Reusing TTY head bolts

[Shannow]

Originally Posted By: jsap
Unlike most, the Mercedes manual gives specifications on how long the bolts can be prior to re-use. New is 160 mm (+-0.8 mm), but max is 162.7 mm BEFORE re-use.

I measured mine as 161, so I'll be reusing them.

I wonder if Mercedes bolts are not TTY (Torque-To-Yield), but instead TTT (Torque Turn to Tighten). Maybe these bolts are hard enough that they don't reach yield-point until 162.7 mm, and therefore are never in the plastic range?


If there's a permanent lengthening, then they are yielding, and Merc's maximum recommended length tells you that you are well within the safe zone.

 

[Silk]

Mercedes give a stretch limit for their head bolts...but even so,we fit new ones.

 

[jsap]

Originally Posted By: Shannow
Nope, they are torqued beyond their elastic limit, and when the load is removed, the strain follows a path parallel to the original elastic line to a point of zero stress, which still has some permanent plastic deformation.

I agree, there is still "spring" on bolts that have yielded, because the portion of the "elastic" range snaps back, minus the plastic elongation.

Originally Posted By: Shannow

This chart you showed us is for A36 structural steel (36,000 psi), so you could probably apply this to Grade 1 / A307 bolts (mild steel). It has a scary looking sideways plastic portion (uncontrolled elongation) before strength hardening. Grade 8 / A354 (150,000 psi) would look different.

Originally Posted By: Shannow
TTY gives good, controlled clamping forces.

As long as the total permanent strain is within the flat area, and safely within it, then multiple uses are possible. A manufacturer's material may not have enough flat area to do more than one.

I'm not sure I agree that TTY, if taken beyond yield, gives good clamping forces. Per the same amount of additional force, the elongation is much greater during the plastic phase (which, for this discussion, I'm defining from yield to ultimate strength). That is, while during strain hardening it is getting stronger, it's also elongating easier with less force required to do so. Remember, steel is like a spider web. We say it's "strong" because before fracture point, it can strrrrretch.

Originally Posted By: Shannow
Nope, regular bolts are "springs", their operation in the elastic range holds the parts together, and when undone the bolt returns to it's normal length...repeat.

TTY uses the stress/strain curve in my above post to give consistent and controlled clamping forces in a production environment.

Again, I'm not sure I completely agree. Stretch bolts are taken near yield, but have NOT technically yielded. The A36 low carbon steel stress-strain curve shows an abrupt yield point. The grade 8 materials will show a different curve, with a more descriptive transition to offset yield.

A detailed diagram will show points for true elastic (before dislocation at the molecular level), proportional, elastic limit (what we often describe as yield), and offset yield. I believe that TTY bolts remain within the offset yield point, and that second use will take the bolts beyond that yield point. This diagram shows the offset yield.


Offset yield generally represents about 2% PERMANENT PLASTIC stretch (again, generally). However, it has not yet technically yielded.

On my Mercedes bolts:

160mm(+-0.8mm) x 2% = 162.384mm to 164.016mm

The max stretch prior to use is 162.7. So, my guess is that these bolts have a composition that has about a 2.5% offset yield.

To bullet this up:

- TTY bolts have not yielded, though they have permanently stretched within the offset yield point.

- Second use will POTENTIALLY carry them beyond yield. This "potential" is probably what drives manufacturers to state their no-reuse recommendation.

Anyway, that's my thought. Finding the stress-strain curve of an A354 steel should get us closer to the truth.

 

[Shannow]

Pretty much in agreeance really.

The torque to yield giving an even clamping force, I'll stick with... the flat part imparts a clamping load (stress x sectional area), that has a reasonable accuracy with a considerable error allowance in tightening angle achieving it.

TTY lets a machine do that quickly, by using material properties to ensure that the clamping force is consistent...snug it, then rotate to a flat part of the curve, where a few degrees either way makes no difference.

Manufacturers really are only worried about the 90% (made up statistic) of users who buy a pre-assembled engine in their car,and throw it out. IMO, selection criteria only come when they have to work out field fixes for problems that are either expensive recalls, or one off engine rebuilds.

BTW Strain calculations on your MB bolt should only be calculated on the working length of the bolt, not the overall length, so it's more than the simple calc of extension on overall length.

