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Accessory studs

RVG8tor

Well Known Member
I bought some Loctite 271 (red) thread locker for installing new studs in the vacuum pad to accommodate a spacer for the SD20 alternator. I read on here that someone received info from Mattituck about using the thread lock on the studs.

Well the old studs (from a new engine) came out with no trouble but there does not seem to be any thread locker used, maybe some signs of anti-seize. So the real question is for studs on the accessory case, should a thread lock be used or if not what else. I am not a gear head and I have spent the last several days learning as much as I can about this stuff on line. I have learned enough to know there is more than one way to skin the cat for this stuff, so please give me your thoughts.

Cheers
 
Overhaul Manual

I am not sure what the current edition of the Lycoming Overhaul Manual is but the 6th edition I was able to locate said this for stud replacement.

"Coat threads of studs with thread lubricant, Specification JAN-A-669, and drive stud to correct depth by using a suitable stud driver."

So the thread that said use Loctite does not agree with the Overhaul Manual unless something has changed since the 6th edition. This also is in keeping with what I found removing the existing studs (no Loctite used).

I would be interested to hear pros and cons of using Loctite if you prefer this method on the studs.

Cheers.
 
I've followed Mattitucks lead on my engines, and not used loc-tite on these threads either - so at least that's two of us Mike.

Paul
 
Studs

I am not an expert but from my experience the Lycoming studs are not necessarily free fitting in at least a Superior accesory case. While replacing magnetos it was necessary to remove the existing studs and replace them with shorter studs.
The original studs came out with difficulty. The new studs, factory Lycoming studs, had to be double nutted and then screwed into the accessory case. This would indicate that there is a locking thread between the stud and the case, not the usual free threading design that we are accustomed to. Has anyone else experienced this?
 
Tight fit

The studs are slightly larger at the end that goes into the accessory case, in my case a 1/4 nut will spin easily on one end and not the other, the large end goes in to the case.

You may be right in that this tight fit acts to lock the stud. I been heavily in to the Education part of building doing stuff on the engine.

Cheers
 
The Lycoming Overhaul Manual specifies a minimum torque spec when installing the stud. I.e. you are OK if it takes at least this much torque to turn the stud when installing it. This ensures that the stud fits tightly enough in its hole so it should stay in place. My manual is at the hangar, so I can't look up the value for you.
 
The studs are slightly larger at the end that goes into the accessory case, in my case a 1/4 nut will spin easily on one end and not the other, the large end goes in to the case.

You may be right in that this tight fit acts to lock the stud. I been heavily in to the Education part of building doing stuff on the engine.

Cheers

I just replaced the studs yesterday on my left mag with new Lycoming shorter studs. The orginal nut freely screwed on one end of the stud but not the other. The threads appeared identical and OD measurement looked the same. But one end would screw into the engine freely and the other needed a double nut. I concluded the threads were fatter or not as deep on one end and it must be a means of providing a lock for the stud. Pretty cleaver. I'm not a mechanic and have never encountered this before. Anyway, the difficult end screwed into the engine with reasonable effort, about the same as it took to get the old ones out.
 
I just replaced the studs yesterday on my left mag with new Lycoming shorter studs. The orginal nut freely screwed on one end of the stud but not the other. The threads appeared identical and OD measurement looked the same. But one end would screw into the engine freely and the other needed a double nut. I concluded the threads were fatter or not as deep on one end and it must be a means of providing a lock for the stud. Pretty cleaver. I'm not a mechanic and have never encountered this before. Anyway, the difficult end screwed into the engine with reasonable effort, about the same as it took to get the old ones out.

The studs come oversize and they have a number on the end indicating how much oversize they are- 3 is .003", 7 - .007", etc. If you have a loose fit on a stud you should go the next size up. You don't need to buy these studs from Lycoming, any auto parts store that has Dorman studs should have the equivalent. All of the hardware specified for the Lycoming is grade 5 so the bolts you can get from Tractor Supply are actually superior (grade 8).

On my Rocket I prefer helicoiling the case and installing 1/4-20 bolts instead of replacing studs. I did this on the governor and on the accesssory case where I took out the studs and machined covers for the magneto holes.
 
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OD of 5/16-18 Stud (31C-16)

Anybody know the specs for the OD of 5/16-18 stud (31C-16) on the bigger side of the stud that goes into the engine block. (My basic understanding is the side going into the engine block is slightly larger to provide locking mechanism.


I just received some and they are very loose. Old ones I pulled out measured about .310 new ones are about .307. I don't think my old ones (on new engine) were oversized, I am thinking these I bought off internet are junk\counterfeit?

Would appreciate someone measuring one of theirs or pointing me to a spec.
 
To be more specific on the driving specs for studs, here is the Lycoming document defining it.

lycoming-torques_zpsd981c236.jpg


Note that the torques are pretty low...:)
 
Studs

Above table, is that fine or coarse threads ?
And really what?s wrong with thread locker ?
 
Above table, is that fine or coarse threads ?
And really what?s wrong with thread locker ?

Table V, studs minimum driving force. As stated before, if you get to the correct depth without reaching minimum torque, an oversize stud must be used.

What's wrong with threadlocker is that you may not be able to remove the stud later without pulling out the threads.

Ed Holyoke
 
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