A return to model building and flying. Part 16

Tale of an old engine

June 2005

Buying fuel at the local modelshop was a head spinning experience, and very unsatisfactory. I was presented with a list of different fuels of all sorts of concoctions, non of which where what I was looking for, namely, 20-25% oil (synthetic or castor) 5% Nitro; not exactly a tall order.

There must have been over half a dozen different mixes, everything except what I wanted. In the end I had to settle for 18% synthetic oil 5% Nitro. I wasn't particularly worried about the oil content, as I am convinced most engine manufacturers quote an overlubricated mixture of fuel to cover their own backsides.

The unknown quantity was the synthetic oil. I remember lots of scare stories about changing from Castor to synthetic, The same stories hovered around diesel fuel, but I used to mix my own brews and know differently: but that is another story. So as always I decided to suck it and see, instead of blindly listening to old tales.

After the previous week running the old Merco 35 in the rat through a tank full of fuel I thought the tightness in the engine would wear off. Wrong on all counts.

I should point out that I have a brace of these engines left over from my early F2B days. I was never happy with them so this time round thrashing them in some form a race seemed like a good use, as I have no intention of using them in a stunter again.

I noticed it was beginning to get some of it's old stiffness back as it cooled down, but nowhere as bad as the situation before I ran it.

During the week after it had been standing for a few days the engine seized solid, taking brute force to turn it over and get some lubrication into it. That seemed to do the trick.

The following Sunday was club race day, so off I toddled with my new toy. The concern kicked in when I tried to turn the engine over: and couldn't. It was seized solid! after several minutes of painful pushing and pulling at the prop it finally turned through 360 degrees. I was still not free enough to be able to flick it, just sticking near the top of it's stroke as if it had hit a patch of glue.

Realising there was just no way I was going to be able to flick the thing over at a sufficient velocity to get it start, I removed the plug and poured copious amounts of fuel into the engine and just kept turning in the hope that it would free up a little. After an arm numbing few minutes, I realised that it was not making the slightest difference. It was then I gave in and solicited the use of an electric starter to help spin it over, even that was having trouble but it did free things up enough to be able to replace the plug and get it to fire and run.

The weird thing is, when the engine is warm it is as free as anything and flips over easily. As it cools it slowly tightens up again.

So I did manage to fly the requisite 100 laps, but with 5.5cc that is developing less power than a cooking 2.5cc motor. The engine being severely restricted by the dustbin silencer and an adaptor I had to make to keep it in place probably didn't help that situation.

The following day the engine was tight again so I took the bull by the horns and dismantled it. These pictures tell more than I can say in words. I am just staggered at the fact that it ran at all in this condition.

merco eng

merco cyl head

merco piston

I am no engineer, but it appears that the old Castor after years of lying around undisturbed had turn into a lacquer tough enough to resist the the combustion process, friction, and not wear off. The deposit on the piston appeared to be the worst culprit, the liner was easy enough to get clean with a light application of metal polish. The piston skirt appears to have changed it's composition, nothing I could throw at it seemed to be able to penetrate the lacquer build up.

I suspect this build up of lacquer was initially caused by running the engine with a rich setting in my stunters, then leaving the engines standing for a few decades.

In the end I tried gently lapping the piston into the bore with a light smear of metal polish trying not to take it all the way to the top of the bore. After cleaning and reassembly it now turns over like a proper motor, with a satisfying Plop! as it goes over compression. Whether I have killed or cured it, I will find out the next time I try to run it?

After seeing what Castor oil can do to the insides of an engine over a prolonged period, it is making me seriously think about changing over to synthetic oils permanently.

At the next opportunity I flew the rat again. The engine had not gone tight on me since I stripped it down and reassembled it, so I was hopefull.

The Merco started without fuss and continued to do so with no sign of any tightening on cooling. Breathing a sigh of relief I had a couple of flights. The engine will just not rev, possible because it was never exactly a high revving motor in the first place, and the silencer arrangement is not exactly what one would call efficient.

Trying to take advantage of the torque I tried coarser pitches, smaller diameters make more noise but didn't increase the airspeed much. During one of these experiments I was surprised and stunned when the model suddenly nose dived into the ground at around 117 kph. The silence was deafening. All I could think was, "Not another Sodding! prop." After the usual jibes about pilot error, 'Yeer right!,' (translation for non street English speakers, "Oh Really!") I started to look for a possible cause.

Removing the hatch revealed the problem in all it's glory.


The pushrod/adjuster solder joint had failed. No one to blame but myself as the brass tube was too short and the the end that failed was only inside by about 2 mm, not much of a margin of safety there. I won't make that mistake again.

Considering the impact the damage was quite light and repairable: with difficulty.


It looks as if the large sponge wheel and undercarriage mounting took the brunt of the impact. Unfortunately the whole model is build around that lower engine bearer, so it required some major surgery to replace it. That is still easier than building a completely new model, plus the fact that I want to get some flying out of this one to decide what to modify on any subsequent ones.

The repairs took some pretty drastic surgury, but where pretty strait forward. I also managed to do some small modifications at the same time like shortening the under carriage to improve the ground handling a little.

The result is not too bad. How it flies, and whether I can get any more grunt out of the engine will have to wait untill I can get to the flying field again.


Part 15 left        right Part 17

Back to the beginning of the saga