Lycoming exhaust stud repair - Helicoil?

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bradg33

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I've got an exhaust stud on one of my GO435s that's decided it doesn't want to stay put any more. We tried an oversized stud (P12), still no dice. My A&P doesn't think a helicoil will work because the exhaust stud is tapered thread and he's not aware of any tapered helicoils. Anyone aware of a tapered helicoil that will work on a Lycoming exhaust stud? Google suggests it’s possible but with minimal details.

If not, any creative options to fix short of removing the cylinder for repair (which I don't want to do unless absolutely necessary)?
 
bradg33 said:
Anyone aware of a tapered helicoil that will work on a Lycoming exhaust stud?
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Don't know about any "tapered" kind, but the use of regular helicoils has been the standard repair on both Lycoming and Continentals. Lycoming had a SL on it at one time and I believe TCM includes it in their manuals. There are also other type insert type repairs for that location as well which usually fall under a Part 43 minor repair. Perhaps give Lycoming or a cylinder shop a call for the current SL if you want a reference.
 
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As far as I know, the studs aren't tapered. The side of the stud that goes into the head is slightly larger however, so it stays put. A straight thread helicoil should work fine.
 
Awesome info, thanks Gents!
 
AKBill said:
Check out Lycoming Service Letter No. L162B
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Lycoming's search function of technical publications sucks. S.L. L162B is listed in their master list of publications, but it doesn't turn up in a dozen searches following the rules on the search page.

Got a link or PDF?
 
Lycoming Service Letter L162B. The file is a bit big--a result of the round-about acquisition process.
 

Attachments

  • Lycoming.SL.L162B.Heli-Coil.Service.&.Repair.pdf
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Digging this up as on preflight I noticed a stud backed out. Looking at the service letter, is this a special helicoil or just a 5/16-18? A quick search hasnt turned up much for the 4028-5 package.
 
The ones that are tapered are labeled NPT
 
The Lycoming studs are a bit oversized on the engine end so that they're an interference fit in the casting. That's so they don't back out when you're removing the nuts. They're not tapered; they just feel that way when they're going in. They'll fit tight in a helicoil, and that's OK.
 
Silvaire said:
The ones that are tapered are labeled NPT
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NPT is national pipe thread. No such stud exists. The studs on the engine side are oversized, not tapered. Just wasnt sure if the correct helicoil to use was a standard 5/16-18. Ordered the Lycoming p03 and p07 studs to see if those work or if I have to go the helicoil route.
 
1708444100019.png

A Lycoming exhaust stud. You can see that the thread on the right is a tiny bit larger in diameter. That's the oversized end that screws into the head casting. No taper.
 
Racerx said:
NPT is national pipe thread. No such stud exists. The studs on the engine side are oversized, not tapered. Just wasnt sure if the correct helicoil to use was a standard 5/16-18. Ordered the Lycoming p03 and p07 studs to see if those work or if I have to go the helicoil route.
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Look again at Lycoming Service Letter SL162B

4029-1 1/16-27 NPT
4029-2 1/8-27 NPT

I know they're not what you're looking for but they do exist.
 
Silvaire said:
Look again at Lycoming Service Letter SL162B

4029-1 1/16-27 NPT
4029-2 1/8-27 NPT

I know they're not what you're looking for but they do exist.
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The SL provides guidance for using Heli-Coil™ inserts to repair damaged threaded holes in Lycoming engines. The letter gives part numbers for the various size thread repair kits, including 1/16" and 1/8" NPT kits.

Nowhere does the SL suggest there are NPT thread studs. It doesn't even mention fasteners.
 
Silvaire said:
Look again at Lycoming Service Letter SL162B

4029-1 1/16-27 NPT
4029-2 1/8-27 NPT

I know they're not what you're looking for but they do exist.
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If my stud was 5/16-18 national coarse as all o-360 exhaust studs are, how exactly would you drill and thread in a 1/16 -27 pipe thread? Sure pipe thread heli coils exist. But not sure how thats applicable here.
 
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Racerx said:
If my stud was 5/16-18 national coarse as all o-360 exhaust studs are, how exactly would you drill and thread in a 1/16 -27 pipe thread? Sure pipe thread heli coils exist. But not sure how thats applicable here.
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It's not. When I replied to the thread I was looking at the original post and didn't notice it was three years old. I wasn't actually talking to you or your application. Sorry, just wanted to point out that there were NPT cases listed in the SL because the OP asked about tapered threads (back in 2021 - lol) again, my apologies for the confusion.
 
Silvaire said:
Look again at Lycoming Service Letter SL162B

4029-1 1/16-27 NPT
4029-2 1/8-27 NPT

I know they're not what you're looking for but they do exist.
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Sorry, but I am very familiar with pipe-thread helicoils. In my 12 years as a foreman in an air brake component remanufacturing plant, we went through thousands of those things, from 1/8" NPT up to 3/4" NPT. They are for stripped or torn PIPE threaded ports, NOT for studs. Lycoming has threaded pipe ports in their aluminum castings, and sometimes those ports get damaged, especially if someone has used an aluminum fitting in them. Aluminum galls in aluminum and the threads tear out when the fitting is removed. Supposed to use steel fittings.

New Lycoming engines come stock with 18mm helicoils in the spark plug ports. The aluminum threads in the head wouldn't last long with the repeated plug removals.

NPT means National Pipe Thread.
 
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Silvaire said:
It's not. When I replied to the thread I was looking at the original post and didn't notice it was three years old. I wasn't actually talking to you or your application. Sorry, just wanted to point out that there were NPT cases listed in the SL because the OP asked about tapered threads (back in 2021 - lol) again, my apologies for the confusion.
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All good. Thought it was odd phrasing by the OP as well. I know it's an old thread I resurrected. Just wasn't sure if the correct helicoil lycoming calls for in the SL was a standard 5/16-18 or something special to accommodate the oversize threads. I've got the p03 and p07 lycoming oversize studs to see how that works for now.
 
Dan Thomas said:
...They are for stripped or torn PIPE threaded ports, NOT for studs...
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Correct but nobody was saying that they were for studs. I was just pointing out to the original poster that there is such a thing as an NPT Helicoil.
 
Silvaire said:
Correct but nobody was saying that they were for studs. I was just pointing out to the original poster that there is such a thing as an NPT Helicoil.
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"the exhaust stud is tapered thread"

Per op
 
I like the Time-Sert better than Heli-coil for most applications. No idea if its permissible for certified aircraft though.
 
moparrob66 said:
No idea if its permissible for certified aircraft though.
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Nope. If it's not in the engine's parts catalog, or listed on an STC that applies to that engine, it's not legal. Homebuilts are an exception.
 
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