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Exhaust stud question.

AX-O

Well Known Member
All,
I had a broken exhaust stud on one of my cylinders. Gforce came by and removed the stud. When we tried to put a new stud in the hole, the stud is like the Energizer bunny, it keeps going and going. The stud won?t stop and goes in very far. If I go in until it stops, the stud is not long enough to hold the exhaust. I checked the part number and it is the correct part. Also, it is the same part number that I used on the other cylinders (and it works). Is there a place were I can find a longer stud? Can I simply use a bolt in that hole? Also, why do the exhaust side of the cylinder use studs and the intake side use regular bolts? Thanks for any help or guidance.
 
first verify

the stud is installed in the correct direction. The stud has 2 different thread pitches. If installation direction is correct, install a .003 oversize stud.

Allen
 
The thread pitches are the same (5/16-18), however one side does seem to be machined to a tighter interferance fit. This is the side with a longer length of thread. Is that what you mean? Is this the side that goes into to the head? Is the stud held in with only the intereference fit, or should it bottom out in the hole?
 
The stud doesn't need to and shouldn't hit bottom. I discovered the same thing while changing studs to install a prop governor. The important spec is the torque that it takes to turn it in- if it fits too loose you'll need to go oversize.
 
Thanks for the info guys. Another dumb question. Why can't I use bolts on the exhaust side? The intake side has them. Could I use a bolt there and safety wire it?
 
No, you should use a stud

Axel,

I was looking at your fastback thread this morning and noticed you referenced this thread, and that no one had answered your question. The short answer is that no, you should use a stud not a bolt. The reason is that when you have large temperature changes the fasteners are designed for a certain tension when cold so that they still have a tension when hot that will work for the design. When you torque a bolt you will be getting a certain amount of resistance that is changing as the bolt tightens related to the amount of thread engagement. When the face of the bolt head contacts the exhaust fitting, the torque also includes the friction of the faces between the bolt and the fitting. The problem is that it is very dificult to get accurate bolt stretch (tension) from a torque value under these conditions. However, when you torque a nut, it has a fixed thread length that does not change, and you can measure this torque as you tighten and add it to the final torque value to get an accurate stud stretch. I hope this makes sense.

Edit: I have no specific knowledge of the design on these particular studs, but I used to design studs and bolting for high temperature oil refinery heat exchangers and the issues are the same. I assume that the Lycosaurus boys did the same analysis, or they would have used bolts.
 
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oversize studs

When installing studs bottoming out is not permited. What you want to happen is to reach the specified drive torque while turning the stud in then stop. If the stud goes in to far or bottoms before reaching required drive torque, an oversize stud must be installed. Oversize studs are available in .003, .005, and .007 sizes. Start with the smallest one (.003) the are usally pinkish in color. If the studhole threads are stripped or damaged in any way, a helicoil repair of the hole is required. Good luck, Russ
 
What Russel said. On the lighter side... I worked on a ratty cherokee exhaust once where someone had really gone oversize. They took 3/8 NC bolts and stepped one end to 5/16 on a lathe. Then they hog-threaded the cyl head to 3/8NC. Actually had threads hanging out in the exhaust port. When I tried to remove the rusty exhaust nuts I was wondering why it was trying to pry the exhaust off. The big studs were backing out pushing on the exhaust flange. Had to use the heat wrench for removal, just like the guys at Midas muffler. Now why didn't I think of that?
 
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