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Pmag conversion from lycoming Impulse coupling

Majorpayne317641

Well Known Member
Does anyone know what lycomings service instruction means when they say the new magneto studs need to have 25in lbs of drive torque? I am replacing my Impulse magneto with a pmag so I'm deleting the extension and studs. I have the new shorter studs and loctite 290 for the install. Now I need to figure out what torque it requires. It is a blind hole and I do not want to damage the accessory case. Link to their service instruction: (closest thing to what I'm attempting to do).

https://www.google.com/url?sa=t&sou...MQFnoECAYQAQ&usg=AOvVaw2oDKlCbXuCk-PUDudK2CJM
 
Logically

To me, this Is the torque of the stud required at the indicated height . If one can not get this level of torque, then they recommend a bigger diameter stud so to reach this torque value. Guess they aren’t wanting a stud that easily turns in its space, but one that is snug @25 INCH lbs.
 
To me, this Is the torque of the stud required at the indicated height . If one can not get this level of torque, then they recommend a bigger diameter stud so to reach this torque value. Guess they aren’t wanting a stud that easily turns in its space, but one that is snug @25 INCH lbs.

By bigger size, there are no custom sized "in between" sized studs correct? Does that mean you have to tap a new sized hole into the case? Maybe I'm misunderstanding that part. I have not tried to screw in the new stud all the way but the first few threads twisted on by hand. 25in lb isn't a lot but I'm not sure I will get that value unless they have some special npt type thread in the case.
 
Studs

Studs are available with oversize THREADS. Standard diameter, the stud thread is machined to create a slightly tighter fit in the accy case.
Check with one of the larger engine parts vendors. They will know what you need. First determine if you really need an oversize.
You will also need a stud driver to use the torque wrench.
 
Studs

To further clarify this an oversize stud will be the same outside diameter on the threaded portion. In the OP case approximately .312 with some variance in manufacturing tolerance. There are two ways that I know of to measure thread dimensions. Go/nogo gages that check the thread by threading the go gage on the thread and then checking that the no go gage does not fit.
The second is a thread micrometer which has custom "jaws" for measuring threads. The micrometer requires specifications for the threads.
The oversized studs for Lycoming are stamped on the end. .003, .006 etc.
Plan on most of $40 for EACH stud.
 
The torque is specified to ensure the stud does not back out. Once tightened, friction between the threads will stop any rotation of the inserted stud and hold it in place. If these are standard threads, which they seem to be, then the torque will only be developed after the stud bottoms out in the hole. So the new studs have to be long enough to reach the bottom of the hole (or at least the threaded portion) and still extend far enough to hold the mag in place.
Adding the Loctite will lubricate the threads and cause higher thread shear values when you go to install the stud. So be careful not to overtighten the stud or you may strip the threads. 25 in-lb is pretty low.
Loctite 290 is like most thread lockers; it looses strength when it gets hot. According the their website, 290 looses about 25% of its strength at 302 degrees F. It starts loosing strength at 140 degrees. But if you get the stud properly tightened the Loctite would be just added security. With the friction from the case threads and the end nut, the stud probably will not back out if you leave the Loctite out.
Just my observations and opinions.

Alan
RV-4
 
Something has to stop it from threading in too far. If not, when you go to put the nut on, it might continue threading in and you would get no engagement on the nut.
 
Studs

Something has to stop it from threading in too far. If not, when you go to put the nut on, it might continue threading in and you would get no engagement on the nut.
Measure the engagement of the old stud and install the new one to match, allowing for the difference in stud length.
 
Mine didn't bottom out...

I'm not an engine guy so take with a grain of salt. Just did this a few weeks ago. Once I got the old ones out, which involved two nuts tight enough together that I was worried about pulling the stud apart and wondering if a welder would come into play. (Good news - once it starts to move the battle was won in my case)

Putting the new ones in was relatively painless. Comfortably over the 25 once I hit the correct length. They hadn't bottomed out on anything, but depth is the limiting factor according to the instructions as I read them.

Makes sense you would have to upsize if they were still loose, but based on my reading before I did mine, my experience is likely typical.

Derek
 
The torque is specified to ensure the stud does not back out. Once tightened, friction between the threads will stop any rotation of the inserted stud and hold it in place. If these are standard threads, which they seem to be, then the torque will only be developed after the stud bottoms out in the hole. So the new studs have to be long enough to reach the bottom of the hole (or at least the threaded portion) and still extend far enough to hold the mag in place.
Adding the Loctite will lubricate the threads and cause higher thread shear values when you go to install the stud. So be careful not to overtighten the stud or you may strip the threads. 25 in-lb is pretty low.
Loctite 290 is like most thread lockers; it looses strength when it gets hot. According the their website, 290 looses about 25% of its strength at 302 degrees F. It starts loosing strength at 140 degrees. But if you get the stud properly tightened the Loctite would be just added security. With the friction from the case threads and the end nut, the stud probably will not back out if you leave the Loctite out.
Just my observations and opinions.

Alan
RV-4

You are exactly right. Which is why I was thinking about how to do this correctly. I know at work I had to engineer a procedure to get red locktite to break lose from a tedious threaded insert. It needed 500 degrees to break free based on the book value and field testing on my work instruction.

The reason why I brought up 290 is that lycoming calls this out in a similar procedure for a magneto adapter that has short studs just like our Pmag. I know I am mixing 2 service instructions, but they each have similarities for the end result I am trying to achieve of a shorter stud retrofit.

The holes are blind, I looked at them from the inside of the accessory case. It is a rough machined casting on the inside back wall case with the threaded blind hole on the outside. Either way with or without loctite, if the stud is installed within the torque value I am sure it won't move. The previous stud seemed to have some sort of thread locker although it never made it all the way down the threads. It required a special stud tool to remove both. One of them went easy, the other took some additional effort, but the tool handled it well.

Link to additional lycoming instruction:
https://www.google.com/url?sa=t&sou...4QFnoECBEQAQ&usg=AOvVaw1pUClrAULmFX-wf9VBD2HZ
 
I stand corrected it is a through hole. The stud has a thread that goes jn snug meeting the torque value. The opposite end threads onto the but holding the magneto down. I had it reversed the first time around. Thanks for the help!
 
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