 

[OVERKILL]

Excellent information Shannow and jsap, I'm learning some stuff here, keep it coming

 

[SteveSRT8]

thanks for those details.

 

[jsap]

Originally Posted By: Shannow
Pretty much in agreeance really.

The torque to yield giving an even clamping force, I'll stick with... the flat part imparts a clamping load (stress x sectional area), that has a reasonable accuracy with a considerable error allowance in tightening angle achieving it.

TTY lets a machine do that quickly, by using material properties to ensure that the clamping force is consistent...snug it, then rotate to a flat part of the curve, where a few degrees either way makes no difference.

Manufacturers really are only worried about the 90% (made up statistic) of users who buy a pre-assembled engine in their car,and throw it out. IMO, selection criteria only come when they have to work out field fixes for problems that are either expensive recalls, or one off engine rebuilds.

BTW Strain calculations on your MB bolt should only be calculated on the working length of the bolt, not the overall length, so it's more than the simple calc of extension on overall length.


Yes, MB's length method is a simplification. Each thread should be examined for uncontrolled elongation, because yield most likely occurs at the root diameter (the valleys between the threads). Not knowing the exact curve of the Mercedes head bolts, I'll just have to observe the threads carefully and be within the specified max length based on their diagram.

As for good clamping force when the bolt has yielded, I agree partially. For static load, yes. The clamping force is good, but only if there is no further dynamic load put on it (which isn't not the case in engines).

For instance, in structural steel statics, once yielded, that structure is deemed unfit for further use. It performed its duty by keeping the structure intact given the force, but it's no longer usable.

In engine application where there are dynamic loads, if bolts are tightened beyond yield, there will be dynamic pressures that, IF exceeds sufficient stress, will easily elongate the bolt. That means gasket failure. That's why TTY bolts are tightened ALMOST to yield (somewhere between the elastic limit and the offset yield point), but not past it. Both in static and dynamic situations, yielded bolts are junk.

But, like I said, I agree with you that in purely static situations (non-dynamic) AND where the bolt has been tightened beyond yield, there is sufficient clamping force. Just not in engines (dynamic loads). And there very few situations where yielded bolts are desirable even in static applications, because there's always the need to account for dynamic loads.

I think there's a lot of misconceptions floating around that TTY bolts are designed to be tightened BEYOND yield. If that were the case, they would be called TTBY!

Rather, TTY is tightened up to yield (more accurately, "offset yield"), but NOT beyond.

 

[mechtech2]

Kestas -
Exactly how do you tighten a used TTY bolt?
Don't tell me that you used the original sequence!
And please don't tell others it is OK!
They might try it!

 

[jsap]

Originally Posted By: mechtech2
Kestas -
Exactly how do you tighten a used TTY bolt?
Don't tell me that you used the original sequence!
And please don't tell others it is OK!
They might try it!


I would agree with you that if the stretch limit is unknown, then do NOT reuse TTY bolts.

But, in Kestas' case, his Mercedes manual provides numbers and procedures. So, as long as there is no unusual stretch at an individual thread, and as long as the numbers are within the specifications, then he should be fine.

TTY bolts are tightened to somewhere BETWEEN these points: elastic limit and offset yield. The delta between these points generally is 2% plastic stretch. If the manufacturer uses low carbon TTY bolts (or, also, bolts of smaller diameter), then the factory assembly process might take the bolts closer to offset yield. However, if higher carbon bolts (or, also, bolts of wider diameter) are used, then it'll be closer to the elastic limit, and so you'll have more turns left before reaching offset yield.

 

[Shannow]

Yep,
in that 2% range, they have yielded. TTY bolts have to have the material chosen that after yield, there is a long plastic deformation range so that clamping pressure can be controlled correctly.

If the MB head bolt uses (say) 40mm of engagement, the 180 turn is 0.5% strain, which is certianly yielded, and in the flat part of the curve.

(best curve I could find, and it's true stress not negineering, but still shows the yield for 4140 steel, rather than my previous structural example).

