R

Reflex

Well Known Member
As I get ready for my first engine start, I noticed a drop of preservative oil on the floor yesterday. I identified the source as the drain back fitting on cylinder #3. I figured I'd just remove, re-dope, and reinstall it today.

I loosened the fitting only 1 1/4 turns and it came out of the hole. The was no dope on the outside of the fitting, but there was dope on the inside. I noticed what appeared to be signs of cross threading on the first two threads. Upon inspection of the cylinder side, it's evident that when the fitting was installed at the factory it was cross threaded.

My question is, can I just chase these threads with a 3/8" NPT tap?

Note: The engine arrived in April of 2020 so I'm out of warranty.

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I would do what you are suggesting. The only caution is that a standard NPT tap may be too long. Have not used a lot of NPT taps.
 
Document and send your findings to Lycoming. At least you want to hear from its tech support.

I had an auto plug adapter that was crossed threaded too. With the ultra slow speed of manufacturing, you think Lycoming at least get the threading correct. Anyway, the warranty coverage and processing were so onerous that I just said F it and purchase new adapters from Aircraft Spruce.

You want something that costs more than a family car to be made better but . . .
 
Luckily this is not a high pressure port and it ought to be fixable. If this is a TB engine, reach out to Jeff he is very good to take of the customer and considering the engine has not run, it may still help especially that it does not cost them much.
 
Bavafa said:
Luckily this is not a high pressure port and it ought to be fixable. If this is a TB engine, reach out to Jeff he is very good to take of the customer and considering the engine has not run, it may still help especially that it does not cost them much.
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Probably should talk to Lycoming before you do anything else…
 
Definitely talk to Lycoming first - but - there looks to be plenty of meat left in that casting to drill out and tap 1/2" instead of 3/8". It is a low pressure fitting after all.
 
Just grab a tap and go slow. Keep it square and use a little oil. the taper will find the good threads and help alignment. Remove the valve cover and flush out.
Once installed, you may never need to remove it. Use Loctite 242 and a new fitting.
 
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gasman said:
Just grab a tap and go slow. Keep it square and use a little oil. the taper will find the good threads and help alignment. Remove the valve cover and flush out.
Once installed, you may never need to remove it. Use Loctite 242.
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What gasman says.
You would expect better from Lycoming but this is no big deal to fix.
 
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