Long story made medium length:
30 minutes into the break-in of this engine, I noticed a growing pool of silver oil accumulating under nose (exhaust). I shut the engine down, and discovered the front bearing had bit the dust. The plug was shot, no surprise there. This was a Thursday night, and maiden flight was scheduled for Saturday. Efforts to find bearings on Friday were without fruit, so I bought another ($99) engine to cannibalize for parts. I was determined to fly the airplane the next day!

The semi- broken in top end was transferred to the new case/lower end (plus a new plug) and another 30 minutes run time on the ground, then the first two flights on the airplane. The plug element was broken after the second flight. The bearings have since been replaced with higher quality bearings from Boca bearings.

After this, I was losing a plug per flight. Examination of the plugs under magnification revealed that the coils were broken about 1 turn from the end of the plug. The engine continued to run until shut down in each case.

The top end was exibiting the BZZZZZZZZ sound indicating a loose con-rod. Two things will blow plugs like this- the mechanical vibration from a bad con- rod, and pre-ignition, usually due to an excessive compression ratio.

Last Friday, I scavenged the con- rod from my parts engine. I also checked for the correct number and thickness of the head shims from both engines (OK) and measured the height of the squish band on the head relative to the liner mating surface. I found the heads to vary by .005". The head that I was running protruded .005" further into the cylinder than the other head, so I swapped heads as well. For reference, the head shims are .005" thick.

Since I changed two variables, I cannot say which one was the culprit in the early plug failures, or if it was a combination of the two defects, but I was able to put 6 flights (1 hour flight time) on the engine yesterday without incident. The plug is still good.

-Robert