Exhaust Studs won’t Stay Put

[snoopyflys]

I am fairly certain this is a no brainer and should no better by now, but I can’t seem to find the correct threadlocker for installing/reinstalling exhaust studs in my Titan ECI XIO-360. I tried the green stuff and let it sit for the required time and when I attempting to torque the nut, the stud turned. Is there a shelf life for threadlockers or maybe I’m not applying it correctly or in the proper amount to achieve the desired result. Any tips/hints/links etc would be greatly appreciated.

[danny]

Hi, I've seen and had the same problem. The solution was oversized studs. I cant remember the sizes but want to say .003", .007" bigger. Anyway it's a direction to look.
good luck
danny

[SkyJunkie]

There are some high temperature thread lockers out there but I would hate to use one on an aircraft cylinder as it might further damage threads the next time the stud is removed. You should not need to use thread locker if you have the correct oversize stud and you install it correctly. I did notice that McFarlane has just about every engine stud needed for Lycoming and Continental engines including the oversize varieties.

[BobTurner]

When you install the stud you should be double-nutting it and torquing it to the same torque as you will use when you put a nut on it. It should not turn when you torque the nut on.

[Bicyclops]

Originally Posted by BobTurner

When you install the stud you should be double-nutting it and torquing it to the same torque as you will use when you put a nut on it. It should not turn when you torque the nut on.

The overhaul manual lists minimum installation torque for studs in the table of limits. If I remember correctly, it calls for the threads to be lubricated. You run the stud into the head using a torque wrench and when you reach the proper depth, the torque must be higher that the minimum. If not, you need the next oversize stud and try again.

Ed Holyoke

Edit: Lycoming table of limits specs min. driving torque of 25inlb for a 5/16-18 stud. Looks like the lubricate threads call was for crush gaskets, not studs.

[Freemasm]

Originally Posted by Bicyclops

The overhaul manual lists minimum installation torque for studs in the table of limits. If I remember correctly, it calls for the threads to be lubricated. You run the stud into the head using a torque wrench and when you reach the proper depth, the torque must be higher that the minimum. If not, you need the next oversize stud and try again.

Ed Holyoke

Edit: Lycoming table of limits specs min. driving torque of 25inlb for a 5/16-18 stud. Looks like the lubricate threads call was for crush gaskets, not studs.

For my benefit, if you don’t mind. Does the Lyc manual call for a max insertion depth or a bottom out/back number, i.e. what is the procedure/assurance that (differential) expansion doesn’t over stress the threads? I haven’t had a reason to look at a Lyc manual in a long time. Thx.

[Bicyclops]

Originally Posted by Freemasm

For my benefit, if you don’t mind. Does the Lyc manual call for a max insertion depth or a bottom out/back number, i.e. what is the procedure/assurance that (differential) expansion doesn’t over stress the threads? I haven’t had a reason to look at a Lyc manual in a long time. Thx.

The table of limits doesn't list installed depth for the studs. The studs have threads on both ends and, generally, an unthreaded portion in the middle. You wouldn't want the stud to either bottom in the hole or to drive the unthreaded part into the female threads. An oversize stud might well have a very tight fit with a standard nut, so you'd want to drive the correct end into the head. There is usually a stamp mark on the top of the stud. I'd figure how tall I want it to stand to allow for the thickness of the gasket, pipe, flat and star washers, and nut with some left over. You could look at the ones that haven't been messed with on other cylinders as a guide too. I'd drive it to the predetermined correct height while looking for the minimum torque. With a clicker wrench it's easy. Set the minimum torque and if it clicks before you get to where you want it, keep driving until it's there. If you hit min torque in the first thread or three, I'd suspect the stud is too far oversize.

Aluminum does expand faster than steel which is why the stud wants to have an interference fit. The minimum driving torque during install would assure that the stud will stay put when hot. If it had a "tight" fit but didn't make minimum torque, it probably wouldn't be secure.

Ed

[BillL]

Originally Posted by snoopyflys

I am fairly certain this is a no brainer and should no better by now, but I can’t seem to find the correct threadlocker for installing/reinstalling exhaust studs in my Titan ECI XIO-360. I tried the green stuff and let it sit for the required time and when I attempting to torque the nut, the stud turned. Is there a shelf life for threadlockers or maybe I’m not applying it correctly or in the proper amount to achieve the desired result. Any tips/hints/links etc would be greatly appreciated.

I would not put just any thread locker on the studs, it will just melt, oil/grease (cheap stuff) maybe so it will coke and seal. Henkel #2620 will go to 650F, it may be OK.

Here is a link for McFarland overside studs if the insertion torque is too low.

[lr172]

Originally Posted by Bicyclops

Aluminum does expand faster than steel which is why the stud wants to have an interference fit. The minimum driving torque during install would assure that the stud will stay put when hot. If it had a "tight" fit but didn't make minimum torque, it probably wouldn't be secure.

Ed

Studs do not need the end inserted into the threaded hole tight in order to hold properly. There is no requirement for this tight fit in order to obtain the rated strength of the stud. In applications like this, the designers will sometimes use an oversize section or thread lock in order to insure that the stud stays in place when the nut is removed for maintenance.

If the issue were expanding aluminum not supporting the threads, how would your bolts hold that are NOT interference fit, like the intake bolts or the case bolts, etc.?

Larry

[Freemasm]

Originally Posted by lr172

Studs do not need the end inserted into the threaded hole tight in order to hold properly. There is no requirement for this tight fit in order to obtain the rated strength of the stud. In applications like this, the designers will sometimes use an oversize section or thread lock in order to insure that the stud stays in place when the nut is removed for maintenance.

If the issue were expanding aluminum not supporting the threads, how would your bolts hold that are NOT interference fit, like the intake bolts or the case bolts, etc.?

Larry

To be clear, my concern was the axial stain. Not just the related alphas of the dissimilar metals but the non-homogeneous heating causing some serious related strain and resulting stress. Same reason for gapping a socket weld, an AN fitting on a hose, etc.

Sounds like you have to trust the assembly quality (part manufacturer and/or A&P) that you’ll run of of threads before bottoming out; the latter of which could cause some expensive and potentially dangerous problems. If someone knows if this assumption is true, please pass it along.