Bolt Tightening: Same Position w/ Same Torque?

[YoshiMoshi]

TL;DR Summary

Will the mark be the same each time?

Say I have a bolt and I thread it into a threaded hole. If I torque it down to a certain torque, then use a paint marker and mark the head of the bolt and the surface that the threaded hole is in with a straight line. Then I unfasten the bolt and refasten it into the same exact hole, and torque it down to the same torque would the line on the head of the bolt line up with the line on the surface upon refastening to same torque?

 

[jack action]

If the parts are very stiff, probably. But if you have "soft" parts (like a gasket for example - which could be made of a softer metal like copper) they often go under permanent deformation on the first installation to the point that re-torquing is sometimes necessary. In such a case, the lines would obviously not line up.

Should I retorque head bolts? Yep, that seems to be a big question on the web and it gets, like most questions on the internet, a variety of answers. In short, at least here in 2020, the answer is no, you don't have to retorque head fasteners, maybe. You see, just like the weather man there's always a chance of rain and with head gaskets there's still times when retorquing the bolts and studs is call for, but it's not very often. If you catch any seeping you can try retorquing the bolts, but if anything gets between the gasket and the block or head surface it may be too late to save yourself from installing a new head gasket. Some manufactured might call out for a retorque procedure, but today, with our modern head gasket materials, it's a pretty rare deal.

After doing some research it seems that the idea of retorquing head bolts originated "back in the day" when composite head gaskets were commonplace. After being torqued down these gaskets would tend to "settle" a bit, which made retorquing them a good idea to prevent seepage and other issues. Modern head gaskets don't have this issue and as such don't require retorquing.

Then there is also the possibility of having dirt in the thread that affects the initial locking position.

 

[Baluncore]

Say I have a bolt and I thread it into a threaded hole.

Then it is a screw, not a bolt. A bolt has a nut to tighten it.

It will not always be the same because the thread and the four contact surfaces will be worn during each cycle.
The bolt will also stretch slightly.
The temperature and the lubrication will also differ.

 

[sophiecentaur]

The bolt will also stretch slightly.

Yes and that will depend on the depth of the 'hole' before the threaded item (i.e. the strain on the bolt). Often, the torque is specified with or without lubrication.
I'm not too sure about your definition of a bolt. Very often you use a spanner on both ends. I thought that a bolt has an unthreaded portion but that may be wrong. Looking in catalogues can also be confusing!

 

[Baluncore]

I'm not too sure about your definition of a bolt.

https://en.wikipedia.org/wiki/Bolt_(fastener)#Bolts_vs._screws

 

[Averagesupernova]

Then it is a screw, not a bolt. A bolt has a nut to tighten it.

Next time I have the cylinder head off of something I'll be sure to ask the guy behind the parts counter for a set of head screws instead of head bolts. I mean isn't it better to stand with the fastener gods and be judged by the fastener world rather than the opposite?

 

[Lnewqban]

TL;DR Summary: Will the mark be the same each time?

Say I have a bolt and I thread it into a threaded hole. If I torque it down to a certain torque, then use a paint marker and mark the head of the bolt and the surface that the threaded hole is in with a straight line. Then I unfasten the bolt and refasten it into the same exact hole, and torque it down to the same torque would the line on the head of the bolt line up with the line on the surface upon refastening to same torque?

My answer is yes, as long as the loads are moderate (material not reaching permanent deformation conditions) and the mechanic is careful regarding cleanness of surfaces and smoothness of applied forces.
I have seen the same exact position for all the bolts and screws in my motorcycles during years of assembling and disassembling parts.

 

[sophiecentaur]

as long as the loads are moderate

It's often specified that cylinder head 'bolts' (for example) should be used only once. I had a real job getting one out after I'd tightened it up to the recommended torque and it parted. I didn't want to take them all out 'cos I was afraid the cylinder head gasket would not work after one flattening. (I was a bit short on workshop tools at the time and eBay didn't exist. So once only seems to be the mantra for those high tensile bolts.

 

[Baluncore]

Next time I have the cylinder head off of something I'll be sure to ask the guy behind the parts counter for a set of head screws instead of head bolts.

There was a time when cylinder heads used studs with nuts on top of the head. The stud could have UNC in the cast iron block, with a UNF nut above. That made it easier to re-torque a head with a composite gasket. That stud was a bolt, because it was tightened with a nut, and the term CHB has stuck.

 

[sophiecentaur]

This could go on for ever but my local fencing supplier has massive wood screws with hex heads and they’re labelled “coach bolts” probably on account of the hex heads.

