Bolts. Especially TTY ones.

[Domdom]

Would one of the more knowledgeable folks here please explain to me why there seems to be a craze now for “torque to yield” bolt usage? Sometimes called stretch bolts.

The first time I came across them was a Ford 1.6 CVH engine where the head bolts were TTY, and pretty much nothing else I came across were. Yet now it seems half the bolts on a car are TTY….why?

More to the point, is there a decent auto data source where this is identified? I understand that re-using TTY bolts is rather unwise, and apparently, replacing them with 10.9 or 12.9 high tensile can be just as bad… But how is a mere mortal like me supposed to know?

 

[MetalMonkey]

But how is a mere mortal like me supposed to know?

You don't.
You have to trust the service procedure, and if it says replace, then you have to replace.
The fun comes when they become NLA on some older vehicles, and then you get into pages of debate on the owners forums.

Bolt torque is a deep rabbit hole, but basically the reason TTY are popular as I understand it, is they are very predictable and accurate in terms of installation and clamping force - so offer advantages from an engineering and manufacturing perspective.
From a servicing stand point, pretty rubbish and more cost. But that goes for a lot of the decisions on modern vehicles.

 

[Mark 5]

The fun comes when they become NLA on some older vehicles, and then you get into pages of debate on the owners forums.

BIB What does this mean please?

 

[Domdom]

You don't.

You have to trust the service procedure, and if it says replace, then you have to replace.
The fun comes when they become NLA on some older vehicles, and then you get into pages of debate on the owners forums.

Bolt torque is a deep rabbit hole, but basically the reason TTY are popular as I understand it, is they are very predictable and accurate in terms of installation and clamping force - so offer advantages from an engineering and manufacturing perspective.
From a servicing stand point, pretty rubbish and more cost. But that goes for a lot of the decisions on modern vehicles.

You mean the ones that "mere mortals" don't have access to? Where do I get such information for my 21 year old Audi? Even the main dealer in Murcia refuses to work on it, stating the parts are NLA. All I want is to know if a bolt can be re-used or if it should be replaced. I do miss the days of a Haynes manual and an Autodata book.

@Mark 5 NLA = No Longer Available

 

[hotponyshoes]

Also, They act a bit like a spring, once past the yield point they can stretch and contract with a consistent clamping force.
So good on something that will expand and contract with heat.

I haven't noticed any particular increase in use on cars but if there is an increase in use it might well be because manafactures are chasing emissions targets so making parts out of lighter materials like aluminium which expands/contracts more than steel?

The bolts themselves can be smaller (sp lighter) for the same clamping force which might also help if you are counting grams?

 

[Munkul]

BIB What does this mean please?

no longer available.

From an engineering standpoint, TTY is awesome. Cylinder head bolts have been TTY for decades on most engines. Once in yield, there's no dispute about calibration of torque wrenches, or friction levels affecting bolt tension. The bolt tension is perfectly set by the material properties of the bolt itself.

You used to be able to measure TTY bolts and re-use if they hadn't stretched beyond a certain point, Nissan used to do this anyway. Seems to be more throwaway these days.

 

[hotponyshoes]

I remember doing a head gasket on one of my first cars. Had to strip it all down after so many miles and re-torque the head bolts.

I'd be much happier now to just buy new tty ones and only do the job once.

 

[minimutly]

They were introduced so production machines could quickly and accurately set "tightness" I believe?
There sometimes exists much myth around them - the early Rover M series were not stretch bolts, but some manuals specified changing them, later the M became T series and these were stretch bolts.
The Rover K were a variation of spring steel, and could be reused, but again manuals stipulated replacement, which was sometimes disastrous since the replacements were inferior.
If the bolt has a spiral on it don't reuse is my rule, if it's a straight shank and hasn't waisted it should be ok...

 

[anto44]

I'd recommend never reusing injector bolts on any modern diesel engines, particularly any VAG engines.