It's a couple of scrolls down for 4140

First application of combustion loads and they will stretch a tiny bit more (which is matched by relaxation of the other components that are bolted together. From that initial application of combustion loads, the bolt then in the joint (head and gaskets), and as combustion load is released, the bolt then establishes a new elastic mode of operation, parallel to the orhginal elastic mode, but offset due to permanent lengthening. Combustion and therml loads are handeled elastically.

On release of the fastener, the elastic strain (0.2%ish) is released, and the remaining 3%(ish) remeains as permanent elongation.

As long as the accumulated 0.3%s are less than the plastic range, and there's enough left for one more thermal/pressure application, then they can be re-used a number of times. Depending on the material, and installation process, that may only be once, or many.

 

[jsap]

I think between you and I, we've licked this one 99%, with only 1% disagreement. Shoots, that's better than most scientists, so we're doing great!

The word "yield" is what's confusing everyone, including our own conversation. The TRUE elastic limit is way at the bottom near the toe of the curve, and the remaining proportional (or linear) part of the curve is only a straight-line approximation of very very very slight permanent deformation (so slight, it's actually difficult to measure). Hence, the PRACTICAL elastic limit is much higher on the curve, closer to the yield point.

When we get near the yield point, that very very very slight permanent deformation begins to be measurable practically. However, it's still not technically yielded until (AND THIS DEPENDS ON THE CURVE, WHICH DEPENDS ON THE CARBON OR ALLOY COMPOSITION) it reaches either the peak before returning down (or in the case of curves that do not return down, a % offset from parallel).

So, while, in a sense, the steel has "yielded" because it experienced MEASURABLE permanent deformation, the properties of the steel does not radically change until past the offset yield point, so it's technically not yet yielded.

In other words, there is a zone of permanent deformation that is after Elastic but before Yield. However, if you start getting into these discussions with PhD's, you might get lucky and find one who likes to stir up controversy. In the end, it comes down to who was the biggest mouth in the room who got to define what, who published the most books, who raises the most funding for research, etc.... That's why these definitions change over time.

This is very subtle, and we only can go further with this conversation by dissecting each actual curve from each carbon or alloy (btw, even the definition of carbon steel as an alloy is controversial, but not really worth fighting for). But I don't have the curves for GM's or MB's or Toyota's head bolts (which are not off the shelf grade 8 bolts), so we'll just have to trust the manufacturer's repair manuals. For us practical folks, if they say replace, well, unfortunately, without better information, we'll just have to replace. If they give limits, then we can follow the limits with confidence, as long as there is no damage to the bolt (local stretching, damaged threads, etc...).

Cheers

 

[Shannow]

I get the drift.

When we did it 20 years ago, yield was the end of the elastic zone, or proof stress for non-straight line alloys...I think the 1% is now cleared up.

I'm hoping that someone sends me an audi one, so I can take it to work and XRF it.

 

[mechtech2]

TTY bolts are common now.
The used ones we see are usually longer than the new replacements.
What does this tell us?

While they are tightened, you can feel them stretch, not act like a simple spring with linear torque increases.

2-3% room for error when reusing them is too fine a line. A new sequence has to be used, at a minimum. What is this sequence? Who knows for each bolt?

 

[Shannow]

Of course they are longer, that's how they work.

It's not 2-3% room for error.

It's 100-500% depending on design...please re-read the above.

 

[mechtech2]

That's where I got the 2-3 %.

I have had to reuse TTY bolts on cyl heads a number of times.
But I do NOT recommend it unless there is no option..

 

[Shannow]

OK re-read and try to understand then.

You are stretching them 0.3 to 0.5% OUT of a potential 2-3%. That's not 2-3% margin for error, there's whole safety factors in there.

Generally, I don't re-use head bolts at all (Holden 6 cyl regular bolts aren't really that reliable second time around), but it is possible, and valid in an engineering sense.

 

[pottymouth]

Wow! The technical info is great but makes my head hurt.

In an ideal world I would love to just replace everything including the bolts, but this is a special case. Although I used to own the car, it is not really mine anymore. It was given to a member of my wife's family who is notoriously unreliable. This car was in great shape when we gave it to her, but has fallen into disrepair in the last 2 years.

