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Exhaust Manifold Nuts

Cardiff Kook

Sisters, OR
My nuts on my exhaust manifold keep coming off. Once about every 40 hrs. Why?

Right now I am missing the outboard exhaust nuts on my #2 and #3 cylinders.

Can I get those from a hardware store or do I need some special hardware? and what tool do I need to tighten the inboard ones?


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Are you using lock washers with the nut? A socket on a swivel and extension works, you may have to grind down the socket some. Snap on had a extension with swivel socket on the end but they stopped making them the last time I checked.
Yes- I was using lock washers with the nut. It just keeps happening.

On one cylinder my mechanic had to put in a helicoil because the stud had been pulled out. If it happens again he said I need a new cylinder. It's an o290d2
If you have the room double nut the studs...stack a couple (one or both even thinner than standard) and tighten one against the other. Not too tight so as to strip the stud threads but enough to jam them together from friction. The idea is not to pull the stud out of the cylinder trying to keep them there, but put the tension mainly between the nuts. As the stud threads wear the single nut has a harder time staying on.

Check the Lycoming overhaul and parts manuals - I think they're available online. Use internal-star style lock washers. I've never had a manifold nut come loose. And note the torque comment above.
I use the internal-star style lock washer on top of a flat washer also. Recheck after a few hours of flight. Also note stud exposed distance you could be pulling them out and that is why the nut comes off.
I'll try - 40-60 inch pounds torque? Any better info? It used to be until the washer just crushed, then someone who had a torque wrench walked up.

More info. I may be low above:

This is as bad as owning two watches.

And, then there's the problem of exhaust flange bending from torque vs adequate exhaust seal....been there as the A&P dealt with it. Exhaust in vice....hammer and torch in hands.

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Read the tech on Nord Locks. Better performance for securing nuts and reusable.

Cardiff, my old 320 loosened exhaust nuts, too. Enough so that they were a preflight item. I’ve used Nord Locks since, and not just on exhaust nuts. They really are a superior product.

I believe the nut torque spec is 204-228 inch pounds and 100 inch pounds for driving the studs.
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The old Lycoming manual had a torque of 160-180 inch lbs. It is left up to the airframe manufacturer in more recent revisions. From my experience when exhaust nuts back off after they were properly torqued it is usually an exhaust stud pulling out. There are Helicoils and oversized exhaust suds. I have found several cracked heads when I started investigating this problem.

STD-1410 Exhaust nut
MS35333-41 lock washer
AN960-516 flat washer
The Lycoming nuts are stainless steel and don't rust like the aftermarket nuts.
No, just 5/16” nuts. No special torque indicated.

What’s your target torque for driving exhaust studs? If minimum for a 5/16” stud is 25 inch pounds and the limit for exhaust stids is 100 inch pounds, what’s ideal? 25 seems low. Perhaps why some exhaust nuts back off?
I usually set the torque wrench to 100 and set the depth. They feel plenty tight in a good head. The only ones I see come out are cracked or the threaded hole in the head is wallowed out. Oversized studs work if the head isn't cracked. I start with the smallest and see how tight it feels and work my way up from there. They come in several different over sizes.
That’s what I’ve done in the past. If the studs have a good fit, torquing nuts to 17 foot pounds shouldn’t be a problem.
We have made it standard practice in our shop that all exhaust nuts (on any engine) get re-torqued after a good hard run up (after cool down). This has put a stop to a number of problems. We are also very reluctant to Helicoil exhaust stud bores on the aircraft, seems the studs will not drive in secure without going too deep and tend to back out. May just have been poor luck and maybe we're just not that good but as much as I love Helicoils, we avoid them in that application unless it is exceptionally small problem with really good access.
Op here- i checked the size of the nuts on various studs. Some are 7/16. Some are 1/2 inch. Many must have been helicoiled…. Is that normal?

I will add it to my “reasons to upgrade to o320” list
Should all be 5/16" on a Lycoming. Takes a 1/2" socket.

I dont understand this statement

When I am listing size of the nut I am referring to the size of the socket that fits. If a 5/16 fits snugly i call that a 5/16 nut. If 1/2 is snug i call that a 1/2 nut.

Wouldnt a 5/16 nut take a 5/16 socket? Or are you talking depth?

I would imagine I am reading your statement wrong.
Only way I do it is new nuts and lock washers everytime. If the head isn't cracked you can put a helicoil in and lock the coil with permatex sleeve retainer 64000. High temp sleeve retainer won't fail in that area.
I dont understand this statement

When I am listing size of the nut I am referring to the size of the socket that fits. If a 5/16 fits snugly i call that a 5/16 nut. If 1/2 is snug i call that a 1/2 nut.

Wouldnt a 5/16 nut take a 5/16 socket? Or are you talking depth?

I would imagine I am reading your statement wrong.

When referring to nut size, the size of the nut is the size of the bolt it fits, not the size of socket needed to fit the nut.
Well dont know if will help here but believe it or not on racecars/streetcars I've owned in the past with headers the bolts almost always loosen up over time and u get an exhaust leak and it sounds like ur great grampas flatbed farmtruck, so we would pull the bolt clean it on a wire wheel and spray the **** out of it with easy-off oven cleaner, let it dry and put backin, its a high heat locktie that WILL come backoff when U want it to.