REFLECTOR: Lycoming head thread

[Keith Hallsten]

I tried screwing a wood “match stick” into the hole and got a length of
about 0.80” of thread marks on it to count.  The small hole on the side of
the cylinder head definitely has 24 threads per inch.  That would be
National Coarse for a #10 screw, so I tried a #10-32 for diameter.  Nope.
It’s definitely smaller in diameter than a #10 screw.  So it appears to be a
#8 – 24, which I have never heard of.

 

The holes on the top of the cylinder head are ¼” – 20 threads per inch –
quite standard.

 

Keith

   

 

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From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of davedent at comcast.net
Sent: Wednesday, July 04, 2007 9:28 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Lycoming head thread

 

keith there are two sizes.  1/4-28 or 1/4-20.  The ones on the face of the
cylinder are normally 1/4-20 for the valve cover and the ones on the fwd and
aft are the 1/4-28.  It's best of you see what screw you can put in, without
forcing it. The plugs that are on the intake are 1/16 pipe.  Check out your
parts catalog, if not sure. They are called out in it.  If you don't have
the parts book give me a call.  I'll be at the hangar for a short time
today.  925-337-0115

Dave

----- Original Message ----- 

From: Keith <mailto:KeithHallsten at quiknet.com>  Hallsten 

To: Velocity Aircraft <mailto:reflector at tvbf.org>  Owners and Builders list 

Sent: Wednesday, July 04, 2007 8:33 AM

Subject: REFLECTOR: Lycoming head thread

 

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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