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headbolts

Which head bolt is better? the old style or the new style?

A good question. And some would say: forget bolts: go with studs.

The old style bolts were in use up to about 1980. They were superceded by the new style bolts. At the time of the supercession, the head bolt torque specifications and procedures were changed; to reflect the difference in the design of the fastener.

There is an ongoing debate among fans of the turbo'd "Bricks" as to which head bolt is better.

I am firmly in the camp of those preferring to use the new style bolts. There is more to it than just "personal preference" as far as my reasons for using the new style bolts.

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To begin with: there is a lot of very good information available on fastener theory, design, and application. For in depth information, I would suggest research on the topic.

...because I do not expect people to just accept what I say. Go prove it for yourownself!

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Here is "my" take on the subject:

There are three basic methods to bolt down a cylinder head:

1)...tightening the head bolt [or stud nut] to a specific torque reading using a torque wrench. This is known as the "Torque" method. It is the method that has been used since engines were first built.

2)...tightening the head bolt to a specific torque reading, known as a 'snug torque' value, to equalize the pressures on the head gasket, and to establish the basis point for the next step; which is to turn the bolt a specific number of degrees...to turn the bolt a specified angle [example: a 90 degree angle tightening is a quarter of a turn]. This is known as the "Angle Tighten", "Angle Tension", "Torque/Angle", or "Torque/Angle Tighten" method. I will refer to this as TAT.

3)...or tighten the bolt to a specified torque value for the reasons stated above; then turn the bolt a specified number of degrees [like in the TAT method] either one time, or do the angle tightening procedure twice [as in one round of 90 degree turns of the bolts, followed by another round of 90 degree turns of the bolts]; the purpose of this method being to stretch the bolt to the point of "Yield". This method is known as "Torque to Yield", or "TTY" for short.

Because the procedure for the "Torque/Angle" and the "Torque to Yield" methods are basically identical, they are often confused and mis-identified. I have been guilty of such confusion and mis-identification. [which is WHY I put together this gallery]

Despite the similarity of procedure, the goals of the two methods are QUITE different. Since both the TAT and TTY methods are derived from, and purposed to improve upon, the original "Torque" method; a quick review of the original method is in order.

The "Torque" method has worked for years, in spite of the main deficiency: the torque reading used is dependent on, and very affected by, the friction of the bolt threads/block hole threads and the bolt head base/cylinder head material under the bolt head base. Upwards of 90% of the applied torque to tighten the bolt can be and actually is used to fight the friction. What that means is that only 10% of the torque applied to the bolt ends up as being used to apply the clamping force of the fastener. Besides being a considerable source of loss of applied clamp, the friction is a variable that causes considerable variation in accuracy of the clamp pressure applied. Reports that I have read indicate an accuracy variation of as much as 35% in clamp force applied using the "Torque" method.

Over the years, the bolts were improved with stronger materials and better thread designs...rolled threads as an example...and improved thread lubricants; all in the pursuit of a stronger and more accurate clamping force applied. "Studs" were developed and implemented to improve upon the basic head bolt/torque down procedure. The classic head bolt or "stud kit" work very well.

The "Torque" method, using either head bolts or studs/nuts, is intended to turn the bolt [or the stud's nut] enough to actually stretch the bolt [or stud] a few thousandths of an inch. It is this stretching [AKA "tensioning"] of the bolt or stud that actually applies the clamping force.

If the bolt or stud is only stretched a certain amount, then when it is loosened it will return to its original length. Stretching the bolt or stud within this area of 'stretchability/return to length' is known as stretching the bolt/stud within it's 'elastic' area. If the bolt or stud are tightened to the point that the stretching cannot "unstretch", the bolt/stud will not return to original length when loosened. This point beyond 'stretch/return to length' is known as the "Yield" point.

Classic head bolts and studs are designed and torque specified to be tightened to a point below the Yield Point, AKA 'percentage of yield"; usually about 75% to 80% of yield.

...in other words, when you torque down a classic head bolt to the specified torque wrench reading, you are trying to stretch the bolt to a point just under the point at which the bolt will permanently stretch out of length. When staying under the Yield Point, the clamping force applied will be at the maximum possible.

...it is this "maximum possible clamping force" that is the reason why many prefer to use either the classic style head bolt, or upgrade to studs.

IF 'maximum possible clamping force' were all we had to be concerned with, then there would be little problem with making a choice regarding type of fastener used: the classic style head bolt or stud would be the premier choice.

BUT....there is more to it than just how much of a clamp is applied. Which is why and where TAT and TTY come into the picture.

The TAT method is actually a refinement of the "Torque" method. Its purpose was to find a way to minimize the variations caused by friction. Knowing the thread size and pitch, it is easy to determine just how many thousandths of an inch the bolt [of a known diameter and material] will stretch if it is turned a certain number of degrees. The angle tightening method is a way of more accurately stretching the bolt. Reports that I have read indicate a accuracy variation in clamping force applied using the angle tightening method to be in the 15% range...a considerable improvement over the "Torque" method.

It is very important to remember that the TAT method, while more accurately stretching the bolt than the "Torque" method, is still a method that only stretches the bolt BELOW the Yield Point:

...TAT stretches the bolt to a percentage of yield...the bolt remains in the elastic area of tension...and returns to length when loosened.

'Torque To Yield' is different from 'Torque' and 'TAT' because:
....with TTY, the goal IS to tighten the bolt to the Yield Point....and actually a bit beyond that into the plastic area of stretch.

