a gas engine via the carburetor and destroyed the cylinder.
I have an engine that has got a scratched cylinder and the seller says that an object that has come from the outside has damaged it. I believe it has been damaged by a part from a broken bearing.
The engine has a rear carburetor.
I fly from a grass field but I have never thought that an air filter is needed. Expecially with an engine that has the carburetor inside the fuselage.
Maybe I am wrong but few people seem to use air filters.

If it's a weedeater or chainsaw carb (Walbro, Tillotson, Zama) the carb has a filter (screen) built into it under the cover. Unfiltered fuel or large particles cannot get into the cylinder via the fuel.

That is right. But in this case it is supposed to come via the air.

I am very interested in what you discover. I fly off of a field that is dirt and grass. Inside my cowl and on my velocity stack, I have dirt/sand. I know it has to be going inside the engine but I have never heard anyone having an issue that is dirt related.

My question to others is, do you fly with an air filter on your gas engine?

I know most of us don't run one, myself included, most of the time...
but they all should really have an air filter of some sort, if we want
to wring maximum life out of them.

All it takes is a single piece of grit, to do the damage you described. (The scratched Cyl)

Did the engine have a broken bearing too?
Or is that just something you thought may have caused the scratch?

If everything else is intact, they're probably right about debris coming into carb.
If something else internal is fubar, then I'd have my doubts.
(Though it's still possible that excessive dirt entry could have
destroyed the bearings, too. Especially in a two cycle engine)

I know folks who will say they've been doing it sans filter for forever,
with no problems at all... but let the right piece of grit get sucked in,
just one time, and BAM.

It was discovered by the supplier when I sent it in for repair. They found a small piece of metal 1/8-inch-wide and 1/4-inch-long.

This is one of those cases where you have to just take their
word for it, since you didn't tear down prior to shipping it..

If they say it isn't from the engine itself, then
it's certainly possible that it got sucked into the carb.
(However likely/unlikely that may be.)

The only thing that's for sure is that you'll never know.
Still... in the future, an air filter would definitely not hurt.
(Well, as long as it stays clean, anyway. Check often)

So tell me,
Who makes an air fliter for a RC engine?

http://www2.towerhobbies.com/cgi-bin....pgm?C=GMJ&P=7

I always used the Bru Lines, personally.
Some of the car engine filters will also adapt nicely.

As for the giant stuff, you may have to get a bit
more inventive, if the manufacturer doesn't offer an option.

It's not hard, really... especially if you add a tube
to get rid of the gas spit that some of the carbs produce.
Just find a piece of rubber fuel line that fits over
the tube, and glue/ziptie/etc... a piece of pantyhose over it ;-)

None of these will fit my gas engine 2.4ci.
Any more suggestions?
I welcome a solution!

If you're serious, and you're talking about the Brison 2.4,
while I can't find anything on the net, it appears
it would be trivial to fit a filter to it.

You could make a simple one of thin sheet aluminum
with a gasket on one side, and the filter media glued
to the other. Or, K&N makes all sizes of filters, down to
5/16 opening (.40 size) and an inch or so long.
Again, if you're handy at all, it would be trivial to manufacture
a mounting plate for your carb, and a short inlet for the filter.
(Don't know how much room you have. May have to play with routing)

I'm truly amazed that the manufacturers don't offer filters
for their engines. Heck, even a weedwhacker has a filter,
and it spends most of its life much farther away from the
grass/dirt than your airplane does on the taxi out, and takeoff
(And that's without a big fan helping to stir stuff up, too.)