JieM, no, it was the old cylinder that broke, the new cylinder is still holding fast.
Hpergm, the prop was a varioprop 4-blader with around 18" diameter and 11" pitch. Static rpm was around 5400 that unloaded to around 7200 at max rpm, in a steep dive, but usually hovered around 6500-6700.
I agree that there are many factors at play, and in no particular order:
Bad design and mettalurgy, we know Saito have corrected this with new piston and cylinder designs (mostly to reduce compression and increase cylinder case mount points
Too lean. It hasnt been tuned since the warmer weather, and this is my bad...
Prop not really the best combo, but was tested before and has worked well.
Engine has been used as a test bed, and lots of changes have been made to it over the last 2 years
Timing is set correctly, but the engine has run for a long time prior to the change, so fatigue could have had a part.
Negative crankcase mod has worked well, and no leaks have been detected at all since the mods, but that means nothing, as a leaking gasket could have caused a lean state...or air getting in somewhere else.
A lot of carbon build up the last time I opened it, particularly combustion chamber, piston crown and exhaust valve, if this had continued to build up, compression would have increased slightly.
I am not condeming either the mods or bad engine design just yet, but one thing I AM aware of, is that this thing would never have lasted this long without the mods.
The plan was to open it this weekend, regardless of whether it had quit or not. Do inspections, carbin build up evaluation, general observations, then we were planning on checking valve lash, putting new plugs and flying as is for the rest of the season. We decided today that it might be worth repairing as long as we just have to put 2 new cylinders, rings, pistons and valves. Then we will have 3 of the new design piston/ring combo's, and send it up again. At the moment it looks like around US$300 all in, so maybe give it another thrash...! :-)