A look at the new O. S. .55 AX high-performance engine
Also included in this column:
• Brodak .25 and wood propellers
• Magical new rust-removal method
• Pitfalls of buying engines on eBay
• Tool source correction
June 2007 91
The Engine Shop Joe Wagner | [email protected]
Brodak’s propellers work exceptionally well for CL. They are well suited to Brodak’s
engines, such as this .25.
O.S.’s new .55 AX’s easily recognizable features are slanted head fins and back-swept,
flexible needle-valve extension.
O.S. ENGINES never runs out of new
ideas, such as the double-ball-bearing O.S.
.55 AX ABL. (The last three letters stand
for Advanced Bimetallic Liner, which is
an improvement on the “ABC” system.
The “ABL” design uses a double layer of
chrome and nickel electroplating in its
brass liner bore. This provides stronger
bonding between the plating and the sleeve
and greater durability in service.)
Externally the O.S. .55 AX is easy to
identify by its slanted head fins, swept-back
needle valve, and rectangular cross-section
muffler. O.S. calls that the “Power Box.” The
new head design seems like an excellent idea.
The taller-in-the-rear fins offer more effective
head-cooling area and permit more airflow
over and through them than the usual
arrangement.
The angled needle now includes an oldtime
feature I’ve always liked and often
wondered why it was discontinued: a flexible
extension. The .55 AX comes with a 31/2-inch
length of high-tensile stainless-steel flex cable.
It can be cut to whatever length you want and
assembled between its knurled knobs with
socket-head setscrews. (A rigid wire extension
can also be used.)
The .55 AX carburetor is easy to adjust. Its
mixture-control feature, although sensitive, is
straightforward. The instruction manual that
comes with the engine explains in detail not
only how to adjust the mixture, but also how to
tell when it needs readjustment.
As do all model engines with aluminum
pistons and brass sleeves, the O.S. .55 AX has
a tapered bore. However, the new “ABL”
process evidently allows closer fitting than
usual. Even though I could feel a “pinch” when
the piston passed through top dead center, it’s
far from a “squeaky fit.”
I had no trouble hand starting the .55 AX
from cold. (The manual advises using an
electric starter, but I can learn a lot about a new
model engine from the “feel” of hand-starting
it. And my heavy-duty leather glove keeps my
fingers safe from propeller damage.)
Further experimentation with Brodak’s .25
CL engine (mentioned in an earlier column)
showed a couple interesting points. For my
own “education” I compared it with one of my
favorite CL Aerobatics (Stunt) engines: the
1956 Johnson .29.
The two engines weigh approximately the
same: 7.1 ounces for the Johnson and 7.6
ounces for the Brodak, including the muffler.
And they deliver almost the same power!
The Brodak .25’s high output is probably
because of its carefully optimized porting and
the greater efficiency made possible by its
lightweight aluminum piston. There’s not so
much power wasted in starting and stopping a
heavy cast-iron piston twice in each revolution.
And the Brodak runs noticeably smoother than
the Johnson.
In comparing these engines I used John
Brodak’s wood propellers. That seemed
appropriate; they are an improved version of
the old Y&O propellers that were such great
favorites with West Coast fliers in the post-
World War II years.
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92 MODEL AVIATION
This twin-ball-bearing PAW .19 diesel
seemed like a bargain at less than $50,
but it needed more than $100 in repairs
to make it run.
A 4-ounce Evapo-Rust sample was all it took
to restore these Fox parts. The fluid is
reusable; later it derusted the ball bearing!
These corroded parts (from a 40-year-old Fox .40 BB engine) seemed hopelessly
ruined, but Evapo-Rust fixed them easily.
Those propellers had been painstakingly
developed by famed Stunt pilot J.C. Yates and
engine maker Henry Orwick, with assists from
Hi Johnson, Bob Palmer, and even some
surreptitious testing in Lockheed’s windtunnel
department by Ken Aymar.
Brodak’s propellers use the same
fundamental design as the original Y&Os, but
with thicker hubs to reduce the “breakage
factor.” (The only drawback to Y&O
propellers was their tendency to break easily in
nose-over landings.)
Recently I received samples of an unusual
rust-removing fluid called Evapo-Rust (E-R).
It’s non-toxic, non-flammable, and affects
nothing but iron oxide.
