With the engine reassembled I did what I should have done in the first place and went to the field to use their test stand so it could all be done at once

I used the same fuel for the same reasons but fitted a 10x6 APC prop because that's closer to what the average modeller would use. The first run was for 2 minutes at 13K with a head temp of 300F which worried me but I was going by the book and it gave the occasional misfire into a 4 stroke (only for an instant). Subsequent runs I chickened out and richened it very slightly so that for the next 18 minutes it ran at 12.7K at 290F.

When it reached 20 minutes total part way through the 4th tank I was at the point (or just past) where the book says they're run in so I began slowly leaning it out. It peaked and held nicely for the recommended one minute at about 14K which I thought was OK for zero nitro and a lot of oil. I might add (because I just gotta say it ) that the needle valve was quite insensitive and gave me a fine control over the mixture (another Myth busted??).

So bring it home and strip it down once again.

First, remove muffler and look at the piston. There was just a hint of the matte finished "ring" at the top. Fair enough, that might get more pronounced with more running.

Turn the engine over with the plug still in and I could feel a pinch so take the plug out and put on the degree wheel. The pinch started at 42 degrees which was virtually the same as it had always been. No problems.

Take off the backplate and it was like brand new, no hint that the rod had touched it and started to give the swirling marks you usually see (the swirls are quite normal BTW).

Take off the head and there was a trace of discolouring beginning on the squish band (I've included a photo of the head because I like the shape and the large tapered squish band) and also on the piston crown.

Time to use the depth gauge on the pinch....hmmmm...I'll come back to that!

Remove the liner and the cross hatching is somewhat fainter, in fact it's almost disappeared from the transfer (thrust) side. It looks fine though.

The piston looked fine on the exhaust side with that barely discernible matte ring as I mentioned before but it didn't extend all the way around. On the transfer (thrust) side there was a very bright polished line about 1/8" down from the crown which varied in width erratically. I'm not too keen on that.

The big end bush in the rod now showed evidence of polishing on the lower (supposedly non load carrying) surface. I didn't expect to see that.

The next thing I wanted to check was the pinch with the loose piston and liner so I reoiled both with straight castor and pushed the piston up with my finger. It nipped at .130" before TDC. The actual measurement was .310" but deck height is .180". This sounds to be very good but brings me to the part where I said earlier I'd come back to it.

Recall (or re-read if you must ) that I went to use the depth gauge to check the nip while the piston and liner were still in the engine. With the head off there wasn't any nip! The piston turned over TDC as smooth as silk. That was something I most definitely didn't expect. This did not happen after that first 45 minutes of rich running. That time there was a definite pinch with the head removed. Now I could only feel a pinch using my finger but assembled there's quite considerable leverage from the prop and the very small rod angle so close to TDC. Even by turning the crankshaft the rod leverage overcame any feel I might have had.

I then reassembled the engine minus head and still there was no pinch so I fitted the head and the pinch returned! Obviously something strange was happening so I loosened the head screws. No pinch. I began retightening in the normal cross pattern a bit at a time (as I'd done before) but this time I turned the piston over TDC as I added a bit of tension to each screw. If a screw caused any pinch I'd loosen it and try another. My assumption here was that any pinch must be caused by some distortion of the liner (considering we're working with virtually zero clearances) but eventually I found a sequence that allowed the head to be fully tightened and yet no pinch.

Why did this happen? I can only guess that because the head isn't made from the same alloy as the piston it then expands and contracts more from hot to cold. The liner has no option but to follow the head because of the clamping pressure between the head and crankcase. I'd be interested to know if anyone else has noticed this.

Now my conclusions from running in as perthe book. From the 20 minutes I saw far more wear than the 45 minutes prior with very rich running. This wear includes the bright ring on the thrust side of the piston, the polishing on the lower side of the rod bush and that strange loss of pinch. What affect these might have on the life of the engine or power is impossible to say, it may have no affect at all and could even have happened if I'd continued running it rich for another 10 hours or more.

As far as I'm concerned if I was to run in another ABC I wouldn't go any leaner than the break from a 4 stroke and for considerably longer than 15 or 20 minutes. I'd put up with 20 minutes bench running but keep the same tune for flying until it had maybe an hour or more before slowly leaning it out. This is only what I'd do and that's all I wanted to find out from my little experiment.