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Exhaust Nut Torque

C

CranelakeMN

Registered User
Crane Lake, Minnesota
Seems to be lots of opinions and ideas out there. The Lyc book I have shows exhaust studs at 40 inch lbs minimum. Does anyone out here know or suggest what the hold down nuts should be torqued to? Thx
 
As loose as they will stay on. Or else the flanges bend(with no blow style gaskets). You can sand the flanges flat again a few times before the must be replaced


Sent from my iPhone using SuperCub.Org
 
Interesting. We smaller Cub owners use brass nuts. Over-torquing results in stripping. But Clint has these neat stainless steel flanges he welds on, and it would take way, way more than 40" lbs to make one banana-shaped.
 
I have not found a current torque value for the exhaust nuts in any current publication from either Piper or Lycoming. The best thing I have found was in the Jan 1973 Table of Limits that has been superseded which gave a value of 160-180 in-lbs.
 
Thank goodness. Spruce has simplified the ordering process a great deal! I just spent $150 on lock washers ahead of my engine install. Worth every penny, too.

If you haven’t tried them? You ought to. They work.
 
Last edited:
stewartb said:
Thank goodness. Spruce has simplified the ordering process a great deal! I just spent $150 on lock washers ahead of my engine install. Worth every penny, too.

If you haven’t tried them? You ought to. They work.
Click to expand...

Timely for me. I've been working on a hardware order for the last week and was trying to figure out which way to go.

Thanks,
Vic
 
My order was quickly filled and shipped from Spruce’s Alaska warehouse. That’s my second order of parts, hardware, and gaskets that have come from the local warehouse recently. Very handy for Alaskans!
 
bob turner said:
Interesting. We smaller Cub owners use brass nuts. Over-torquing results in stripping.
Click to expand...
I use these nuts on the Lycomings as well. They are deeper than the steel Lycoming nut, lock washer and flat washer and they don't rust from the heat. Just run a course thread tap through them, since they are intended for the fine threaded Continentals. They are long enough to cover the entire stud preventing heat and rust erosion of the stud.
 
I Have seen a lot of rusted studs over the years. Some rusted so much that the nuts will not back off and pull the studs out with them.
 
I insisted on the muffler terminating that AD. Nobody was doing it except me, and I hated the job. Never seen a Super Cub without stainless nuts - but have the Heicoil kit ready anyway.
 
I also recommend the stainless nuts and I also put a little dab of hi temp RTV inside the nut. This coats the stud as you install the nut.
 
Thread resurrection since I helped a friend with exhaust stud replacement. I did some research, including here, and finally found the Lycoming info from a reference in my IO-390 maintenance manual to the linked document. My take is the driving torque for the studs in 40 lbs/inch. Torque for the exhaust nuts is 17 lbs/ft. I hope that’s right. We used standard studs because the had just a few foot pounds of resistance. Oversize studs would have been tighter, but there’s no upper limit in the torque specs for these studs, just a minimum.

https://www.lycoming.com/sites/default/files/SSP-1776-5 Table of Limits - Complete.pdf
 
Funny, I just pulled a 100 hr inspection on an IO-390 and had this document opened on my computer.
906 ALL 5/16-18 Exhaust Port Studs (Driving Torque) 40 in. lbs. min.
ALL 5/16-18 Nuts to Attach Exhaust Stacks to Cylinder Head 160-180 in. lbs.
 
Where did you find exhaust nut torque? I’ve looked a few times and don’t see it so I used standard values for 5/16 nuts, 17-19 lb/ft. Exhaust studs are in special values. That’s what I was most interested in. Mike gave me an assortment of oversize studs and I had no idea how to determine what to use or why. Now I do.
 
Item 906 just under the stud torque.
Screenshot_20220827-125450.png
 

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Yours is correct, I was on Part IV Vertical Engines and you are on Part I Direct Drive Engines. I kept part ! in my paper copy and threw the other sections away for this very same reason a long time ago. I had forgotten about that.
 
Ahh, I get it. I was looking at the geared engine chart at first. Not the most intuitive document. I was thinking while torquing those bolts that I wouldn’t have been much different if I was doing it without a torque wrench.

Those aluminum interference threads are interesting. I had enough torque feedback to stay with standards. How much harder would .030 over have been? That has to be a decision you make from experience, and I had none. The project was a result of a loose nut and a missing stud on one flange. When I dropped the stacks I found the one remaining stud wobbled. I removed it with very light pressure and found the stud was about 2/3 as long as the correct part and didn’t have the interference threads. I suspect the missing one was a match. Who does that crap? After chasing the holes with a bottom tap the new studs went right in. In this case I like a little more nut torque to keep the studs in place, because 40 inch pounds isn’t much torque. I enjoyed the project, and the opportunity to learn something new.
 
I keep a couple of oversized studs on hand just in case. Back in the day did a lot of helicoils and studs but mainly factory new engines or at least cylinder so don't get into it much. Got my but
t kicked by a transient Super Cub resently that I had to drop the exhaust on and every nut I torqued pulled the stud out of the head. Reminded me why I don't normally take on new customers.
 
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