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Fresh cast iron threads gulling and locking black oxide socket cap bolts

dgfoster

Diamond
Joined
Jun 14, 2008
Location
Bellingham, WA
The additional grip strength offered by the minor peaks of the thread is minimal. If you use good quality fasteners, you'll get better holding power by lowering the "H Limit" of the tap to shrink the gap between screw and cut thread. It'll put more emphasis on thread quality and cleanliness, but more than make up for a small increase in drill size.

Tapping Tip - How H-Limits Relate to Class of Fit (Part 2) - GWS Tool Group

But if the threads are still getting pulled out in use, you might seriously consider going with helical inserts from the beginning. It'll do a lot more for thread robustness than just about anything else aside from changing from a cast iron to an alloy steel.

If these threads are commonly failing, using a Helicoil or other brand insert should improve strength and life of the threaded hole. That is why Helicoils were developed in the first place—-not as a repair but as an enhancement to threaded hole in aluminum aircraft engines.

Denis
 

tylersteez

Aluminum
Joined
Jul 9, 2019
The additional grip strength offered by the minor peaks of the thread is minimal. If you use good quality fasteners, you'll get better holding power by lowering the "H Limit" of the tap to shrink the gap between screw and cut thread. It'll put more emphasis on thread quality and cleanliness, but more than make up for a small increase in drill size.

https://www.gwstoolgroup.com/tapping-tip-how-h-limits-relate-to-class-of-fit-part-2/

But if the threads are still getting pulled out in use, you might seriously consider going with helical inserts from the beginning. It'll do a lot more for thread robustness than just about anything else aside from changing from a cast iron to an alloy steel.

If the rest of the cast iron part is still good and just has stripped out threads then I do put helicoils in as a repair. These threads usually fail from operator error (incorrect length bolts with minimal thread engagement, cross threading, etc). I'm not against the idea of using helicoils from the start to eliminate the possibility of failure from the start as well as the ease of repair if a coil was to fail, however it would be a hard sell to the boss, he would look at it in as not factory correct and not entertain the idea.
 

dgfoster

Diamond
Joined
Jun 14, 2008
Location
Bellingham, WA
If the rest of the cast iron part is still good and just has stripped out threads then I do put helicoils in as a repair. These threads usually fail from operator error (incorrect length bolts with minimal thread engagement, cross threading, etc). I'm not against the idea of using helicoils from the start to eliminate the possibility of failure from the start as well as the ease of repair if a coil was to fail, however it would be a hard sell to the boss, he would look at it in as not factory correct and not entertain the idea.

Might not be "factory correct" unless you consider the mil-specs and aircraft and aerospace common practice of specifying helical coil inserts in their highly-engineered parts:

Aircraft Hardware – Threaded Fasteners Part Eight (Installation Part Two)
"They are used as screw thread bushings. In addition to being used to restore damaged threads, they are used in the original design of missiles, aircraft engines, and all types of mechanical equipment and accessories to protect and strengthen tapped threads in light materials, metals, and plastics, particularly in locations which require frequent assembly and disassembly, and⁄or where a screw locking action is desired."



Helical inserts and tooling comply with the following standards and Specifications:

  • MS122076; NASM122076; NASM122275. Insert, corrosion resistant Helical Coil Fine Thread (Inch Series)
  • MS21209, NASM21209. Insert Screw Thread – Self Locking (Inch Series)
  • MA3279. Insert, Screw Thread, Helical Coil (Metric Series), Screw-Locking
Aircraft Fasteners are authorized distributors for the premier manufacturers of these products, including SPS, KATO and Acme.

Denis
 

1yesca

Stainless
Joined
Jun 1, 2004
This is along the lines with what I was figuring, I'll step up the drill bit size .002"-.005" and see what that does for me. These specific bolt holes always get stripped out by the machine's operators so I was skeptical of oversizing the hole and losing thread depth

studs would fix the stripping problem if there's room for them and it won't affect a period correct thing [ ha ha machine P C ] i have seen on the MB sl v-8 motors that the exhaust bolt holes [aluminum heads ] had tread inserts in them from the factory
 

tylersteez

Aluminum
Joined
Jul 9, 2019
Might not be "factory correct" unless you consider the mil-specs and aircraft and aerospace common practice of specifying helical coil inserts in their highly-engineered parts:

Aircraft Hardware – Threaded Fasteners Part Eight (Installation Part Two)
"They are used as screw thread bushings. In addition to being used to restore damaged threads, they are used in the original design of missiles, aircraft engines, and all types of mechanical equipment and accessories to protect and strengthen tapped threads in light materials, metals, and plastics, particularly in locations which require frequent assembly and disassembly, and⁄or where a screw locking action is desired."



Helical inserts and tooling comply with the following standards and Specifications:

  • MS122076; NASM122076; NASM122275. Insert, corrosion resistant Helical Coil Fine Thread (Inch Series)
  • MS21209, NASM21209. Insert Screw Thread – Self Locking (Inch Series)
  • MA3279. Insert, Screw Thread, Helical Coil (Metric Series), Screw-Locking
Aircraft Fasteners are authorized distributors for the premier manufacturers of these products, including SPS, KATO and Acme.

Denis

I don't doubt their legitimacy and strength for a second, its just him and most people see helicoils as a quick repair option and think of solid threads in the part themselves as the absolute best option. if the topic of helicoils ever get brought up I'm going to explain to him their legitimacy. I appreciate the info you provided
 

tylersteez

Aluminum
Joined
Jul 9, 2019
In all honesty, the bars these threads are used to hold to the machine can be as large as 20'+ long 1-1/4" square barstock, studs would make holding them in place on the machines a lot easier for the operators who are switching them out. again, its just not how the factory did it so its a hard sell to management.
 

dgfoster

Diamond
Joined
Jun 14, 2008
Location
Bellingham, WA
I don't doubt their legitimacy and strength for a second, its just him and most people see helicoils as a quick repair option and think of solid threads in the part themselves as the absolute best option. if the topic of helicoils ever get brought up I'm going to explain to him their legitimacy. I appreciate the info you provided

Ya, there are bosses who greatly benefit from the knowledge of their employees and then their are bosses who boss.

Denis
 








 
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