Model Forum / Radio Controlled / Land Models / September 2004

If your sure its the connecting rod thats worn and not the crankshaft  I
would get that part and then send the motor off for a recrimp job.
There are several good people on eBay that do this and it works pretty good
and is cheap if your on a budget.
(Had several friends have this done with worn motors and they worked well)
You could try just the new connecting rod before you send it off as well and
see if it rids the problem.

Just an observation on piston/liner wear. I've read and heard loads of
opinions on how to judge a good fit, and diagnosing wear marks etc.

In the late seventies through to the early 80's I was into a sport called
FAI Combat which involved .15 sized aircraft engines. One of my favourites
was one called a "Cox Conquest", pretty much a clone of the Italian Rossi.

The Cox had a shocking (and partly deserved) reputation for mech failures,
in my experience mostly around the front bearing. Anyway, to get to the
point (at last:-) during FAI combat it was very common for the motors to run
lean toward the end of the "tank" because the tank was a pressurised bladder
which could minutely lose pressure toward the end. My Conquest pistons would
routinely have awful looking black scorch marks running from the crown to
skirt, typically worse around the exhaust port area. People would look at
them and shake their heads. They would turn them over and comment on the
(lack of) compression. People would snigger and sneer, Rossi and Nelson
owners would stand in small groups muttering to each other .... OK not
really, but you get the picture - my piston/liners would be simply discarded
by most people.

But with the (increasingly tricky) job of starting accomplished, the jaws
would drop. Those engines would simply *scream* - the performance was
completely off the scale (for the time), low (ok, almost non existent)
compression notwithstanding. Or indeed, maybe *because* of the low
compression. I didn't know (or care).

So, I say to you, the *only* metric for piston/liner wear is performance,
pure and simple:-)

Mike

Did the engine stop when hot and idling? Flame-out on idle and difficulty in
starting can be caused by very lean LSN settings. Richen the LSN and HSN and
make sure the carb is properly primed before starting.

With the glow plug in place and tightened down, do you feel any resistance
when turning the engine over by hand?
Drop a few drops of after-run oil down the glow plug hole and then refit and
tighten down the glow-plug. Is there any more resistance when turning the
engine over by hand and can you hear the "pop" in the exhaust as the engine
goes over TDC?