I don’t know how many angels could dance on them.

 

[Baluncore]

“coach bolts”

Those wood screws are also called "coach screws", et al.
https://en.wikipedia.org/wiki/Screw#Lag_screw

A "coach bolt" here has a domed head with a short square neck to prevent the bolt turning while you tighten the nut. AKA "cup head bolt".
https://en.wikipedia.org/wiki/Carriage_bolt

I blame the industrial revolution and WWI for the global variation in fastener terminology.

 

[hutchphd]

Always a "carriage bolt" in my USA vernacular

 

[sophiecentaur]

Which reminds me of the local newspaper headline: "Nut screws washers and bolts" which referred to some inappropriate behaviour of a mental patient in the town laundry.
(I'll get my coat).

 

[Flyboy]

... If you keep the same stack of non-compressible hardware on the same location, yes, it will return to the same location.

I know this because that is the British way of doing castellated nuts and cotter pins on aircraft. I was introduced to this wonderful bit of knowledge by Hawker during a stint in Wichita a few years back. Here's how it works:

You have a new bolt you need to install in, say, a Hawker 700 (location is irrelevant). You first get all new hardware (washers, nut, bolt, etc) and you install them hand tight, then torque to spec. Then, you mark where the castellations on the nut are, remove the entire stack, drill the hole for the cotter pin, then reassemble the whole stack again, then torque it until the hole lines up. Pin it and you're done.

This is distinctly different than AN or NAS hardware where the hole for the cotter pin is drilled at the factory and you have an acceptable torque range. Torque to the lower end of the range, then advance to the point where the hole lines up with a castellation on the nut, then cotter pin it. If you can't get it to line up without exceeding the upper limit, throw the thinnest washer available at it. Usually that's enough of an offset to get the hole to line up.

 

[Bystander]

It's often specified that cylinder head 'bolts' (for example) should be used only once.

"Torque to 'yield/failure'," yes; "real" bolts, no.

 

[Baluncore]

... drill the hole for the cotter pin, ...

That opens another can of trans-Atlantic worms.

Long ago, a split pin was used to retain a wedge or tapered cotter, so the split pin was called a "cotter retaining pin". That got shortened to "cotter pin" in the USA.

Tapered cotter pins are still used to attach the pedal crank arms on bicycles.
https://en.wikipedia.org/wiki/Cotter_pin

 

[Flyboy]

That opens another can of trans-Atlantic worms.

Long ago, a split pin was used to retain a wedge or tapered cotter, so the split pin was called a "cotter retaining pin". That got shortened to "cotter pin" in the USA.

Tapered cotter pins are still used to attach the pedal crank arms on bicycles.
https://en.wikipedia.org/wiki/Cotter_pin

Interesting. They didn't spend a lot of time on etymology at A&P school... the were more focused on teaching us how to use the stuff!

As another interesting aside, the stuff I'm most familiar with for pins is the MS24665 pins, which come in a huge variety of sizes.

 

[Averagesupernova]

There was a time when cylinder heads used studs with nuts on top of the head.

Yep. Not that long ago that I dealt with one. Can't recall what it was though. A diesel head I will never reuse head bolts. But have on many gassers. Had a Chevy 305 once upon a time the bolts just continued to stretch and stretch. Never did torque to spec so they all got replaced.

 

[jack action]

Then it is a screw, not a bolt. A bolt has a nut to tighten it.

I prefer this distinction:

What is the difference between a bolt and a screw?

Historically the difference between a bolt and a screw was that the screw was threaded to the head whereas the bolt had a plain shank. However I would say that now this could cause you a problem if you made this assumption when specifying a fastener. The definition used by the Industrial Fastener Institute (IFI) is that screws are used with tapped holes and bolts are used with nuts.

Obviously a standard 'bolt' can be used in a tapped hole or with a nut. The IFI maintain that since this type of fastener is normally used with a nut then it is a bolt. Certain short length bolts are threaded to the head - they are still bolts if the main usage is with nuts. Screws are fastener products such as wood screws, lag screws and the various types of tapping screws. The IFI terminology and definition has been adopted by ASME and ANSI.

I never heard of an engine built with cylinder head screws, even if there are rarely nuts involved.

 

[Flyboy]

To me, the difference between a bolt and a screw is entirely how you interface with it to apply torque. Screws have a recess of some sort that you insert the tool into. Bolts are inserted into the tool to interface with it.

