I've retitled the thread because it seems too many focussed on the words "break in" and therefore missed the point. If there's anyone alive today who hasn't seen the original thread it's at
http://www.rcuniverse.com/forum/m_28..._1/key_/tm.htm . The main reason I've started this thread is to condense the 5 (so far!) pages of the original (BTW, I loved the part about "it's hard to hit a moving target" ).

But to sum up, we've all heard that if a new, unrun ABC is run too rich from the getgo or even during the break in period for any length of time at all then there's a likelihood that this rich (as in 4 stroking rich) running could cause immediate and permanent damage. This damage can be anything from excessive wear on the piston through to breaking rods because of "compression-tension" cycling of the loads. However, I'd never seen anything by any manufacturer specifically warning against running in a 4 stroke, they all seem to say run it rich but there's never a proper definition of "rich". The closest I've seen was a point barely above the break from a 4 stroke into a 2 stroke mode. Now this is fine if you've run engines before and know what the 4 stroke sound is like but what does a newbie do? All he knows is that the engine is running and makes a noise. Now very few newbies will go out and buy a Jett, Nelson or Profi as their first engine so all this talk about damage is directed against sport type engines.

OK, so that's what we've all heard but I've had to wonder if there's any truth behind it or is it all just hearsay? Years ago I decided that if I ever got a new ABC (cheap ) I'd consider it expendable and put my money where my mouth is and see what happens by running it extremely rich right from the start. And not for just a short time, I wanted to really wring it's neck over an extended period to give it every chance to show damage. It had nothing to do with trying to run it in, simply to find out if it would be damaged. From what I'd learned playing with model engines for over 45 years plus over 21 years as an aircraft engine fitter (including 3 years as an apprentice doing nothing but theory and practical training) I was fairly confident it'd survive ok but I wanted to know for my own satisfaction. If I was wrong then I'd have learned something.

Preamble (rambling?) over, down to business. I picked up a brand new Bluebird 51 for $40 (about US$30) from "you-know-where" so here was my chance. I only had one shot at doing this experiment so I had to eliminate as many variables as possible. I chose to use a 9x6 wood prop to keep the loads down because I was more interested in piston wear than rod wear. I chose to use a fuel with 24% oil (1/3rd being castor) and zero nitro for two reasons. The first was that I wanted the engine to run as cool as possible and didn't want a low oil content contributing to excessive wear, the second reason was I already had it mixed

The first tank was done in 3 runs of 2 minutes each in a solid 4 stroke at 11.7K with a head temp of 175F. The second tank was slightly richer at 10.3K and 160F and the third tank at 8.7K and 145F (when I could hold the head for maybe 5 seconds before it became uncomfortable). By now it had a total of 18 minutes. To check for any wear I'd twice taken off the muffler and looked at the piston through a magnifying glass and couldn't see any signs at all. It still looked brand new. The pinch still seemed to start at the same place but at this point I had an idea about how to check it a little more scientifically. I took out the plug and mounted a degree wheel in front of the prop. This allowed a very fine feel for the start of the pinch and was occurring at 45 degrees BTDC. For rod wear I put the piston into the pinch to lock it in place then checked the freedom of movement at the prop tip. This was about 1/16" and never varied.

I continued running it very rich until it had done 45 minutes at which time I figured that surely there'd be signs of damage if it was ever going to have any so I stripped it down for a close look (and here I'm going to cheat and do a cut and paste from the first thread) but note that where I use the word "wear" I'm not talking about damage, I'm talking about things you'd expect to see in an engine that's been run.

On the piston, the only evidence of any wear was the beginnings of the ring you usually see close to the crown. Most of this was on the transfer side of the piston with only a hint of it beginning on the exhaust side. The photo shows the transfer side. There was no detectable play in the wristpin bosses in the piston.

The liner was starting to show signs of smoothing out because the cross hatching was getting faint. One area between the front transfer and the boost port had taken on a very high polish like a mirror.

Now for the rod (and I know it's not one of the squeaky tight high performance engines). It's difficult to show in the photos because of reflections etc but the upper surface of the rod bush is taking on a nicely polished finish while the bottom half still has the original machining marks. This tells me there was definitely none of this "piston jamming in the pinch and the rod dragging it down" going on. In other words, no "compression-tension cycling" of the rod. One myth busted at least and it came as no surprise to me.

(End of cut and paste ).

One thing I'll add here which I didn't think important at the time was that with the head off I turned the piston just into the pinch and measured the point it began to pinch with a depth gauge. This was .075" BTDC and was the equivalent of about 30 degrees BTDC (I also used a shorter backplate screw to stop the liner from lifting).

My conclusion from what I saw in this engine? Absolutely nothing abnormal. My opinion is that it was barely showing signs of even running in. Once again though, I'm not advocating this as a way to run them in, simply that in this test with this engine (which seems to be a typical off the shelf ABC sport engine) running slobbering 4 stroke rich caused no harm at all.

The next post will be the results of running it in as close as possible to the "do it my way" thread