 

[bricol]

Lass here seems to have had the garage who did her timing belt change reuse the crank bolt . . done up with a windy gun as thats the way they do them . . . they explained to her, a qualified mechanical engineer, why she wouldn't understand that as a correct engineering procedure . . .

 

[roofman]

The stretch bolts on the M9r engines (renault) come out exactly the same length as a new unfitted one, dont know what they mean by stretching.

 

[Mark 5]

You don't.

You mean the ones that "mere mortals" don't have access to? Where do I get such information for my 21 year old Audi? Even the main dealer in Murcia refuses to work on it, stating the parts are NLA. All I want is to know if a bolt can be re-used or if it should be replaced. I do miss the days of a Haynes manual and an Autodata book.

@Mark 5 NLA = No Longer Available

Thanks.

 

[Mark 5]

no longer available.

From an engineering standpoint, TTY is awesome. Cylinder head bolts have been TTY for decades on most engines. Once in yield, there's no dispute about calibration of torque wrenches, or friction levels affecting bolt tension. The bolt tension is perfectly set by the material properties of the bolt itself.

You used to be able to measure TTY bolts and re-use if they hadn't stretched beyond a certain point, Nissan used to do this anyway. Seems to be more throwaway these days.

Thanks.

 

[fizzy]

Back in the 80s my mate's father was a senior engineer in Port Talbot steel works. He used to talk of some gigantic bolts they used on the rolling mills. They would stretch them hydraulically. Then spin the giant nut up and release the hydraulics. One very tight bolt always to the same setting.

 

[Erie Fred]

Back in the 80s my mate's father was a senior engineer in Port Talbot steel works. He used to talk of some gigantic bolts they used on the rolling mills. They would stretch them hydraulically. Then spin the giant nut up and release the hydraulics. One very tight bolt always to the same setting.

Yes, large diesel engines hold the conn rod caps and main bearing caps on like this.

I worked on an old (1943) 4 post press, 1000 tons (made a layout to move it)
In the manual, it discussed how the main tie-rods where tensioned.
These columns were 8" dia by 20' long, and had a hole thru their length.

You are to install a heating rod, measure the length change, and then spin the
nuts down by hand. When cooled, they were very much in pre-loaded tension.
IIRC 2.5 times working load (1000 tons).

 

[Domdom]

Aha!!! The light bulb just came on

If I understand correctly, a normal TTY bolt will stretch at a certain torque so that tightening it further (up to the point where the bolt breaks of course) doesn't actually exert any more clamping force. So it can't really be over-tightened.

This might go some way to explaining why replacing a TTY bolt with a 12.9 one is not good as the 12.9 could easily be over-tightened. This was a really confusing part for me, I couldn't fathom why a stronger bolt would (could) be undesirable.

 

[fizzy]

Yes, large diesel engines hold the conn rod caps and main bearing caps on like this.

I worked on an old (1943) 4 post press, 1000 tons (made a layout to move it)
In the manual, it discussed how the main tie-rods where tensioned.
These columns were 8" dia by 20' long, and had a hole thru their length.

You are to install a heating rod, measure the length change, and then spin the
nuts down by hand. When cooled, they were very much in pre-loaded tension.
IIRC 2.5 times working load (1000 tons).

As I have always said - there is very little truly new out there. Every invention we have is based on the works of others!

 

[cocker]

?? If a bolt is done up tty , the thread is stretched ?
Is the theaded hole it went into not deformed/stretched ?

 

[hotponyshoes]

?? If a bolt is done up tty , the thread is stretched ?
Is the theaded hole it went into not deformed/stretched ?

No because TTY bolts are not usually fully threaded.
The non threaded part is usually a reduced diameter section and that bit is the 'stretch zone' so the dimenisons of the threaded part remain the same.

 

[fizzy]

No because TTY bolts are not usually fully threaded.
The non threaded part is usually a reduced diameter section and that bit is the 'stretch zone' so the dimenisons of the threaded part remain the same.

That is one thing I hate seeing with ANY bolt. Threads inside the non threaded hole.