Many rust removers have been marketed in
the past. (Naval Jelly is one well-known
brand.) All those I’ve tried were acidic. They
converted the iron oxide to phosphates or other
water-soluble compounds and weren’t
particularly safe or “friendly” to use.
E-R isn’t like that at all. It works by
chelation: a hard-to describe process in which
an unwanted metallic compound is chemically
“coaxed” into dispersing slowly. E-R does that
in eliminating rust.
I could have used rusty X-Acto blades to
test E-R. (It works fine for that!) However, to
provide a serious test of its capabilities I
sought out and acquired a truly rusty engine: a
Fox .40 BB that contained the most badly
corroded internal parts I’ve ever seen in a
model power plant.
I had a difficult time taking the Fox apart! I
never did get its ball bearing out of the case or
the wrist pin out of the connecting rod.
I rinsed all the rusty parts in a bath of hot
water and dishwasher detergent to eliminate
any oil or sludge. Then I put the Fox sleeve,
shaft, and piston-rod assembly in a plastic cup
and poured E-R over them until they were
completely submerged.
E-R works slowly. Nothing much seemed
to be happening in the first couple hours. From
time to time I gently agitated the Fox parts in
their E-R bath and checked them.
After awhile I could see the corrosion
gradually disappearing. In approximately 24
hours it was gone. And best of all, no pitting
remained; the parts returned to smooth metal.
Encouraged by my test results (and having
nothing to lose anyway), I dunked the Fox’s
case in the plastic cup to see whether E-R
would do any good to the ball bearing. It was
so rusty it would barely rotate.
(Ordinarily I would have pressed it out and
replaced it with a new one. However, the outer
race of the bearing was rusted solidly into the
case, and I had no way of removing it without
ruining both parts.)
It took two days of submergence in E-R,
with agitation every few hours, but the bearing
rust eventually vanished. No, the outer rim of
the bearing didn’t derust and come free from
the case. The E-R solution couldn’t penetrate
the interface to do its magic work. (Same for
the area of the wristpin inside the connecting
rod’s “little end.”)
The once-rusted ball bearing spins freely
again. It’s not perfect, but the old Fox is
runnable now.
E-R can be reused many times. (It darkens
as its rust-removing capacity is used up.) It
doesn’t harm paint, plastic, rubber, or metals
such as aluminum, zinc, or magnesium alloys.
E-R is available in quart bottles from Fulton
Enterprises, Inc., 108 Walter Davis Dr.,
Birmingham AL 35209; Tel.: (800) 766-6695;
E-mail: [email protected].
Friends and acquaintances of mine had
advised me several times in the past few years
not to buy model engines off of eBay. They
told horror stories of misidentified engines,
hidden damage (disguised by careful
positioning in the photos shown on the Web
site), and no recourse from the seller.
Yes, I believed them. Yet when I took a
quick look at what was being offered on eBay
in the way of model engines recently, I could
not resist bidding on one of my favorites: a
PAW .19 diesel (twin-ball-bearing model).
The bid was temptingly low—I “won” the
engine for less than $50, including shipping.
When the PAW .19 I’d “won” arrived, I
learned how much truth lay in my friends’
advice. Externally the little diesel didn’t look
too bad. Inside was another story! I had it
rebuilt (by “Doctor Diesel” Eric Clutton), and
the parts and repair service came to more than
$100.
I suppose used model diesel engines are
more likely to be damaged and/or worn out
inside than glow engines. It seems that too
many fliers misunderstand their diesels. They
run them overcompressed, with too lean of a
mixture, or use electric starters, all of which
can ruin a model diesel in a short time.
When properly handled a model diesel
should last almost forever. This is because
that type of engine runs cool and its
recommended fuel contains ample lubricant.
In a previous column I mentioned reshaping
engine intake “lips” using a tool I call a “burr
knife,” and I included a picture of one that I
had recently bought via mail-order from
Harbor Freight Tools. I recommended the
company as a source in the column.
Alas, I made two mistakes there, the first
of which was to used “old-time
nomenclature.” Nowadays that tool is called a
“triangular scraper” instead of a burr knife.
Second, the tool I bought was a close-out
special. Harbor Freight doesn’t carry it
anymore.
However, I did find an online source:
Amazon.com. It doesn’t market a triangular
scraper directly but acts as an ordering agent
for a company called “Value.” The catalog
code is MSM 1280, and the tool costs
approximately $6. MA
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