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What missing from this threaded hole in my O-235-C1?

lndwarrior

Well Known Member
AS pat of my ongoing troubleshooting of my engine dying on roll out, I noticed this threaded hole on my #2 cylinder. The threads are so clean it makes me think there was something threaded in here until recently.

Also, after 600 hours I pretty sure I would have noticed this before now.

Cylinders 1,3 & 4 all have what looks like a permanent plug installed in this location.

I've checked the Lycoming Parts Manual, the Overhaul Manual, and the Service manual and there is no record, diagram, or part number for anything in this location.

If you know what this hole is for, or have any info on what goes in here, I would greatly appreciate it if you could educate me. Added appreciation bonus if you have a part number!

TIA
Gary
 

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That is the casted boss for CHT sensor. In the old days, they made different cylinders for different engines and if that engine didn't specify the sensor they either didn't cast in the boss or drill and tap it if it was cast. That little hole on the undrilled version is just a cast in guide hole for where to drill and tap. It is NOT a plug in an already drilled hole. Today they are smarter and realize that cost can be saved by making only one variety that can be used on all engines.

Not surprised it is clean, as it is hard for dirt and oil to go uphill into the threaded cavity.

Larry
 
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CHT sensor well and should not have any influence on your engine roll out issues. But you may want to start troubleshooting with plugs, wires, and firing system components.
 
While you're in there...

It would be a good idea to replace that nylock nut with an all metal type locknut, and any others that may be lurking firewall forward.
Apologies for the drift.
 
That is the casted boss for CHT sensor. In the old days, they made different cylinders for different engines and if that engine didn't specify the sensor they either didn't cast in the boss or drill and tap it if it was cast. That little hole on the undrilled version is just a cast in guide hole for where to drill and tap. It is NOT a plug in an already drilled hole. Today they are smarter and realize that cost can be saved by making only one variety that can be used on all engines.

Not surprised it is clean, as it is hard for dirt and oil to go uphill into the threaded cavity.

Larry

Makes sense. Thanks Larry!
 
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