Know nothing about trimmer engine conversion - RC Groups
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Nov 01, 2012, 04:57 PM
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Know nothing about trimmer engine conversion

Hi all, I want to convert a trimmer engine, actually I have a homelite 26 but I heard that the ratio power/weight was bad, I could buy another one but. At first what are the best trimmer or chainsaw engines.because I want to make a good plane which would be cool to fly. Aerobatic if possible

Also I don't know anything about these things can anyone explain me how to convert them.

I don't want to buy a conversion kit. So how can I make these pieces if I need some?

And how am I mounting it by the way? I want to make it lightest as I can.

Thank all
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Nov 01, 2012, 06:29 PM
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earlwb's Avatar
One needs a lathe and a drill press and some other tools such as drills and taps, files, etc.

First one strips off all the extra stuff on the engine that they won't need.

Then you need to figure out a way to mount the engine to the airplane. This would typically be a flat aluminum plate drilled for mounting to the back of the engine and in some cases a set of standoff spacers, if the engine has a rear carburetor on it.

You use the lathe to make the prop hub for the engine so you can bolt on a propeller. If you stay with the magneto, you could leave the flywheel on too. But nowadays most folks go with a electronic ignition instead as it saves the weight of the flywheel too. But if you keep the magneto and flywheel, you can use the lathe to trim off the blower or fan blades and make the flywheel more light too. You'll need to check the flywheel balance when done too.

You can put a longer throttle arm on the carb and or choke too.

You may need to go to a resistor spark plug to help reduce radio frequency interference as well.

You can also look at the engine and machine off extra protrusions and metal you don't need for it as well. Things like the cooling shroud cover mounting protrusions, etc.

You can get a lot of ideas for it here as well as more information about it too:

Here are some of my conversions:

Nov 02, 2012, 10:03 AM
Registered User
thank you , maybe i will have other question tonight about it i have started to remove things that i dont need but i almost removed the flywheel and the magneto , i have seen that u didnt removed it , why did you prefer to keed them?Is it not weight that we could remove instead?
Nov 02, 2012, 10:22 AM
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earlwb's Avatar
At the time I was using the gas engines, I preferred the magneto system. No fuss no mess, no batteries to charge, et cetera. Back then the electronic ignitions were more simple and they used a mechanical spark advance. I had seen other people having issues with them so I decided to stick with the good old relaible magneto ignition.

But nowadays the electronic ignition systems are much easier to use and you can save a lot of weight ditching the flywheel and magneto system.
Nov 02, 2012, 11:41 AM
Registered User
Are they easy to install?
Nov 02, 2012, 02:27 PM
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earlwb's Avatar
Quite easy. You find top dead center (piston at the top of the cylinder). Then mark a dot where you want to drill a shallow hole for the magnet in the prop hub. Then place the magnet in the hole and with the ignition system hooked up with a spark plug, move the sensor over the magnet to get a spark, so you make sure you don't need to flip the magnet over as some sensors are polarity north pole south pole sensitive. Then you can glue the magnet in. Next position the sensor so that when the piston is at Top Dead center, the sensor is at 28 degrees ahead of it (static advance timing). You can get some images of degree wheels off the internet and print out one, glue it to some stiffer cardboard and use it to make the measurement with. You want to place the magnet where it makes it easy to mount the sensor of course.

It is used something like this for example:
Nov 02, 2012, 04:41 PM
Registered User
i saw something about balancing the flywheel or something why is this for?
Nov 02, 2012, 05:23 PM
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earlwb's Avatar
You only need to rebalance the flywheel if you have machined off some of the excess fins and stuff on it. But even then it may be balanced Ok, so it may be quite minimal if anything. But sometimes there are protrusions or something that when machined off might affect the balance.
Nov 03, 2012, 11:14 AM
Registered User
Did you keep the exhaust??
Nov 03, 2012, 05:12 PM
hutchinstuff's Avatar
Earl, did you covert any of yours to E ignition? Thinking about doing this but might prefer to just use the mag.
Nov 03, 2012, 10:26 PM
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earlwb's Avatar
I have a Echo four stroke engine that I am finishing up. I plan on using a RCEXL CDI module for it. I only need to mount the sensor and magnet and make a engine mount for it. I already made a prop hub for it a while back.

