adding threads to bolt

Ted in NE-OH

Well-known Member
I'm replacing the front axle pivot bolt on an AC G. I may have to add additional threads to a 3/4" bolt. If the bolt were a grade 5 or 8 would this be hard to do? May need only 1/2" more threads.
 
I do it all the time.
Just run a die on it to where you need it. Keep oil on it and work the die back and forth to keep the chips broken off. Easier then starting new threads because they are already started straight.
 
Like said by other it can be done. Use a good set of dies and use plenty of thread cutting oil and as you cut keep running die back and forth.
 
Reason I said "no way" is, once I ruined a metal cutting bandsaw blade by trying to saw a grade 8 bolt (didn't look at marks on the bolt head).
 
There again, It has to do with the quality of the tool . I have blades that will cut them with no problem. The cheap Bi-metal ones won't. Back to the old saying you get what you pay for. I have some solid carbide drill bits that will drill about anything, but don't even ask what they cost.
 
Go with a grade 5. 8 would be better but you will probably ruin your die. Gs aren't too heavy on the front. The thread on bolts are rolled on and with that work-hardens the G-8s, especially where the thread runs out.
 

Most bolts the threads are rolled not cut, so the shank of the bolt OD. is smaller than the OD. of the threads.

Dusty
 
Grade 5 or 8, don't matter, a good quality die should easily cut the requisite # of threads. Be sure to lube the object being threaded with cutting oil, or hog lard, if you can't find the cutting oil.
 
Yes lard, it works good. That's all Dad ever used. Must be good as I have his old tap & die set that I know is over 70 year's old, and it still cuts excellent.
 
You can look up the hardness of bolts. If I remember correctly Grade 8 bolts can be close to Rockwell 30 hardness. You could probably buy the proper length bolt online and have it delivered to you sometime next week for around $5.

Could you stack some flat washers under the nut or under the bolt head so you could use the longer bolt without modifying it? That would retain the full strength of the bolt.

As others have said, bolt threads are roll formed before heat treating. Cutting longer threads on a bolt does work, but it introduces stress risers that weaken the bolt. In an application where there can be a lot of shock I would try to avoid using a modified bolt, or have a plan to replace it before it breaks.
 
I would find a machinist and have them turn additional threads on a lathe.
If you decide to thread with a die--Nut Dies are not designed to cut new threads, even in standard steel. They are designed to "clean up threads".
Just use the correct die.
 
I have had them like that but also han some just the oposite in that the OD was bigger than the threads so could be correct size, too small or too big to do it.
 
In the past 30 some years I don't remember how many times I have taken a 40 dollar band saw blade out to the tool shop one day and the next day be ask for another because the blade is almost smooth with the end of a grade eight bolt laying on the floor. I keep a can of tap magic for the harder metals like grade eight bolts.
 
I have "Snap-On" tap and die sets, from #4,6,8
10, machine screw threads, course and fine, up
through 1-1/4, course and fine. All of mine will
cut threads in grade 8!
 

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