The Engine Shop
Joe Wagner | [email protected]
Maintenance and usage thoughts for diesel fans
In past columns I’ve recommended Tower Hobbies’ After Run Oil (ARO) for keeping engines from getting sticky in long-term storage. It certainly does that job well! However, I’ve learned the hard way that ARO should never be used in a model diesel.
I made that mistake last fall with four of my diesels. When spring came, none of them would start. Rinsing their interiors with fuel or solvents did no good. Evidently ARO contains a powerful antioxidant that bonds to steel surfaces. Yes, it does a fine job of preventing rust and keeping moving parts free—but it also stops the ether in model diesel fuel from igniting.
To get my engines running again I had to take them apart and scour their pistons and cylinder bores thoroughly with steel wool and WD-40. Now they’re okay; and I learned a lesson from that.
Later I took one of my old diesels that had not been run for years out of storage. As I expected, congealed oil had made it hard to turn over. But a squirt of fresh diesel fuel in its intake and exhaust loosened the moving parts nicely in just a few minutes.
Modelers often used ARO in their engines to prevent rust in ball bearings. That’s less of a problem now. Many manufacturers have switched from earlier bearings that used brass ball retainers to all-steel types. Those eliminate any chance of brass catalyzing the methanol in glow fuel into acetic acid and causing rust.
It’s an excellent idea to keep all brass out of and away from glow fuel. The Sullivan company knows that and now supplies nylon fuel tubing with its tank kits, along with the traditional brass tubes.
I like model diesels a lot for powering my RC airplanes. Their ability to uncomplainingly swing large-diameter propellers makes them especially suitable for today’s “3-D”-style RC flying. Diesels are quieter than glow engines, too.
Ace’s Pipe Thread Compound seals air leaks in needle valves (such as those the Cox Texaco .049 often suffers) and makes a good substitute for gaskets.
Clarence Lee can repair stripped glow-plug threads to be like new by installing a HeliCoil insert. This is not a do-it-yourself job!
At the Toledo Show last April, Eric Clutton—aka Doctor Diesel—demonstrated one of the Progress Aero Works (PAW) .19s he imports. He attached a long piece of vinyl tubing to the engine’s muffler outlet and submerged the other end in a bucket of water (to avoid polluting the indoor environment with exhaust fumes). “It silenced the engine so effectively, I could hear the connecting rod swishing back and forth!” he said. To learn more about the British-made PAW line of model diesel engines, check www.eifflaender.com and Eric Clutton’s site at www.cafes.net/doctordiesel/. You can learn a lot from those sites.
Model engines do sustain damage. CL airplanes flown over concrete can break pieces off their power plants in “unplanned landings.” Damage such as broken mounting lugs can be repaired fairly easily with a product called J.B. Weld. I’ve fixed several broken lugs for friends through the years, and the repairs have held up well.
My method for repairing broken lugs with J.B. Weld:
- Clean the break thoroughly and rough-file the surfaces to provide a good bond.
- Make a slightly oversize wooden mold to fit around the broken area and clamp it in place.
- Mix the two-component material well and fill the mold cavity with J.B. Weld.
- After three or four hours the J.B. Weld will be solid enough to cut the mold away; at this stage it can still be carved with an X-Acto knife.
- Do most shaping before the weld fully hardens (full cure takes about 24 hours, depending on temperature).
- Finish with careful work using needle files and drills to restore the part.
Although uncommon, loose glow-plug threads can spoil an engine’s performance. The usual sign is compression leaking out around the base of the plug. Trying to stop the leakage by tightening the plug very hard can strip the head threads, and replacement heads can be hard to find—especially for older engines.
My friend Clarence Lee (who wrote the model-engine column for Radio Control Modeler magazine for decades and now writes for Model Airplane News) can fix loose or stripped plug threads to be as good as new using a HeliCoil insert. The insert looks like a short, tightly wound spring and is made from wire with a diamond-shaped cross-section.
HeliCoil repair steps (overview):
- Drill and tap out the bad plug hole oversize using a special HeliCoil tap.
- Use a special tool to screw the HeliCoil insert into place. The insert’s diameter is larger than the newly tapped hole, so it compresses inward as it threads in, bringing its diameter to the proper size and locking it firmly so it cannot back out.
- Use another tool to break off the crossways tang of the HeliCoil so the plug can be installed.
This repair requires special procedures and tools. I have a well-equipped metalworking shop here, but I let Clarence handle my plug-hole problems.
Probably the most common source of unreliable model-engine performance is leakage of one kind or another. Air will leak past the needle-valve seat or at the needle-valve-to-carburetor joint and spoil the mixture. It can enter through almost-invisible slits in fuel tubing, a tiny gap between a carburetor base and its seat in the engine case, or through loose-fitting needle-valve threads—and whenever that happens, the results are rarely beneficial.
Crankcase pressure can leak out through a cracked gasket or a slightly loose case-cover screw. If it has the slightest opportunity, combustion-chamber pressure will do the same. That's why it's vital to check your engine's screws for tightness from time to time. If you find a loose one, there's a good chance there's a leaking gasket underneath it.
In previous columns I've described how to make custom gaskets. Another way of stopping pesky leakage in model engines is the inexpensive Pipe Thread Compound available from Ace Hardware dealers across America.
This gummy but nonhardening substance comes in a brush-top plastic jar. I apply it with a toothpick; it doesn't take much. I've found it easy to use and effective in model engines. It can stand temperatures up to 500°, and it doesn't seem to be affected by fuel or common solvents.
Ace's Pipe Thread Compound is an excellent solution for air leakage through loose needle-valve threads. The Cox reed-valve engines with tank mounts have given many 1/2A model fliers plenty of trouble finding an optimum needle setting. You wouldn't think the infinitesimal "spiral tunnel" between the spraybar threads and the needle could allow enough air through to mess up the mixture, but it can—and does.
To seal a needle-thread leak:
- Remove and clean the needle.
- With a toothpick, spread a bit of Ace Pipe Thread Compound over the lower half inch of the needle threads.
- Do not put any compound in the threaded hole (it will clog the fuel orifice).
- Screw the needle into place. Excess compound will squeeze out—wipe away any large amounts with a paper towel. A small collar of compound around the top of the spraybar is harmless and probably helps block air leakage.
This technique is equally effective in other needle-valve assemblies where the needle itself is threaded. Pipe Thread Compound can also seal gasket leaks—probably not head gaskets, which are exposed to high heat—but it works great on mufflers, carburetor bases, and around the setscrews that retain many carburetors. I've seen enough air sneak in next to one of those setscrews to make it impossible to shut off a model diesel by closing its throttle.
I won't have that problem again now that I've discovered Ace's Pipe Thread Compound.
Transcribed from original scans by AI. Minor OCR errors may remain.