She now wants to sell the car and has a buyer already arranged. I volunteered to make the repair and must front the cost of all materials on the promise that I will be reimbursed after it is sold. There is a better chance I will walk on the moon than ever see a penny. Bolts for one head will probably run me about $35. It's not a lot of money, but due to the situation I am really trying to limit my losses here. It's not about me being cheap. If I were cheap, I never would have given her a car that was worth 4 grand at the time.

Still, I don't want some unsuspecting buyer to end up with a blown headgasket in a couple months. I'll see if I can find a specification for acceptable length. Thanks to all for the input.

 

[mechtech2]

Don't use the original tightening sequence.
It is likely to be too much.
There is no acceptable used length - don't waste your time looking.

 

[Shannow]

If you want to feel a little more secure (and it's not your engine and bolts), but still a Chrysler product and recommendation.

http://www.enginebuildermag.com/Content/...00000005691.pdf

Quote:
Cylinder Head Re-Torque
Procedure For 2004-2005 Chrysler 2.4L VIN B, J, X, 1 & 8 Engines
There is updated information on cylinder head re-torque procedures for 2004-2005 Chrysler 2.4L
.
.
.

procedures, remove the cylinder head cover.
4) Using a six-inch wobble plus extension friction ball and shallow socket, and following the torque sequence shown in Figure 2 (below), loosen one bolt at a time to 0 torque and then torque that same head bolt to 60 ft.lbs.
5) Repeat step 4 for every head bolt one bolt at a time in sequence.
6) Verify that each head bolt is at 60 ft.lbs before performing next steps.
7) After all head bolts have been verified to be torqued to 60 ft.lbs follow the torque sequence and turn
the head bolts an additional 90°(1/4 turn).
8) Install the cylinder head cover
and remaining components and run engine and check for leaks.


Quote:
Cylinder Head Installation For 1999-2006 Chrysler/Jeep 4.7L VIN J & N Engines
The cylinder head bolts on 1999-2006 Chrysler/Jeep 4.7L VIN J & N engines. are tightened using a Torque-To- Angle procedure commonly referred to as TTA.
Even though the cylinder
head bolts are tightened using torque and an angle procedure, the bolts are not a Torque-To-Yield TTY, which are a single use design.
Therefore,TTA bolt re-use is possible if each bolt is “qualified” (examined for neckdown) first.
If the threads are necked down, the bolts should be replaced. One method of checking for neck-down is to use a like-size nut and check to make sure you can easily thread it along the
entire threaded area of the fastener.
Once reuse or replacement is determined, follow the steps listed below to install the cylinder heads for these engines.

1) Clean the cylinder head and cylinder block mating surfaces.
2) Position the new cylinder head gasket on the locating dowels.
When installing cylinder head, use care not to damage the tensioner arm or the guide arm.
3) Position the cylinder head onto the cylinder block. Make sure the cylinder head seats fully over the locating dowels. Note: The four smaller cylinder head mounting bolts require
sealant to be added to them before installing. Failure to
do so may cause leaks.
4) Lubricate the cylinder head bolt threads with clean engine oil and install the ten M11 bolts finger tight.
5) Coat the four M8 cylinder head bolts with Mopar Lock & Seal
Adhesive (or aftermarket equivalent), and then install the bolts finger tight.
6) Tighten the bolts in sequence shown above in Figure 4, using the following steps and torque values:
1) Tighten bolts 1-10,
15 ft. lbs. (20 Nm).
2) Tighten bolts 1-10,
35 ft. lbs. (47 Nm).Tighten bolts 11-14, 18 ft. lbs. (25 Nm).
3) Tighten bolts 1-10, 90°. Tighten bolts 11-14, 22 ft. lbs. 30 Nm).

 

[jsap]

That's a great article. I wonder how many other manufacturers use TTA (Torque-To-Angle) rather than TTY.

It seems the Chrysler head bolts can definitely be reused, as long as the bolt's overall length is within spec (if not specified, I'd stay under 2% stretch) and there is no neck down at any thread (confirm by threading a nut all the way through, but you'll always be uncertain of the top-most thread where the nut can't go over). Because of possible neck down at the top most root, I personally would only re-use TTA head bolts once.

It might be difficult to understand the theory of TTY vs TTA (also called TTT for Torque Turn to Tighten) bolts, so I drew these up on some sticky notes. I hope they help.