C-90 Exhaust Flange Stud

[JP]

What is the thread size on the exhaust flange studs on a C-90? Need to do a helical insert and am looking for the correct size, etc.

 

[bob turner]

Did one last week and I still can't remember! 5/16-18? Brain is getting old.

 

[JP]

That seems to be the consensus--5/16-18.

What threw us off a bit is the coarse side of the thread on a stud has a slightly different pitch. It is clearly an 18 and a 5/16, yet a 5/16-18 nut won't fit on the stud.

Here is the way it was explained to me.

On a regular thread you have a given angle from vertical; on a stud thread that goes into the block the angle is greater so that it engages the internal threads tighter. This makes sense because you are screwing and unscrewing the nut on the fine side of the thread and you don't want to disengage the coarse side.

Amazing, if you ask me.

 

[behindpropellers]

Usually the studs that go into the case have a coarse thread side and a fine thread side for the opposite. The coarse side usually goes into the softer aluminum castings, etc. When you buy the studs for the engines you can buy them oversize to help keep them in. So....the threads do not have a different pitch, they are just a hair oversize.

Tim

 

[JP]

Pitch is probably the wrong word. The way it was explained to me, a thread has a peak and a valley. The angle of the slope going from the peak to the valley is at a greater angle on the coarse side of the stud to give it some resisitance to being unscrewed when you are removing one of the flange nuts on the fine side of the stud.

One thing is for sure, every inch of an airplane is a potential learning experience... :-?

 

[behindpropellers]

http://www.sacskyranch.com/acatalog/Sacramento_Sky_Ranch_Exhaust_289.html

Here ya go JR!

Scroll down

:lol:

Tim

 

[JP]

Man, I forgot about Sky Ranch--they always do such a nice job of describing things and include great photos. And lots of useful information on their site.

I really like that tool company link you sent--the helical repair kit looks like a darn good deal.

 

[tempdoug]

continental part #22022 brass exhaust nut 5/16-24 fits A65 - 0-300

 

[bob turner]

Or make your own out of brass hex stock - mine are quite long, to avoid corrosion on the bare ends of the studs.

 

[tempdoug]

I was at napa the other day and picked up a box of 10, 55 cents each and no $10.00 ups charge, there the long ones also. doug

 

[JP]

Would that be National Airplane Parts Association? :lol:

 

[JP]

Doing the helicoil (I used Recoil) was a piece of cake and I am very pleased with the results.