Service Bulletin 388C
One issue inherent to helicopters is that of high engine operating temperatures. Helicopters that are hovering or not moving forward fast do not have a lot of cooling air available. The problem is increased when student training is involved due to the amount of hovering practice. It is also worse in hotter environments like Florida or Arizona. These higher operating temperatures could lead to coking formation in the valve guide area that might eventually stick a valve.
Lycoming's service bulletin 388C addresses the issue of valve guide clearance. This inspection helps prevent engine failure due to sticking valves which can be the result of excessive carbon buildup between the valve guide and valve stem. It also helps to detect excessive wear (bell-mouthing) of the exhaust valve guide that could lead to broken exhaust valves. Lycoming recommends that helicopter engines have this inspection performed every 300 hours or earlier if valve sticking is suspected. Revision "C" which was issued Nov. 22, 2004 combines Service Bulletin No. 388B and Supplement No. 1 to Service Bulletin No. 388B and adds dimensions for fabrication of a tool to measure wobble.
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