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Do bolt threads grab at the same spot every time? #5330979 01/24/20 10:26 AM
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motor_oil_madman Offline OP
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Am I nuts or is this true. Say you need something to be positioned at a certain spot, but you don't want to over tighten it. If you take it back out and rethread it again will it take more or less turns to get to that ending spot?


2007.5 dodge cummins 6.7 liter. Chevron Delo400 15w40. 7000 mile or 250-300hr intervals.

Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5330981 01/24/20 10:32 AM
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supton Offline
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Should be the same, but you'd have to tighten it to the same amount, and hopefully nothing ground away. Or would cause the surface to sit more proud now, after having its tension released and then reapplied. And that the bolt is not taken past its yield point. And I'm assuming nothing else changed, like the same bolt & nut being reused.


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Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5330991 01/24/20 10:42 AM
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hallstevenson Offline
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Yes, it will. What might need positioned at a specific spot in a nut/bolt application ?

Can you add a washer ? It will still be mostly luck to get it exactly where you want. Try different thickness washers, put a washer (or two) on the nut side if you don't want to see the washer.

Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331007 01/24/20 11:10 AM
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I'm talking more about npt fittings, where a hose barb needs to be positioned a certain direction to prevent a tight bend.

Last edited by motor_oil_madman; 01/24/20 11:10 AM.

2007.5 dodge cummins 6.7 liter. Chevron Delo400 15w40. 7000 mile or 250-300hr intervals.

Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331010 01/24/20 11:17 AM
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kschachn Offline
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Even if it did "stop" at the same position (or if you stop it) there is no guarantee it will be leak-free if you've already tightened the joint once. Tapered pipe threads rely on the interference between the threads for sealing, and every time you tighten the joint you can potentially deform the threads. This requires you to tighten it a bit more the next time.

FWIW NPT are not "bolt threads" as in your title. And they don't "grab" as much as they interfere and seal in that way. If you really, really need a joint to be positioned in a specific geometry then you don't use NPT, you use some other method such as some sort of union.


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Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331011 01/24/20 11:18 AM
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hallstevenson Offline
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Well, you said bolt thread....

With NPT and a pipe fitting, "tight" just means "doesn't leak" so once it's not leaking, if you have to turn it more to line up, you generally can do so. Depending on how much more, you might need a larger wrench or longer handle.

Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331048 01/24/20 12:15 PM
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A_Harman Offline
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Yeah, for tapered pipe threads, you can tighten to get the fitting in the proper position for efficient hose routing. Sometimes you'll need a bigger wrench, but if you are working with iron pipe, it can stand the stress.


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Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331104 01/24/20 01:38 PM
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Pipe Threads are tapered and require some form of thread sealer. You tighen until firm, then additional as needed to align, then stop. If you reverse, you need to remove it, clean and reapply. Y=If you over tighten, you put hoop stress in the fitting and it WILL crack.
Rod

Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331109 01/24/20 01:44 PM
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Most threads are single start, but if you Google "Multi start thread" there is a lot of interesting info out there.

Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331128 01/24/20 02:08 PM
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Yes, normally.

Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331144 01/24/20 02:32 PM
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Dave9 Offline
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Close but not quite. In some cases the threads will wear slightly and will need a small additional rotation to reach the same torque, or if the threads are now contaminated with rust or grease, anti-seize, etc, this can also effect turns to reach the desired torque value.

As far as making a seal on fittings, in some cases you use tape or pipe dope. In some cases the first time you put the parts together, the seat was new and the first installation deforms them a bit so you may need to use some emery cloth or whatever to true the mating surface.

Last edited by Dave9; 01/24/20 02:34 PM.
Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331159 01/24/20 02:51 PM
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I would adjust the amount of tape sealer so that you get the correct tightness at the correct angle.


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Re: Do bolt threads grab at the same spot every time? [Re: motor_oil_madman] #5331204 01/24/20 03:41 PM
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duh
Oy vey!!!
NPT connections are designed to be torqued until they do not leak in service.
You have additional adjustments of approximately 1/3 to 1/2 of a turn in to position the fitting. Any more than that and you risk splitting the female NPT connection. Bear in mind that NPT fittings, when properly taped, will seal at a much lower torque value that people generally anticipate. So when I say 1/3 to 1/2 half turn after sealing occurs, please take into account that you need to know the exact moment sealing has taken place when rotating the fitting in order to apply this rule of thumb.

An alternate and likely easier solution is to use a swivel elbow.
Like this:


Screenshot_20200124-164032.png

2001 Mitsubishi Galant 2.4 - 165k+
Grandma's creampuff...
Re: Do bolt threads grab at the same spot every time? [Re: Imp4] #5331271 01/24/20 05:30 PM
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heynow Offline
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Originally Posted by Imp4
duh
You have additional adjustments of approximately 1/3 to 1/2 of a turn in to position the fitting.


If this were true, half the fittings installed would not be able to be clocked correctly
More like approximately one full turn of adjustment after the joint is sealed.

Last edited by heynow; 01/24/20 05:32 PM.

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Re: Do bolt threads grab at the same spot every time? [Re: heynow] #5331325 01/24/20 07:04 PM
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Originally Posted by heynow
Originally Posted by Imp4
duh
You have additional adjustments of approximately 1/3 to 1/2 of a turn in to position the fitting.


If this were true, half the fittings installed would not be able to be clocked correctly
More like approximately one full turn of adjustment after the joint is sealed.

Adjust the tape, switch to a paste sealer. Tap the female threads deeper, same with the male threads.
There are several ways....

As always, you do your thing and I'll do mine.
Cheers!!!!
cheers


2001 Mitsubishi Galant 2.4 - 165k+
Grandma's creampuff...
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