Alt Kit Pivot bolt with a "Drunken Thread"!
7-2014 R. Kwas, updates on-going
Reference info: Bolt Ratings
Background: I buy the quality Holo-Krome pivot bolts for the Sw-Em Alternator Kits in batches...typically in boxes of 50. I then add the washers, treat the threads with Anti-Seize or even ACZP, and thread them loosely into the threaded insert of the bracket. This quality check verifies all critical characteristics of the bracket, such as alignment of the pivot bosses as well as threads of the insert, and bolt engagement.
While preparing a batch of brackets, one of these bolts just wouldn't thread in(!)...substituting another bolt, it threaded into the bracket without difficulty, showing that the problem was with the bolt itself...[a burr or something else wrong with the thread, I immediately thought]...but a close inspection of the end of the bolt and threads revealed nothing immediately obvious.
I tried the bolt in another bracket with similar results: It wouldn't thread in!
Holding the bolt up to another to check thread pitch didn't reveal anything either! Threads of the two matched perfectly!....what the?
Two Alt Kit pivot bolts. Top one threads into 3/8" 16 thread perfectly,
bottom one would not.
Performing the typical thread comparison any mechanic might: Perfect pitch match!
[Insert Tim Allen grunt with a rising tailing note here, indicating extreme bafflement!]
...but they did look mysteriously different in the "angle of the thread"...that is, it seemed like the threads still had an issue...I consulted an expert...
Taking the bolt and a "normal" one for comparison to my dad, fastener expert and who happened to also be retired from Holo-Krome Engineering department, I presented them, along with a 3/8'-16 nut to him asking him to examine and explain. It took him only about 15 mS after looking at the two to say: "Ronny, that's what they call a "Drunken Thread". HUH?!? (This is apparently an industry term!)
His Explanation: Apparently, when the bolt blanks are rolled along the threading dies as shown below, occasionally, a blank will drop into the space between the dies not vertically, but on an angle. As the dies then move and the blank rolls between the two, displacing (not cutting!) metal to make the threads, two thread-starts can occur, and a Double Lead/Drunken Thread results. Process control is supposed to prevent, and inspection after the threading operation is supposed to catch and weed these rarities out, preventing them from making it into the box and to the customer, but apparently one got through and to me. Thanks for the explanation, dad!
Cold Forging thread forming after heat treat, shown happening correctly at this video: https://youtu.be/3kxcw08p_oY?t=2m10s
Occasionally, it happens wrong too, resulting in a Drunken Thread!
...so I checked off the "Learned Something New Today" box on my calendar!
Note: Since Alt Kits are prepared and shipped with pivot bolt completely threaded into place, there is no possibility that a customer of the Sw-Em Alt Kit ever receives a kit with a pivot bolt with a Drunken Thread, but I thought I would show, and explain it here for anyone interested.
Reference Information [My Additions. RK]:
...from a popular reference site: http://en.wikipedia.org/wiki/Screw_%28simple_machine%29
The fineness or coarseness of a screw's threads are defined by two closely related quantities:
In most screws, called "single start" screws, which have a single helical thread wrapped around them, the lead and pitch are equal. They only differ in "multiple start" screws, which have several intertwined threads. In these screws the lead is equal to the pitch multiplied by the number of starts. [Intentional!] Multiple-start screws are used when a large linear motion for a given rotation is desired, for example in screw caps on bottles, and ball point pens, and for instance, some drywall screws, but unintentional Double-Starts can occur as a result of manufacturing faults, as explained above!
Pivot bolt of the Sw-Em Alt Kit is clearly supposed to be a Single Start 3/8" UNC bolt, but because of the manufacturing error, Double Starts do occur and one was inadvertently included in the bolt stock, but found and excluded from the kit during the quality check.
...under Lead and pitch:
from a discussion at http://forums.pelicanparts.com/porsche-911-technical-forum/193003-sae-vs-metric-bolt-strength.html
US Grade 5 (three lines on
head) = approx. metric property class 8.8 Regular (nothin' fancy...quality general
US Grade 8 (six lines on head) = approx. metric property class 10.9 Better (the good stuff...for your Brakes!):
US Grade 2 (no lines on head) = Junk low carbon bolts...china's finest!...good for holding together lawn furniture!...well...not really, because (the only way you could get this trash metal to rust any faster than leaving it in the back yard is by dropping them in the ocean!) ...never to be used on your Volvo! ...but nice because you can buy them long, and easily cut them to length (with a butter knife).
Slotted Head bolts...and Square Head Bolts...may have been good for holding sides on a Conestoga Wagon in 1880, or slats to your white picket fence, but when I find 'em now, they go in the metal recycle can.
External material sources are attributed. Otherwise, this article is Copyright © 2014-2016. Ronald Kwas. The terms Volvo and Holo-Krome are used for reference only. I have no affiliation with either of these companies other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here. Holo-Krome makes high quality fasteners, but I guess anyone can have a bad day. (H-K Quality Control Dept are you reading this?) The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion. As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles!
You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source. If you donít, youíre just a lazy, scum sucking plagiarist, and the Boston Globe wants you! As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment.