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Need screw hole size and pitch

A

AN23

Well Known Member
Reinstalling engine after IRAN. The screw I know goes back into the marked hole refuses to go back in without feeling "wrong". Before I potentially mess up the threads, I wanted to ask if someone could just confirm the size and pitch. Once confirmed, I'll run a tap thru it. Many thanks.
IMG_5867.jpeg
 
No way to see what your going "into", but looks to be a 10-32 (3/16 dia. by 32 threads per inch) . Pretty typical for baffle installation hardware. By the looks of the washer stack-up, it may have a thin lock washer on it. If it is going into a self locking nutplate, it will feel slightly "wrong" as you put it since the locking portion is simply distorted (crimped) threads. It looks pretty well in full depth in the picture, are you sure it isn't protruding into something on the backside?
 
fixnflyguy said:
No way to see what you’re going "into", but looks to be a 10-32 (3/16 dia. by 32 threads per inch) . Pretty typical for baffle installation hardware. By the looks of the washer stack-up, it may have a thin lock washer on it. If it is going into a self locking nutplate, it will feel slightly "wrong" as you put it since the locking portion is simply distorted (crimped) threads. It looks pretty well in full depth in the picture, are you sure it isn't protruding into something on the backside?
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Those go into the case casting.
 
FYI- Most of the time when you have threads cut into aluminum you can expect them be coarse thread unless theres some specific definite reason for them to be something else. That's because aluminum is a much softer material than steel and it's easy to wreck it with fine threads like AN bolts or screws..
 
Coarse thread screws on the outside of Lycoming engines. 1/4-20 most likely just like the ones that hold the baffle to the cylinder heads. There is one screw on the back of the engine for a spacer that is #10-24.
 
A thread chaser would be a much better option than a tap. It straightens the threads, instead of cutting new ones over old ones.
 
Cth6 said:
Those go into the case casting.
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Well, I have an O320H2AD in mine and it doesn't have those provisions..didnt recall seeing case mount holes in the other O320 variants, but true, all case threaded holes are typically 1/4-20, same as the cylinder attach points.
 
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