Captive bolt? What is it?


#1

I saw a reference to the phrase “captive bolt” , about a starter motor. Apparently this vendor’s starter motor sported a captive bolt, which somehow made it easier to install. While the competitors product didn’t. I’ve heard of a captive nut, but never a captive bolt.


#2

It might refer to an escaping prisoner!!


#3

I’m thinking it’s a bolt which “stays” with the starter, meaning you don’t have to find room to install that long bolt, if underhood clearances are tight

To be clear, you probably CAN’T remove it from the starter, not without damaging the bolt or the starter

I don’t know if you’ve removed many transmissions . . . but here’s a trick I’ve used a few times, when it came time to reinstall

I fed each bolt through the mounting holes in the transmission, then put an o-ring on each bolt, so that they wouldn’t fall out. Didn’t have to worry about lost bolts, bolts falling out while installing the transmission, etc.

I’m not sure if that’s what a “captive bolt” is, but I’m thinking it’s a fancier and more professional version of what I just described


#4

Sometimes the bolt head is fitted into a cavity that prevents it from turning making it effectively a stud. Othertimes it’s just captive in the hole like this:

Versus a PEM (pressed in) stud:


#5

I agree. Sometimes bolts are held “captive” with one-way push-on metal or plastic retaining washers.

On the other hand, captive nuts (I call them “caged” nuts) often slide into a groove that not only keeps them from falling out, but keeps them from turning, too. that makes the need to hold them with a wrench not necessary.

CSA


#6

Hard-to-catch bolt:


#7

I agree with the “hard to catch” part

But the “captive” part possibly has some disturbing connotations :frowning2:

I’m sure you only meant positive things, though :slight_smile:


#8

Slaughterhouses use it to kill Captive bolt @ Wikipedia


#9

I’ve seen that technique, but it might be problematic with a starter motor b/c it has to mount flush. I got the impression it was functionally equivalent to a stud sticking out of the business end of the starter, so during the install you could push that stud through the hole in the engine flange and the motor would sort of stay in place, given you time to thread a nut on the other end. The alternative would be holding the motor against the engine and threading a bolt through the engine flange, hoping to hit a threaded hole in the starter motor case. The stud method (captive bolt) would make it easier to do the intall as long as there was room to position the motor in place w/the stud sticking out like that. I’m guessing it was referred to as a “captive bolt” b/c it wasn’t a stud, instead an actual bolt captured in a way it couldn’t fall out, as shown in TT’s first photo. Thanks for the comment, I think that more or less solves the terminology ambiguity.

I’ve never installed an automotive transmission @db4690, but I’ve read about a method similar to what you say, not using bolts, but “dowel pins” of some sort. Seems similar and good ideas. The whole process of sliding a transmission into place so it is properly aligned with the engine seems a very awkward thing to accomplish.


#10

The dowel pins you’re referring to are not actually a method

Many engines have dowel pins on the block. I believe one of the purposes is make installation of the transmission easier. Another purpose is so that the engine and transmission are lined up correctly. I haven’t worded this very well, and one of our engineer regulars can please phrase it better and/or correct me :smiley:

Dowel pins won’t help prevent dropped bolts, though

And those dowel pins are generally not threaded, btw. So the engine either has them, or not

I suppose you could make your own dowel pin, though, by lopping the head off of a long bolt, making a slot, and then threading it into the block. But I wouldn’t even bother with that, unless installation of the transmission was a real chore, and the engine never came with factory dowel pins. I haven’t had to do that, yet


#11

When I first saw the posted question I thought it meant something like this:

https://www.accu.co.uk/en/6-captive-screws?mkwid=sj0g4qY2l&pcrid={creative}

Could it be that a captive bolt is like a captive screw, except bigger? What is the difference between a screw and a bolt anyhow?

(I have learned here that “captive bolt” also means the animal slaughter device. I wonder if that is what Dick Francis calls a “humane killer”.)


#12

It’s simple George.

You enter in your search engine, Captive Fastener.

Then click on Images.

And you get something like this.

https://search.aol.com/aol/image?q=captive%20fastener&s_it=searchtabs&s_chn=prt_nav-control-5-g&v_t=comsearch-aolcustomfirefoxtoolbar

Tester