Threads have nothing to do with the difference. There's bolts with threads all the way to the head, and bolts with an unthreaded shank. Same with screws. You can also have different thread types, like you have machine screws with fine threads and you can have lag bolts that are intended to grab into whatever you're putting them into.

 

[Baluncore]

Screws have a recess of some sort that you insert the tool into. Bolts are inserted into the tool to interface with it.

The gender of the driver is an interesting distinction.
Bolts are male, screws are female.

I go with bolts having a well-defined standard parallel thread, that can be tightened by turning a nut with the matching thread.
You must turn the head of a screw to install it. A screw has no nut with a standard parallel thread.

 

[Nugatory]

"Torque to 'yield/failure'," yes; "real" bolts, no.

Even with fasteners that are not torque to yield, I am cautious about reusing heavily stressed fasteners: check for stretching, run a nut the length of the threads. And I don’t care how good it looks, I am not going to reuse a con rod big-end bolt.

 

[Lnewqban]

With so many different applications for fasteners, making a black and white definition must be debatable.

https://www.boltdepot.com/Hanger_bolts_and_Dowel_screws.aspx

https://www.boltdepot.com/Sex_bolts_and_Mating_screws.aspx

https://www.boltdepot.com/Socket_screws.aspx

 

[Averagesupernova]

With so many different applications for fasteners, making a black and white definition must be debatable.

Ding ding ding ding ding! I would agree with that statement whole heartedly.

 

[Averagesupernova]

Here's a link to a company that calls a particular fastener a stove-bolt. By the illustrations, they have all the qualities that have been described for being a screw. https://www.aftfasteners.com/stove-bolts/

 

[Baluncore]

By the illustrations, they have all the qualities that have been described for being a screw.

They write; "Stove bolts are sometimes referred to as a machine screw".
Stove bolts have a parallel UN standard machine thread. When you put the nut in the picture, they clearly become bolts, and so would then be subject to the US tariff for bolts, not screws.

"Distinguishing Bolts from Screws".
https://www.cbp.gov/sites/default/files/assets/documents/2020-Feb/icp013_3.pdf

 

[Flyboy]

The gender of the driver is an interesting distinction.
Bolts are male, screws are female.

I go with bolts having a well-defined standard parallel thread, that can be tightened by turning a nut with the matching thread.
You must turn the head of a screw to install it. A screw has no nut with a standard parallel thread.

Counterpoint: Machine screws. They have well-defined standard parallel threads that can be tightened by turning a nut with the matching thread. They're very common. A good example that I remember is the AN515 or MS35206 family.

Further counterpoint: Lag bolts. They do not have an accompanying nut, but are called bolts.

That's why I went for the driving mechanism as the defining point.

 

[Baluncore]

That's why I went for the driving mechanism as the defining point.

But screws are defined as being driven from the head end; that is opposite the point.

You are free to call them what you want, bolts or screws. If you deviate from your national customary practice, you will be misunderstood, to your cost.

In the USA, the border authorities would class them as screws by following down this list until the determination was made at paragraph 2.
1. An externally threaded fastener, which because of head design or other feature, is prevented from being turned during assembly, and which can be tightened or released only by torquing a nut, is a bolt.
2. An externally threaded fastener, which has a thread form which prohibits assembly with a nut having a straight thread of multiple pitch length, is a screw.
3. An externally threaded fastener, which must be assembled with a nut to perform its intended service, is a bolt.
4. An externally threaded fastener, which must be torqued by its head into a tapped or other preformed hole to perform its intended service, is a screw.

My summary:
A bolt has a nut with a straight thread of multiple pitch length.
A screw can only be tightened or released by turning the head.

The OP question assumes the phase angle of the screw to the clamped material be repeatable. If it was a bolt, then the differential phase angle of the nut to the bolt would be repeatable, but that requires the nut have a defined work face, and not be installed upside down.

 

[sophiecentaur]

Tapered cotter pins are still used to attach the pedal crank arms on bicycles.

The very worst invention ever. Great when you have engineering awareness but they only work properly when the user realises that fitting / filing is usually required. The bikes I had as a lad all had wobbly cranks. Cotterless cranks rule these days.
So the bicycle cotter pin would have been a 'bolt'?

 

[Baluncore]

So the bicycle cotter pin would have been a 'bolt'?

Yes. It is described in paragraph 1. An externally threaded fastener, which because of head design or other feature, is prevented from being turned during assembly, and which can be tightened or released only by torquing a nut.

 

[sophiecentaur]

Categorisation is a dodgy business. I have a DC motor with an armature which is threaded at one end, to keep the whole thing together with a nut. An 'armabolt' perhaps.