Yeah originally, years ago, we either converted the engines to glow fuel power, or used the magneto ignition. Sometimes we would trim the flywheels, sometimes not, it depends. Electronic ignition at the time wasn't all that good. CH ignition modules were better, but they used a mechanical advance setup couple to the throttle. I didn't really like that method much. When the CDI ignition modules came out with microprocessors (tiny computer) inside them, then they could delay or retard the timing using a program inside. This made the electronic ignition modules much more easy and nice to use then.

Now then some guys used the stripped down magneto setup and those systems don't weigh much more than a CDI system does, if that much. But those have to be custom machined to fit a particular engine.

Many years ago, the largest model Rc engines one could get around here were the Fox .78 and the OS Max H80 engines. So we converted a number of trimmer engines to run on glow fuel. You make a plug adapter for the glow plug. Then a adapter for a Perry carb or something else that would work for you. Then a prop hub for the prop, and a engine mount of some type. Since it was glow, you could really remove a lot of stuff off of the engine. Anyway, that worked quite well actually. I had made several engines like that many years ago and flew some larger size planes with them.

I think the engine manufacturers took notice and started making bigger engines for the RC plane market. I remember OS coming out with their .91 engine and it was hugely popular with the .60 size scale plane folks. It was way back then when I saw one of the first Quadra gas engines that a guy had in a huge Bud Nosen Mr Mulligan plane. It was pretty awesome at the time.
Nov 04, 2012, 05:00 AM
Bellanca Kruesair
epoxyearl's Avatar
Along the lines of modifying everything we can get our hands on,I did a Stihl 4 cycle engine ,the year the Great PLanes Stearman was introduced..

I used a 17 x 5 prop,and had to reduce the size of the carb venturi to get a reasonable fuel draw-(kinda like running with the choke on.) I ended up with a restrictor with a 10 mm opening.

It isn't the most powerful engine,but it's the best sounding one !.

I left it all heavy,with original ignition....And since you mix the oil and gas,it can run in any attitude......inverted is okay.

There's some question about mounting 4 strokes with separate oil and gas inverted because of the oil location,and possible lubrication problems.
Latest blog entry: Rut-roh !
Nov 04, 2012, 05:27 AM
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earlwb's Avatar
Epoxyearl, You are correct in that the engine I have converted uses a oil sump. So it cannot fly inverted very long. So inverted maneuvers would not be good for it. So some type of a plane with a flat bottom airfoil would be the way to go with it. I can use the stock Walbro carb on mine though, as it has a tiny venturi size on it already, I forget at the moment, but maybe it is a 6.5mm or 7mm size.

Nastagol, most people use the stock muffler that comes on the engines when they do a conversion. But many of the aftermarket RC gas engine mufflers do fit the engines too. Generally you can measure the mounting hole spacing and you can find one to fit that. Some folks have made their own exhaust and muffler system too. Many RC flying fields are becoming much more restrictive on engine noise. So using a good muffler doesn't hurt anything at all. Those OEM utility gas engine mufflers tend to be next to worthless for noise reduction anyway. But they are handy for initial engine conversion test runs though.
Nov 04, 2012, 07:36 AM
Bellanca Kruesair
epoxyearl's Avatar
Earl-I don't know if you've had the base off your engine,but I had disassembled an engine that had a "header tank" inside the crankcase....The oil was free to circulate by pump,and there was a smaller sump that collected the oil when inverted,and simply metered it through a small hole to provide inverted oiling.

The engine was sold by an Italian firm,whose name I can't recall..It was on a 'fun -cart',and the kid had ruined it by running with out an air filter.It had full oiling and electric start,hydraulic disc brakes....High dollar !

It could have run inverted for a bit,with no damage....
Latest blog entry: Rut-roh !
Nov 04, 2012, 11:00 AM
Registered User
earlwb's Avatar
I need to finish this engine soon. But I did sort of stall out on what to do for a engine mount. I either mount the engine to the back of the firewall on the plane so the crankshaft protrudes through; or I make a L shaped aluminum bracket with braces to use to bolt it on to something. There are three bolts on the front of the crankcase and there is one bolt off the base of the cylinder on the back that I could use too.

The OEM carb is a Walbro WYL 240, which has a 7.5mm venturi. The carb is pretty cool in that it uses a rotating barrel throttle like most of our glow engines use.

Oh yeah I forgot that I need to trim the excess plastic off of the heat isolation block for the carb too.

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