of this site may recall some problems I have experienced with
my ST G21/46 F2B stunt motor. Nobody else seems to have suffered
the indignities that have plagued me with this fine motor.
All reports read along the lines “model X performing
to F2B standard with a ST46 now purring its way around”.
I assume ST means Super Tigre and not Stalker! So what form
did my problems take?
Most common symptom was the motor starting
off beautifully, then leaning out and refusing to break back
rich, eventually leaning out to give a 9 minute run with the
motor so lean that the model lands under power! Next symptom
was the motor running nicely, but losing power whenever the
nose was put up, as in a loop: a fatal problem for vertical
eights and hourglass. Another symptom was the motor going
too rich, then too lean, and so on, in a generally temperamental
and frustrating way.
In response, I pulled out the tank 3 times,
pulled it apart looking for blockages, changed the uniflow
vent position, etc, etc. Tried new fuel, old fuel, very oily
fuel, more nitro, new plugs: in short, nothing worked.
Now being something of a loser for some time
now, I have been watching for a word that describes this problem:
it is INTRANSIGENT. Once I reach the need to use this word,
I am finally on track. It means that the solutions I have
tried are not connected to the problem! How is that for intellectual
So now I am left with blaming the motor. Problem
is, there isn’t much in an ST46 to go wrong: there are
only 3 moving parts. I couldn’t really blame the conrod
or the crankshaft, so it came down to the piston.
The ST46 has an aluminium piston with a single
iron ring. I know nothing about piston rings, but assume they
are made of some form of cast iron like meehanite. They are
the very devil to remove, as they are likely to snap or take
a permanent set in either of which case they are ruined. The
original ST ring was actually found to be broken, as assembled
by the factory. This was rather distracting, and sent me off
trying rings from 3 other manufacturers.
Suffice to say, none of these gave me a good
running motor, certainly not one that “purred its way
around”. It appeared there were different ways of tempering
the ring, and of making it truly circular. What shape it assumed
after opening it out to drop in the ring groove, seemed to
be an entirely separate matter.
Now to progress this diatribe, with the second
last ring I tried, I thought I detected an improvement the
more the motor was run: ie, it appeared that perhaps the ring
required a lot of running in. So out of the model and onto
my test stand for 5 hours running at the Bindoon light aircraft
Every 45 minutes I peeped in the exhaust port
and noted the condition of the ring. There appeared to be
a bright rub mark about 3mm long at the top of the ring, which
moved around a bit during each run. Now when I say “at
the top of the ring”, you need to note that the ring
is only about 1mm thick, so that only .3mm depth of the ring
was actually rubbing. OK, well Supercool is nothing if not
tenacious, so I ignored this and continued to the end of 5
Then into the model, to find nothing had been
gained; the run was as bad as ever. Well, with the State Champs
only a month away, and with another defeat at the hands of
Peter White looming, something had to be done.
I recalled purchasing some diamond lapping
paste about 4 years previously, at the Claremont Timber and
Woodworking show. It was used for sharpening woodworking chisels,
and at the time I thought it might come in handy for setting
up my combat G15’s. It was cheap, Chinese, but mostly,
it was there. Later I was to learn from Stan Pilgrim that
it comes in various grades, right down to fine enough to act
as a polish. I had no idea what grade my purchase was, but
then ignorance was always my greatest strength.
So I stripped the poor Tigre down for what
may easily have been its last overhaul before becoming a boat
anchor. With the piston out, I was able to examine the ring
a little more scientifically. The previously mentioned rub
mark was present, plus two others, on either side of the ring
gap. But that was all. The rest was black with carbon, suggesting
that there was no contact with the cylinder wall at all. Not
good; it’s not supposed to be like that.
Photos by Grant Lucas...8MP Canon digital camera
Could this have been the reason for my poor
motor runs? Only one way to find out. I took out a brand new
ring, and replaced the bad one on the piston. I then smeared
some diamond paste on the ring, and on the bore of an old
ST46 cylinder, which was fortunately hoarded in my motor junk
With only a very few rubs of the piston up
and down, the lap marks showed that this ring also was neither
round nor square: ie, it was crap also. So I kept lapping,
with piston rotation to help make the ring round. Within only
a very few minutes, I produced a very nice, uniform pattern
on the ring surface, which suggested that the ring was both
round and square. Hopefully I had not damaged the ring groove
with the diamond paste, but there was no guarantee of that.
Examination of the cylinder bore showed where
the previous ring had polished high spots in the chrome. So
in with the piston and some more diamond paste, and within
seconds these spots were gone as well. The piston was then
washed, with the intent of removing the diamond paste, and
the motor re-assembled.
I gave it 30 minutes on the test stand in my
backyard, and hoped the lady next door did not enquire as
to why her washing smelt of castor oil. At this point Ian
Thompson wandered in to pick up some team race props, and
took a few giant lungfuls of the aromatic blue fumes to make
Then into the model and off to the flying field
at Whiteman park for some test flying. Does this story have
a happy ending? Well, so far, the motor has been just superb,
running the same way everybody else’s ST46’s run.
At the moment, I am using my own 12X6 and 15% nitro to give
me plenty of power thru the wind, with lap times of 5.7 on
65’ lines. Slow compared to the top guy, who seems happy
at 4.6’s, but I am old and feeble, and if I can get
away with slow, then I am content with that.
Now if I am not an orphan in respect of these
problems, here is what I suggest. Rings appear to be very
hard to make, and need to be lapped in by hand if they are
to work properly. For a mere $50, I will lap in your ST46
piston ring for you. This will save you $10 for buying your
own diamond paste. I will also drill a hole in the front of
the crankcase, so you can get (expletive deleted) the gudgeon
pin out easily.
Now, lets see how the State Champs shape up