REFLECTOR: Lycoming head thread

Keith Hallsten KeithHallsten at quiknet.com
Wed Jul 4 10:33:29 CDT 2007


Does anyone know what thread is in the screw hole in the side of a Lycoming
cylinder head?  I need to attach a clamp to support the lower ignition
harness as it "turns the corner" at the #5 cylinder on my IO-540 parallel
valve nominal 260 hp.  

There is a hole that appears to be sized for a #8 screw facing the firewall,
but when I tried an #8-32, it will only thread in for most of one screw
revolution.  It's hard to see, but I suspect that the thread pitch is larger
than 32 threads per inch (that is, fewer threads per inch).  However, #8-32
is in the "National Coarse" series, and I don't know of a commonly-used
coarser thread standard.  

This hole looks as if it was intended for the attachment of baffles.  It
could be that there is some paint or something in the hole that is causing
the resistance, but I don't want to chase the threads with a tap until I'm
sure that they aren't some other pitch!

If the thread really is an oddball, I'll also need suggestions of where to
buy a couple of appropriate screws! 

Thanks for any help you can offer

Keith Hallsten  
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