Hey, @M20Doc (and anyone else interested in the Lycoming Valve Guide "Wobble Test"...
I read SB 388C, the Lycoming 400 hr "Wobble Test" service bulletin very closely as I was planning on buying the test rig when I discovered this statement:
"400 hour intervals or earlier if valve sticking suspected until exhaust valve guides are replaced with guides made of improved material. (Refer to latest revision of Service Instruction No. 1485.)"
I then looked up Service Instruction No. 1485. It says:
"TIME OF COMPLIANCE: Half way to recommended TBO or at 1000 hours of operation, whichever occurs first or earlier at owner’s discretion or anytime valve sticking is suspected.
\As a product improvement, Lycoming has introduced a new exhaust valve guide material with an increased chrome content for improved wear characteristics. The incorporation of this guide material has significantly reduced the amount of valve guide wear (bell-mouthing) seen by some operators of Lycoming engines. Improved exhaust valve guides were initially incorporated into some cylinder assemblies beginning in April 1996.
Since March 1, 1998, all engines, cylinder kits and spare exhaust valve guides shipped from Lycoming contain the improved material. Cylinder assemblies which incorporated the improved “Hi-Chrome” exhaust valve guides when the engine was shipped from Lycoming are identifiable by the letter “C” stamped inside a circle on the boss for the drain back fitting. The individual guides made with the improved material can be identified by a 5° chamfer at the top of the guide (See Figure 1)."
So, for those who are interested, it appears that if your cylinder assembly is newer than 1998 and the guides are therefore the improved ones, the "Wobble Test" is recommended every 1000 hrs instead of every 400 hrs....
Alex