WHY?

To stretch the bolt beyond the permanent elongation point seems counterproductive: you are going beyond the maximum clamping force capability [at 75-80% of Yield] and on to the point of having over-stretched the bolt.

Like I said: there is more to it than just maximum applied clamping force.

With the classic style head bolt or stud, the tensioning below yield does provide a very steady clamping force. The steadiness of the clamp is good. The constant-ness of the clamping force is good.

..and is also a source of problems.

With the use of aluminum cylinder heads, the needs for clamp changed. Instead of a 'maximum clamp applied' that is constant and unchanging being optimal, a steady clamp that can allow for the thermal expansion of the aluminum head without exceeding the compressibility of the head gasket became more important. The classic head bolt or stud does expand and allow for some thermal expansion, but the flexibility was not quite optimal. When the aluminum head warms up at full operating temperature, it grows in size, putting more bolt tension on the head gasket, and on the aluminum itself. This results in brinelling of the head surface where the fire rings of the head gasket are located, overcompression of the HG itself, and distortion of the aluminum around the head bolts. If the HG cannot handle the excessive pressure that occurs, it will remain permanently thinner; so that when the engine cools down, the HG does not provide as good a seal between the head and block. Over time, this will lead to HG failure.

Enter TTY:

By stretching the bolt beyond the yield point, and into the plastic range, the maximum clamp applied is reduced; but by being in the plastic range, the bolt can and does give more with the thermal expansion of the aluminum head. The clamp is not as great; but it is steadier throughout the temperature range...a very important thing when using aluminum for head casting material. HG longevity is increased; and brinelling and distortion of the head is reduced.

...[sounds good to me]...

There is one other good result of using TTY bolts: a further reduction in the variation of applied clamp force between the bolts. Reports that I have read indicate that TTY bolts reduce the variation to the 7% range; a very considerable improvement over the classic style "Torque" head bolts. This means that using TTY bolts can provide a very improved uniformity of clamp around the head.

...[and that sounds real good to me as well]...

So far, it sounds like going with TTYs is a good choice; and that Volvo did that when they superceded the old style bolts with the new style bolts. And based on appearances of the new style bolts, and the revised tightening procedure, it sure looks like they are TTYs.

AND, up until recently, I viewed them as TTYs, called them TTYs, and defended their use as being better than the old style bolts based on the assumption that they were TTYs.

I WAS WRONG!!!

THEY ARE NOT TTYs!!!

As stated earlier, the tightening procedures for TATs and TTYs are nearly identical: an initial torque; usually followed by another torque; then a final angle tightening. Often, TTYs receive two angle tightening rounds, but not always.

Normally, TTY bolts are a one time use; but that is not necessarily always the case. So, the fact that the new style bolts CAN be reused up to four times [as per the green manual] is not absolute proof that the new style bolts are NOT TTYs.

The biggest reason that most [myself included until recently] consider the new style bolts to be TTYs is the appearance of the bolt itself: it has the reduced diameter section between the head and the threads: like most TTYs have. Combine the appearance of the bolts themselves with the angle tightening procedure and the reasonable conclusion to be drawn is that they are indeed TTYs.

BUT.........[and this is one of those Bertha Class size of 'buts': a really BIG one].........
...there is one very important thing that needs to happen when tightening down a TTY bolt that does NOT happen when tightening down the new style bolts on a Volvo redblock:

....THE BOLTS DO NOT YIELD.

In all the motors on which I have tightened down the new style bolts, following the factory procedure [15 lbs-ft, 45 lbs-ft, angle tighten 90 degrees], I have never felt the bolts yield.

And when a bolt yields, you can FEEL it "yield". It feels like you have started to pull the threads: like it went soft. I HAVE overtightened bolts on other things; I KNOW the feeling of the yield. And when you feel a bolt yield when it should not have, it gives you a certain kind of sick, sinking feeling in the stomach...

The new style bolts are not TTYs. THEY ARE TATs.

...The final angle tightening part of the procedure is to improve the clamp force accuracy.
...The reduced diameter section of the bolt body is to provide the flexibility needed to respond to the thermal expansion of the aluminum head; without having to go with a 'one-time-use-only' bolt.
...The new style bolts can be reused up to four times, IF there is no evidence of the bolt having stretched! By its very nature, a TTY bolt will (more than likely) BE stretched when loosened.
...The new style Volvo bolts are not supposed to stretch: if they do, you replace them.


In Conclusion:

That is how I see it; I have stated my reasons.

I will continue to use the new style bolts: BECAUSE they are better than the old style bolts at APPLYING and MAINTAINING a MORE UNIFORM CLAMP [throughout the temperature range] on my aluminum cylinder heads.

Footnote for the skeptical:

...in the Penta redblocks, the same HG and the same part number head bolts are used. Penta gives a head bolt tightening procedure of: 15 lbs-ft; 45 lbs-ft; angle tighten 120 degrees. The Penta manual also states that those head bolts CAN be reused up to four times, if they have not stretched.

Hmmmm...sounds like even a 120 degree angle tightening is less than the yield point.

UPDATE: A point that I have confirmed in practice: NO YIELD at 120 degrees angle tightening.

Anyway....just something for the skeptics to ponder and mutter about.
the two choices
the two choices
another view
another view
the difference in the drive end
the difference in the drive end